Intermittent EV

A few months ago I was cruising along and my EV suddenly lost power. It was like someone had pushed in the clutch. I lifted the bonnet but the problem seemed to have gone and off I went again. Then it happened again last week. I suspected the throttle pot, as I know they can wear or get dirty over time. The speed controller can sense if this pot is open circuit and will shut down for safety reasons. So I pulled off the connectors to the throttle pot and sprayed some circuit cleaner on them. This seemed to fix the problem for a days.

Then just today it became really bad. Stop-start, stop-start. I could hear the contactor clunking so I knew the 12V circuit was OK. Other possibilities were a loose connection in the battery pack or a bug in the (rather expensive) speed controller. So I popped the bonnet and started poking around. Then I noticed something. By moving one wire I could hear a small relay clicking on and off rapidly. This relay (I think, it’s been a while since I built the EV) supplies a “go” signal to the speed controller. As I moved the relay it would click on and off, and the motor power would cut off.

Then I wiggled the solder connections to the relay and sure enough one of them was loose. Ah-ha! A few minutes later the problem was solved. A bad solder joint is not bad for 3.5 years electric driving I guess. Shows how easy Electric Cars are to fix as well.

28,000 Electric km

I haven’t written much about my EV for a while as nothing much has happened. It just goes and goes, and has just clocked over 28,000 electric km since it was converted. This post is a collection of notes from my EV driving in 2011.

It’s been two years since I installed Lithium batteries and they have operated faultlessly. I haven’t touched them. It’s a bit deceiving actually, guess I should check the terminals or something. But when it just goes and goes you get a bit complacent. I don’t even have an ammeter or voltmeter at the moment.

This picture says it all – spider webs where the petrol used to go!

However I realised I hadn’t taken the car in for a “service” since I finished the conversion to electric drive 3 years ago. This is because there isn’t much to service in an EV, no oil, water, spark plugs, timing belts, heads to crack, water pumps, hoses, fuel pumps, or exhaust systems to corrode. The only wearing parts are brakes and tyres. So I took it down to my friends at Woodville Park Autos and they rotated the tyres, checked the brakes and replaced the wiper blades. That’s the maintenance for 3 years and 28,000 km on an EV!

Earlier this year I did 108km of city driving on one charge, and the car still felt just fine. So I am not sure what the range actually is. One of those things I don’t really want to find out!

The 6.7 inch Advanced DC motor is adequate for a 60 km/hr commuter vehicle on the Adelaide plains. It’s quick off the line on the flat but struggles up hills at say 80 km/hr. OK for our terrain but I think I would put a 8 inch motor in next time. However this would require bigger Lithium batteries to provide enough current.

The feeling of “electric cruising” on a warm summer night is quite magical. Windows down, a warm breeze in your face and no motor noise.

My daughter turned 16 recently and had her very first driving lesson in the EV:

It’s much like an automatic to drive so a nice easy way to start. I am sure EVs will play a much bigger part in her life time than ICE vehicles.

Driving the Nissan Leaf

Yesterday I took part in a Green Zone Drive event and had a test drive in the Nissan Leaf in the Mebourne CBD. I attended along with Michael and John, of the 3 Day EV fame. Also present were some other low emission vehicles and the Mitsubishi MIEV.

Our guide for the drive in the Leaf was Paul, a very helpful EV enthusiast from Nissan. Paul especially enjoyed meeting Michael and I as we both drive (home converted) EVs every day.

So off I went for a test drive in busy Melbourne CBD traffic! For about 10 minutes we drove around city blocks and had the occasional chance to put our foot down and test the regen. Compared to my home-made EV the Leaf was very smooth and refined. It was silent to drive and accelerated well. The AC motor gives more mid range acceleration than my DC motor conversion. The Leaf has lots of cool flat-screen diagnostics showing kW used, regen stats, an estimate of remaining range, and how much power the air conditioner is using.

Michael, John and I made some comparisons between the Leaf and the MIEV, for example dashboard electronics, size, and performance.

However I think these are small issues. The real comparison here is between petrol and electric. Car companies marketing factory built EVs should be hammering home the advantages of Electric Vehicles such as low “fuel costs”, trivial maintenance, low reliance on foreign and depleting oil supplies and zero emissions. I should know – we have 25,000 Electric kilometers on our home built EV now. It is really hard to go back to petrol after driving electric……

Some more photos. John is 6 feet tall, and had adequate leg room in the rear of the Leaf:

Here is the Leaf just in front of us while driving the MIEV. We nearly had an silent drag race on our hands…..

The engine bay looks like a regular 4 cylinder car. In fact the silvery casting on the left is the same cylinder head cover as used in some 4 cylinder Nissan engines. The 12V lead-acid battery seems like a throw-back, but I also have one in my EV to power the 12V electrics.

There are three phase and single phase charge connectors giving the car a fast charge capability, although overnight charging is recommended by Nissan to maximise battery life. I charge my EV overnight. It’s no big deal once you realise “refueling” an EV is more like charging a mobile phone than filling up at a petrol station. Just plug in when you get home and come back in the morning.

For me the Leaf has the feel of a “mainstream” medium size vehicle. If you travel less than 100km a day buy one and say good bye to petrol, servicing and emissions. It could be the “break through” EV.

EV Efficiency

Paul pointed out an interesting fact: EVs are more efficient in city driving than on the highway. This translates to increased range in the city. I think this is because the major energy loss in EVs is wind resistance which goes up quickly with speed. Everything else (e.g. the battery to wheels power train) is already very efficient at any speed. Efficiency is one reason we use electric motors in fixed industrial applications like factories rather than petrol engines. In ICE vehicles the major losses are in the power train, i.e. petrol to wheels efficiency is at best 20%. Power train losses dominate with low speed stop-start driving.

Here is a classic example. In an EV the motor stops when you do. So it’s efficiency sitting at traffic lights is 100%. No energy consumed for no velocity. An ICE vehicle wastefully idles at traffic lights, so it’s efficiency is 0. Fuel is being used to propel the vehicle at zero velocity.

EV Pricing and Market Traction

The MIEV and Leaf are being pitched at around AUD $48,000, about the same price as a Prius here. In the same market this sort of money will buy you a very nice mid-size European car. I would like to see EVs for a drive away price of less than $40,000. This would differentiate these EVs from the Prius, and get over the “range anxiety” hump compared to hybrids. When everyday people “get into” EVs and start talking to each other network effects will take over.

As I have pointed out before, an electric vehicle is implicitly simpler than an internal combustion vehicle and should cost less to make and sell. Like a lot of consumer electronics hardware, the price can be driven asymptotically down with increasing volume.

So it’s a volume game. Market traction and competition will push the price down. At this stage Manufacturers are dipping their toes in the water. But at some point competition will get real, consumer demand will increase, volume will increase, and EV prices will drop sharply.

2010 EV Festival

This years Australian EV Festival was held in Adelaide. Friday was a conference day, with a speakers from Universities, wind power and charge point companies, car companies, state government, and EV drivers. Many, many really smart people. Lots of great ideas. I talk about some of them below.

The photos are from Saturday, where there was a public display of home converted and commercial EVs. This photo is Rosemary talking about our EV to Michael Harbison, the Lord Mayor of Adelaide, who is quite keen on Electric Vehicles:

Peak Oil is Main Stream

I was surprised at how many speakers cited Peak Oil. This is a big change from a few years ago. For example Tom Kenyon MP had the opinion that EV take-up will occur due to sky-rocketing oil prices over the coming decade, due to world wide economic recovery and diminishing oil supplies. Environmental reasons, though important, just don’t motivate the same way the price of petrol does.

Range Anxiety

I heard the term “range anxiety” for the first time from Peter Pudney. This is a great term – describes the main concern the general public has when thinking about EVs. It evaporates after you drive an EV for a few days. I don’t even know what the range of our EV is, we just drive wherever we want each day and plug it in each night.

It’s like a threshold, once your EV has enough range for the sum of your daily trips, range is a non-issue. This depends on your city and your commute. Peter presented some great graphs on this. For Adelaide (1.2M people), 92% of people have a total daily drive of less than 100km. This is the range of todays average Lithium powered EV.

I guess 100 years ago early ICE drivers had similar anxiety over finding gasoline stations.

Charging Points – Do We Need Them?

Charge Point Australia had a interesting talk about their very high tech charging points (charging bollards). They even use mesh networking to connect their smart charging bollards, have a network management system, billing system etc. Reminds me of the technology used in the Village Telco and Flusko projects.

Anyway the idea behind smart charge points is that I can use GPS and my smart phone to direct me to a vacant charge point so I can recharge my thirsty EV while I shop for a few hours. The electricity then gets billed to me, and I get all sorts of stats on economy, a map of where I have charged etc.

Only, it’s not a problem that I need solved, at least for day-day driving. I am a real-world EV driver, with a modest EV, living in a medium size city, and I don’t need to charge during the day. Charge points might be useful for short country trips, but we only make these a few times a year.

See the 92% figure from Peter Pudney above – most people won’t drive far enough to need a charge point. Now for larger cities in todays Gen 1, 100km range EVs I can see some need for charge points. However the 200km EV (possible today, likely tomorrow) will make public charging even less common.

Actually I have a bigger problem with the notion of charge points. There is a myth that we “need” charging infrastructure before large scale EV adoption can happen. It’s just not true, and this myth works against EV adoption. It’s EV-FUD based on range anxiety. Even in the largest cities vast chunks of the population drive less than 100km/day.

Honestly, our experience is that the “problem” of charging is largely solved by the humble General Purpose Outlet (GPO). I have about 30 in my house.

Fast Charge

There were some interesting ideas around fast charging (20 minutes or so) stations. They could look like regular gas stations, and people could buy a coffee or check their email while they wait. A big problem is where to find the huge amounts of electricity needed to fast charge each car – e.g. 50kW per EV, or 500kW for 10 cars. This sort of power could be a show stopper. It’s the average power drawn by 500 houses. It means rewiring a city and dealing with massive peak loads.

If you want this technology for long distance EV driving, then you need huge amounts of power available in the between cities. Right where the power is not available at the moment. That’s a lot of expensive infrastructure.

To me, this sort of thinking is an attempt to map Internal Combustion Engine (ICE) thinking to EVs. We currently use gas stations, so we must continue to use gas stations. We can currently “charge” quickly with fossil fuels, so we must continue to “charge” quickly.

I bet none of these guys actually drive an EV. We just plug our EV in at night like a mobile phone. It takes 5 seconds to plug in and walk away. Would you take your mobile phone to a special place and wait 20 minutes just to have a fast charge? No, neither would I. Do you like going to gas stations? No, neither do I. It’s so much nicer to refuel your car at home.

Refuelling an EV is already a better experience than refuelling an ICE vehicle.

Supply of EVs is the Problem

There is a lot of demand for EVs that people can buy and drive away. Several presenters pointed out that supply of EVs, not demand, is the problem. So where are the EVs?

Mitusbushi have done a great job by having a number (112) MIEVs on the road here in Australia, most of them have been leased to governments for trials. However the price ($60,000 for a 3 year lease plus, I presume, a residual) is very expensive, over 3 times what a equivalent internal combustion car would cost.

As several people pointed out, it’s cheaper to buy a new internal combustion car, through away the CO2 generator, and convert it to an electric car with equivalent performance. When asked about the price, the expense of the battery pack was mentioned. However I can (and did) buy an equivalent battery pack for $6,000. In quantity 1.

The University of Western Australia presenter described just how disruptive this technology is. The Australian government gets $15B from fuel tax – and we have some of the lowest fuel taxes in the world. Car dealerships depend on servicing income, which evaporates with EVs that require near-zero servicing. So you can see why big car companies would be in no rush to deploy EVs.

As other EVs come to the market, I think competition will fix the price issue. There are many new EVs planned for release over the next few years. I figure with steady competition, the $15,000 EV is coming.


A big thanks to the people who helped put on the EV Festival – especially Eric Rodda who I know worked very hard for this event. Here is an under the bonnet photo of Eric’s very nice EV conversion, which he recently completed:

Some University students built this novel, self balancing EV:

David Sharpe drove his EV to Adelaide from interstate. He has 3 Zivan chargers for fast charging and an auxillary generator for emergencies. He typically drives for 1-2 hours, then charges for one hour. A novel, relaxed way of travelling long distances.

Equalising Lithium EV Batteries

In December 2009 I installed a new Lithium battery pack in our EV. The pack has been working really well, not a single battery problem for 10 months. The previous lead acid pack was always causing problems, possibly because we pushed them a bit to get the range we needed. I didn’t mind so much, as I like tinkering with the EV. Other people I know have had better luck with lead acids, in particular when the depth of discharge was low. However a Lithium powered EV is a much more practical vehicle for the every day driver.

We have done up to 75km on a single charge. I have no idea if that was close to the limit as I don’t have an amp-hour meter. In fact I don’t even have a volt or ammeter at the moment. We just get in and drive, then plug in at night to charge.

Anyway after 10 months and around 7000km total on the Lithium pack it was time for some maintenance – I wanted to see if the pack was equalised. That is, make sure all of the batteries are in a similar state of charge.

Manual Charger

All 36 Lithium cells are in one series string. Wired across segments of that string I have three chargers. The 96V charger handles 28 cells, and each 12V charger 4 cells each. The 12V chargers always finish first, and never trip the over-voltage cut-out. The 96V charger takes a longer, and one of the cells always trips the over-voltage cut out. This means at least one cell is a little “ahead” of the pack, hitting 4.1V and dropping out the charger.

My Battery Management System (BMS) uses CM090 BMS modules on each cell. These BMS modules have a series regulator feature. As the cell reaches full charge, the series regulator starts to bypass current around the cell. Each regulator can bypass about 800mA. Electric car chargers are designed with a certain charging “profile”. They start out with a bulk charge current (say 10-20A), then towards the end taper that current down to 1A or less. The low finish current allows the series regulator to bypass current (up to 800mA) around the fully charged cells allowing the slower cells to catch up.

However my 96V charger was designed for lead acid cells so it doesn’t have the right “profile”. It tends to charge at full current (8-10A) rather than “tapering” the current at the end of the charge. When the first cell hits 4.1V the BMS system shuts down the charger. So I am not getting the benefit of the series regulators and there is a chance that some cells may be out of balance.

A single EV charger designed for my pack would get them all to 100% every time I charge, by tapering the charge current at the end of the charge to 500mA.

The picture below shows the CM-90 BMS modules fitted to 4 Lithium Cells. The red LEDs indicate that each cell is over 3.6V. This means they are at about the same state of charge. At the end of a charge we want all red LEDs lit on all cells.

series regulators in action

Balancing Cells

To balance my cells I first charge them normally, then connect a simple low current “manual” charger. This charger is a Variac followed by a full wave bridge rectifier and an Ammeter. It delivers an average DC current of 0-2A that can be controlled by the voltage on the Variac. However it has no automatic cutout so you need to be very careful when using it. I always connect the AC plug of the manual charger to the AC socket on my BMS controller, so it will be switched off automatically if a cell hits 4.1V.

To balance the cells I dial up a small current like 0.5 to 1A then wait for the LEDs on each cells series regulator to come on. The red LED indicates the cell is above 3.6V. The trick is to bring them up gently (500mA to 1A) so that no cells hit 4.1v – at that point the BMS controller drops out the charger.

However my cells are rated at 100A, so charging them at 1A can take a while. For example if one cell is 5AH down, it could be 10 hours at 0.5A. If I up the current to 5A the series regulators on the full cells won’t be able to bypass the current, those cells will hit 4.1V, and the BMS system will cut the power to the charger.

So to speed up the process I connect a 12V charger across blocks of 4 cells at a time. This way I quickly charge any low cells at 12A, which takes around 30 minutes.

In the picture below a 12V charger is connected to a block of 4 cells in the middle of the pack:

topping up a block of four cells

After a few hours I had all LEDs lit on nearly all cells. To bring the last few up I connected the manual charger at 500mA to get the last fraction of an AH. After about 30 minutes all cells had all LEDs on.

In the picture below just one red LED is unlit:

last cell

I estimate that the worst cells were no more than 10AH down (about 10%). I don’t know if they get more out of sync over time, or if they get 10% out and just stay there. The cells are likely to vary in capacity, so 10% doesn’t sound unreasonable over 9 months.


EV Page
Electric Car BMS Controller
Lithium Batteries for my EV

Science Alive

Our little EV has been on display at Science Alive – a local 3 day science show. Our local branch of the Australian Electric Vehicle Association (AEVA) was asked to set up a booth. A good chance to show off Electric Cars to the general public.

Rosemary at Science Alive

At the show we set up a slide show of photos on a big LCD screen, for example shots of the car under construction and the various components and tools we used. We also typed up 2 page flyers on Rosemary’s Electric Car (yes, I have admitted it is hers!) which talks about our real EV life experience and busts a few EV myths like:

  • Electric Cars are Expensive
  • Electric Cars are Slow
  • They don’t have enough range
  • But we need infrastructure like charging stations
  • The power stations will be overloaded
  • Charging is slow

The cool thing about our EV is it gets used every day. We greatly prefer it to our other, infernal combustion car. This gives us real life EV experience that we like to communicate to people. Much better to hear it from real people using a real EV every day than the media or car companies.

The booth was staffed by local AEVA members over 3 days, including Rosemary and I. Rosemary manned the booth on Friday when all the school students came through. She found the girls to be more interested in the the car – they asked the best questions.

Rosemary with students

I spent Sunday at the booth. I thought the LCD slide show worked really well to explain things that weren’t obvious from the assembled car, like exactly how the motor fits to the transmission. There were a lot of common questions, which could probably be handled by signs at the next event:

  • The range (80-130km for our car). This is a major preoccupation for non-EV drivers. However we find our EVs range isn’t an issue for us in day-day driving. The question is more an indication of the current state of education on EVs. Once you drive one, a lot of concerns like range and charging go away.
  • Where is the motor? Our motor is small (7 inch diameter by 14 inches) and obscured by batteries. A picture or arrow might help.
  • Kids instinctively want to touch, even though there is a do not touch sign. Little kids can’t read. This is a concern when you have 120V and 500A within a few cm of little fingers.

Thanks especially to Eric Roda and Edward Booth for the hard work they put into this event. Science Alive was a good warm up for the National EV Festival which will be held here in Adelaide on November 5/6 this year.

Blown Armature

The other day I was driving my daughter to a friends house in our Electric Car. I hammered it a bit taking off from the lights. Suddenly the engine seemed to shudder and loose power. It felt like a petrol motor that was “missing”. I stopped, pulled over and popped the bonnet. I could smell burning. Revving the motor I could see small sparks flying out near the brush end of the motor. Oh dear.

I managed to limp home at low speed and it was up on the ramps early the next day to drop the motor and take it down to my local electric motor shop. Eventually it was decided that the armature was kaput. Some dead windings and a 1 ohm connection to ground (normally both motor terminals are isolated from the chassis).

I phone up the good people at EV Motors Australia and they were surprised – the exact same motor has been running for 19 years in their EV with just a change of brushes. They said “the only thing that can hurt these motors is over heating”.

Oops. A suppressed memory came bursting forth!

You see just after we put our EV on the road 18 months ago the motor had a couple of “near death” experiences. At one point it popped out of gear at 80 km/hr. You can’t hear the motor at that speed over the road noise so we over revved and over heated it before we realised it was out of gear. After that the motor was making a sewing machine noise as the “commutator bars” had been raised. My friends at the local electric motor shop tut-tutted but machined the commutator which brought it back to life. However I was warned that I might be on borrowed time. About 18 months and 16,000km as it happens.

It popped out of gear because your friend and author of this post forgot to re-attach a small bracket between the gearbox and chassis. This bracket stops the gearbox twisting under torque. It was this twisting that threw my car out of gear during acceleration and started this sorry saga.

The second near-death experience was accidentally driving around in 5th gear. “What’s this red light on the dash?” asks Rosemary after the light in question had been lit for 20 minutes. That would be the over-temperature light, Rosemary. Please pull over quickly when you see that! Fifth gear at low speed (we normally drive in third) means lower revs and hence more current and heat. Electric motors have so much torque they happily pull 5th gear at 0 RPM. It’s just that they get hot after a while.

Here is the dud armature and a close up of the commutator. The carbon deposits on one bar indicate the dud armature winding (so I’m told, I’m not sure why).

A new armature was procured for $950 and fitted, and I put the motor back in the car. The photos below shows all the bits that spin, in the order they are assembled. When I started my EV conversion I had no idea what these bits looked like so I have labelled them for you.

The Adaptor Plate allows the electric motor to bolt onto the bell housing. The other parts in the picture come from the original car. Many EVs are clutchless, but I elected to keep the clutch. Although I would go clutchless next time – it’s just simpler. Without the flywheel it’s surprisingly easy to change gears without a clutch (I have tried this on other EVs). Gear changes are rare in an EV anyway, I just use 3rd for all forward driving.

The next picture shows the Adaptor Plate bolted to the motor and flywheel to the coupler.

The pressure plate is then bolted to the flywheel with the clutch plate held inside like a sandwich. The transmission spline pokes into the centre of clutch plate once the assembly below is bolted onto the transmission. Here is the whole assembly:

In this photo I have connected the power cables so I could give the motor a test run. Like unit testing software – it’s a lot easier to fix something now before it is “integrated” with the rest of the car. Important test – make sure the motor runs forward. The first time I ran my electric car it went backwards, and I had 5 reverse gears and one forward gear!

Then I hoisted the motor up into the car and bolted to the bell housing of the transmission. Here is an earlier photo from the EV’s life that shows the motor assembly secured to the transmission. That little motor accelerates our EV faster than the original 1.3 litre petrol engine.

Tip – design your battery racks so the motor can be easily pulled out. Mine just fits but I have to knock a rubber boot off a CV joint every time I drop the motor which is annoying. This time I did one silly thing – bolted the motor in with the hoist chain still wrapped around it. Here is the view of the bottom of the motor looking up:

There were some alignment problems when my adaptor plate was machined. I fixed it with a special jig and ground some alignment marks to help me re-assemble with correct alignment. Without correct alignment the gearbox shaft and motor shaft are not aligned and at low revs it makes a “creaking door” sound as the pressure plate and clutch orbit slightly different centres. At high revs such a mis-alignment would eventually destroy the gearbox or motor.

With our beloved EV off the road we had to resort to the petrol car. You know we had forgotten how painful visits to a petrol station are. And expensive! After a few years of driving an EV we have “unlearned” the habit of obtaining expensive energy from petrol stations. That method of refuelling seems so painful compared to just plugging in each night at home and walking away. How would you feel if you had to take your mobile phone to a special place twice a week to charge it? That’s what the petrol station model of energy distribution feels like to us now. A bad habit we are happy to be rid of.

Rosemary was very unhappy about being forced to use our “infernal combustion” car for two weeks. However the good news is that she is now happily back in her beloved EV:


David’s Electric Vehicle Page

Debugging the EV

While I was away in Europe our EV stopped working. My wife was sitting at some traffic lights, the lights went green but nothing happened. So my wife and my daughter had to push the car home, fortunately just a few 100m from where we live. Much to the amusement of the 4 year old sitting in the back! Worse, they were forced to use our other petrol card for a week until I returned home. Once you’ve driven electric, you find yourself going to some lengths to avoid using petrol cars.

The fault was demonstrated to me after I hopped off the plane yesterday. The car would move OK, then after applying the brakes it wouldn’t move anymore. No power to the electric motor. Also the vacuum pump sounded “slow”, or under powered. So I suspected a 12V system fault. After switching car off and jiggling a few wires around near the 12V battery it would run again.

However after a 34 hour trip from Stockholm I wasn’t at my mental peak, so I decided to work on the car after some rest. Australia is a long way away from anywhere, but I find the trips are getting easier with practice. Movies on the plane help pass the time, and I always find nice people to talk to. After two wonderful weeks in Europe attending Cebit in Hannover, visiting Berlin and Stockholm it seems strange not having to put on a big jacket to go outside and I keep wondering “where is the snow”? More on the European trip later.

After a good nights sleep I looked at the EV again. The 12V battery voltage looked OK at 12.5V. I switched the car to “on” (which makes all the systems live) and the battery voltage went to 13.8V as the DC-DC converter started charging it from the traction battery pack (in an EV the DC-DC converter takes the place of an alternator, supply 12V power while driving and charging the 12V battery).

So I poked around with my new clamp ammeter, fast becoming my favorite toy. Makes current measurements so much easier. This means measuring current can be part of basic, every day diagnostics rather than being a special event that requires breaking a wire. Makes current measurements as fast and easy as voltage which leads to another “dimension” in our visualization of circuits. I bought the clamp ammeter for tracking down to phantom loads on my household power, but have found many other uses, like checking the EV charge current.

The wire from the battery to the 12V system of the car showed 0.7A, which was about right – this powers instruments, the dash etc. However the current coming from the DC-DC converter read 0A. When I checked the battery voltage again it was 12.0V and dropping.

So I measured either side of the lug connecting the DC-DC converter to the battery and it had several volts across it, and when I wiggled it could feel it was loose. Bingo – bad connection to the battery, meaning 12V was failing, eventually to the point the main contactor wouldn’t close so the car wouldn’t move. With a gentle tug the lug came off:

Bad solder joint or maybe fumes from the battery affected the joint. The diodes you can see prevent the battery from discharging into the DC-DC converter when the EV is off. I had a bunch of schottky (for low voltage drop) diodes handy so used several in parallel.

Five minutes to diagnose and one minute to re-solder the bad connection. Zero cost. Not bad for 18 months of EV driving. My electric wiring skills aren’t great, and my EV is a one off, hand made prototype, so this sort of problem wouldn’t occur in a factory EV.

The Amazing Louis Palmer

Last night I went to an excellent talk by Loius Palmer, famous for driving the Solar Taxi around the world in 2007 – 2008. His stories of the trip were very funny and it was just amazing to see what one very motived guy can do. He has also rode the length of Africa by push bike, and flown across the USA at 60 km/hr in an ultralight. An example of a life well lived. If you get a chance to see Louis talk in person or on video I highly recommend it.

He was in Adelaide to help promote the Zero Race, an 80 day race around the world in zero-emission vehicles. In particular the talk was to help our local team entering the race – Team Trev. Trev is a very novel 2 seater car that weighs only 350kg and is assembled from a novel composite material that is simply folded like cardboard. The light weight makes it extremely energy efficient – about 1 cent/km (compared to my EV at about 3 cents/km) for electricity.

Louis has a passion for Electric cars. In particular “solar cars” – electric cars charged by PV arrays on domestic roofs. The key take away was that solar powered EVs like mine are totally possibly today. You should be able to buy a small electric car with a 200km range for $10,000, then run it for free from a $5000 PV solar array. He is puzzled why the big manufacturers don’t get it, and expects the small innovative companies will eat their lunch. I agree.

Louis had some great graphs that showed the big problems with current “green” technologies, like hybrids (glorified petrol cars, excessively complex), biofuels (not enough land in the world), and hydrogen (uses 3 times the electricity/km of battery electric cars). All duds.

He sees China as the best hope in a world naively sailing towards Peak Oil. They lead the world in Solar Cell production and have over 100M electric vehicles (mainly scooters). Australia, on the other hand, leads the world in per-person carbon emissions. Although I am happy to say that South Australia (my state) is getting very close to 20% renewable electricity thanks to a bunch of wind farms going over the last decade. We also have great PV feed-in tarrifs (I get paid 55 cents/kWh for electricity I export to the grid, and pay only 20 cents/kWh for import). Australia also has a pretty good rebate scheme for home PV solar arrays, that cover perhaps $4500 of the typical installation cost.

Louis had some great quotes, for example “if we can afford to bail out banks and big car companies, why can’t we afford to bail out our children?” During his Solar Taxi travels he noticed a lone solar panel on a roof in Canada. He asked the owner, an indigenous Canadian lady, why she had such a panel. She said, “We natives think 3 generations ahead”.

Electric Car BMS Controller

This post is about a Battery Management System (BMS) Controller that I have designed, built and tested for the Lithium batteries in my Electric Car. I recently installed some SkyEnergy Lithium batteries in my EV. Lithium batteries are sensitive to overcharge and undercharge so some sort of Battery Management System (BMS) is required.

A BMS Controller (aka BMS Master Unit) disconnects the chargers when the batteries are full and warns you when they are empty. For a bit of fun I designed most of the logic using transistors and diodes rather than using a microcontroller or regular logic chips. It has been designed on fail-safe principles to best protect my expensive new Lithium battery pack.

The BMS controller works with BMS modules that sit on every cell. I use the CM090 BMS modules from EV Works. Here is a photo of 4 of the BMS modules installed on 4 cells:

The BMS modules are the red PCBs with the components in translucent plastic. Note the thin blue wire connecting each module. This wires snakes through every one of the 36 BMS modules and forms a “dead mans switch” normally closed loop. If any one of the modules detects an over volt (4.1V) or under volt (2.5V) condition it opens the loop. The BMS Controller then does something sensible with this open loop information. When charging this means stop charging. When driving this means stop driving. Otherwise you can kill your cells or in the worst case even start a fire.

Every cells is slightly different, so you need one BMS module per cell. The BMS modules also function as series regulators. As the cell reaches 3.6V during charging the BMS module starts bypassing some of the charge current. This helps bring all cells to a similar state of charge.

Re-cycling Lead Acid Chargers

Now most people installing Lithium batteries would go out and buy a commercial BMS controller and a suitable charger for their battery pack. However I made a promise to my long suffering, EV supporting (and EV-driving) wife that the entire Lead-acid to Lithium conversion would cost no more than $6,500, which was about the cost of the Lithium cells and CM090 BMS modules.

That meant I had to re-cycle the chargers I had laying around, which amounted to a 96V (8 by 12V) AGM charger and a bunch of 12V dual-stage Jaycar M-3612 chargers. Lead acid chargers usually charge to around 14.7V per 12V battery. So the AGM charger that is rated at 96V actually charges to around (96/12)(14.7) = 117.6V.

I ordered 36 Lithium cells which have a nominal voltage of 3.2V. After some reading and a few emails to the helpful EV-works guys I determined that the Lithium cells are just about fully charged at 3.6V. So we need a charger (or chargers) capable of a total of 36(3.6)=129.6V.

This just happens to break down nicely to the AGM charger (117.6V) plus one 12V charger. However each charger will charge at a different rate. So this means I need two BMS Controllers, one for the 32 cells connected to the AGM charger, and one for the 4 cells connected to the 12V charger. Each would have a separate loop of BMS cell modules to monitor.

BMS Controller Requirements

I came up with these requirements for the BMS Controller:

  1. During charging independent monitoring and cut off of the two BMS module loops.
  2. When driving sound a buzzer if we get a low voltage situation. This buzzer shouldn’t fire when charging. I like the idea of an annoying buzzer. My wife was quite good at “driving through” visual warnings of low batteries with my lead acid pack!
  3. When a BMS module loop opens charger power should be cut and stay cut. It shouldn’t start charging again if the cell voltage drops (which they do just after charging), as I don’t want the whole system cycling off and on all night.
  4. It should handle up to 100 ohms of loop resistance from the series connected CM090 BMS modules.
  5. It shouldn’t draw significant power from the car’s battery pack or 12V battery when not in use.

Analog versus Microcontroller

Many people have developed Lithium BMS systems. An obvious choice for each cell module is a little microcontroller that communicates with it’s peers using some sort of serial bus. Then as well as BMS functions you could report the voltage and temperature of each cell, and do all sorts of other magic. The EV Works guys have taken the KISS approach and used an analog design, which I guess is a bunch of voltage references and comparators plus a big transistor to dump the heat during series regulation. They argue that in the heavy duty EMI environment of an EV it’s too easy to crash a microcontroller and screw up digital communications. Software can also be a source of problems. Would be a bummer to set instead of reset a bit and burn down your garage from an overcharge fire.

I have experienced EMI problems in my EV – my microcontroller based voltmeter/ammeter instrumentation goes nuts when I accelerate and often resets itself while I drive.

However there are plenty of cases where microcontrollers have worked for years in high EMI environments. For example a petrol car’s fuel injection computer operates just fine while located a few inches away from a 10kV spark ignition system. So I think the analog idea has some merit and is likely to have less bugs and be easier to develop, although I wouldn’t rule out microcontrollers. Using analog instead of a microcontroller also has a kind of retro appeal.

So I decided to try using transistor and diode logic, and no microcontrollers. I almost got away with it, too.

State Machine

I like state machines, they are a relatively error-free way of designing complex logic. I use them widely in software and hardware projects. This project needs a simple state machine, but the implementation (using discretes) is complex. So before heating up the soldering iron it’s a good idea to have a very clear idea about the logic. This approach let me break down the circuit into clear logic blocks (flip flop, inverter, diode gates), each that was implemented as a separate chunk of hardware. Divide and conquer.

When power is applied we start charging (CHARGE State), and any break in the BMS module loop causes us to switch off the charger (OFF state). Once we have switched off we stay off, no matter what happens to the loop. Two states can be expressed with a 1 bit flip flop, which brings us to the schematic.

Schematic description

Here is the BMS controller schematic PDF. I also have the gschem files if anyone wants them.

There are two BMS Controllers, one for each loop. The BMS LOOP2 Controller at the bottom is the easiest to understand. Q6 and Q7 form a Set-Reset flip-flop. You pull the base of Q6 low to set the flip flop, and the base of Q7 is taken low to reset the flip flop. When power is applied, C6 sets the flip flop, putting it in the CHARGE state. Led D12 lights when the flip-flip is set (CHARGE state).

When the flip flop is set, the collector of Q7 is close to 0V. This pulls the base of Q9 low via D18 and R22, allowing current to flow through the relay that controls the AC power for the LOOP2 charger.

Note that the relay current flows through the BMS module loop. This is an extra layer of safety – even if the flip flop gets stuck on, if any of the BMS modules open the loop then charging will stop. The BMS modules can handle 100mA and the relays chosen use 70mA. A fuse in the BMS cell module loop is a good idea.

When the BMS module SLOOP2 opens R19 pulls the base of Q8 high which causes Q8 to switch on. The collector drops to near 0V and pulls the base of Q7 low via D13, resetting the discrete flip flop. The system is now in the OFF state and even if SLOOP2 closes again power to the relay and hence charging will be disabled.

D14 and D15 in the base circuit of Q8 make sure that Q8 stays off when nominal currents are flowing from the relay circuit through RLOOP2. For LOOP1 (32 BMS modules) RLOOP1 was measured at 20.5 Ohms, with 70mA flowing 1.4V is induced, enough to switch on Q8 if D14 and D15 weren’t present.

I built the first version using a piece of PCB with squares cut using a Dremel:

But this soon became unwieldy and I was concerned about shorts. So I rebuilt it using veroboard which was neater and I hope more reliable in the long term:

The left hand side controls the AGM charger, the right hand the 12V charger. Only one side would be needed for a single charger. The electronic parts total just a few $, the relays are probably the most expensive part.

This board was mounted in a plastic box with the relays, AC power sockets, terminal strips etc. Crimped automotive lug-type connectors connect the BMS Controller with the sense loops in the car, 12V ignition power, and car ground. These connectors can be removed in a few seconds so I can take the BMS Controller inside to mess with it. Until recently, this happened a lot!

Taming a too-smart AGM charger

The BMS LOOP1 controller is a little more complex. After trying the BMS controller I noticed the AGM charger would stop charging after about 90 minutes with a “battery error” warning. I think this is because of the different charge characteristics of Lithium batteries compared to lead acid. Lithium cells seem to charge very slowly between 3.3 and 3.4V until very near the end of the charge. For example a cell might remain at 3.34V for hours while it charges. The voltage of a lead acid pack moves more linearly as a the pack charges, unless the pack has dead cells in it. My AGM charger is “smart” and thought it had a lead-acid pack with dud cells. So it stopped charging and raised an alarm.

The AGM charger could be reset by power cycling it, so I included a timer to do just that. What I needed was an oscillator with an on time of 90 minutes and an off time of 30 seconds. I looked around for an analog design but chips like the NE555 are limited to a few 10’s of minutes due to analog effects like capacitor leakage. So I relaxed my analog-only approach and used a PIC microcontroller, here is the PIC source code.

In the BMS LOOP1 circuit D9, D10 and R12 form a two-input AND gate, ensuring the AGM charger (Charger 1) is only turned on when both the PIC timer output and flip flop are set. Diode AND gates effectively clamp the “low” voltage to 0.6V, so D8 ensures the base of Q4 is less than 0.6V keeping Q6 off.

BTW it is really, really important not to confuse the SLOOP1 and SLOOP2 wires when installing in the car – we don’t want the wrong charger switching off!

Drive Mode

The schematic discussion above deals with Charge Mode, where the idea is to stop charging when the BMS modules open the BMS loop due to one of the cells being over 4.1V. In Drive mode, the BMS module loop opens when any cell drops beneath 2.5V. This will probably happen during acceleration with nearly-flat cells, due to the internal resistance of the cells.

In Drive Mode we don’t want the flip flop or relays to be active, we just want to sound a buzzer when either SLOOP1 or SLOOP2 opens. So in Drive Mode we just power the inverter circuits (Q3 and Q8). Diodes such as D19, D22, D21, D1, D6 and D20 isolate the Drive Mode circuit from the Charge mode circuit.

In Charge Mode the circuit is powered from a 12V plug pack. In Drive Mode the 12V “Ignition” rail from the car powers the circuit. This ensures there is no drain on the cars traction or accessory battery when we are not driving. The “Ignition” circuit is just the 12V rail of the car that becomes active when you switch the ignition key to “On”. The term “Ignition” is a bit of a misnomer in an EV but you get the idea.

Testing and Development

I developed the BMS Controller circuit over a few weeks as a pleasant part time activity. At first I didn’t pass the relay current through the loop, I had it wired in the collector lead of the flip-flop Q2. However one day I had a near-death experience where I had somehow zapped Q3 without knowing so the flip-flop never flopped into the OFF state. Luckily I was monitoring the batteries at the time and no damage was done. However it convinced me to add the redundancy of passing the relay current through the BMS module loop.

Initially I had the AGM charger connected to 32 cells and one 12V charger connected to 4 cells. However I noticed that the AGM charger took much longer to finish the charge. The reason was the current from the AGM charger was only 7A, compared to about 12A from the 12V chargers. With 32 cells the AGM charger was seeing about 32(3.3) = 105.6V so it was “tapering” the charge current to the lead acid cells it thought it was charging. So I tried connecting the AGM charger to 28 cells and added another 12V charger. This brought the AGM charger current up to about 10.5A – much better. The 2nd 12V charger was added to the LOOP2 circuit, as I reasoned that identical 12V chargers charge at about the same rate.

The only problem with running the AGM charger on just 28 cells is that it doesn’t taper the current towards the end of the charge. The series regulators in the BMS modules have only limited bypass capability (700mA) so for effective equalisation we would like a final charge current of just a few A. I’ll need to watch that over time, to see if the cells drift apart in state-of-charge. A proper Lithium charger would be adjusted to taper the current off the whole pack to a point where the BMS modules could effectively equalise on every charge.

Here are the chargers mounted in the back of the EV. Just two were mounted when this photo was taken:

And a close up looking down on the (three) chargers:

Jaycar MB-3612 Charger and Lithiums

The low cost 12V lead-acid chargers I have used are really well suited to blocks of 4 Lithium cells. Curiously, these chargers work better for Lithium cells than for the lead acid batteries that they are designed for (see EV Bugs post)!

These chargers have a simple two-stage tapered current design. When the voltage they see is close to 12V, they charge at about 12A. As the lead acid battery charges and the terminal voltage rises this drops linearly to a few A as the battery reaches 14.7V, which results in rather slow charging and no equalisation for lead-acid batteries.

However this profile just happens to be very good for Lithiums. My Lithuim cells stick to around 3.2-3.3V for most of the charge (hours), then they quickly shoot up to 3.6V (plus) in the last 20 minutes. However as the voltage rises, the MB3612 tapers it’s current off. This allows the BMS module series regulators to shunt current around cells that are a little ahead of the curve. The result is the over voltage cut out never fires for the 12V chargers, and the cells are always nicely equalised. I guess the small number of cells under each charger (just 4 per charger) also helps.

In practice the LOOP2 controller never switches to the OFF state, as the 12V chargers stop charging before any one Lithium cell reaches 4.1V. The LOOP1 AGM BMS controller always switches to the OFF state, as one of the 28 cells charges a little earlier than the others. The lack of tapering of the charge current means full charge current is applied right to the end, when the 28 cells are at about 101V.

Note: my MB-3612 12V chargers have been modified to allow series-string connection (removed ground wire from negative lead, making the output fully floating), and tweaked to charge to 14.7V instead of the rather low 14.1V they were initially set to (trim-pot in middle of PCB).

Next Steps

The BMS controller has been working nicely since I put the Lithium-powered EV on the road 2 weeks ago. Next I would like to design a PIC or Atmel based voltmeter/ammeter/Ah meter. As you can see I enjoy messing with electronics and the EV, so I am just looking for an excuse to build some of my own instrumentation.

An Amp-Hour counter would be really useful for Lithiums as they are not afflicted by the Puerkert effect like lead acid batteries. This would make a AH counter a really useful indicator of “fuel” remaining.


Thanks Mithro for lending me his PIC programmer and PIC collection when he migrated to Sydney – has been very useful for this and a few other PIC projects.


Lithium Batteries for my EV – converting my EV from lead acid to Lithium
David’s EV Page
EV Bugs Post