Over the last month I have been installing a pack of SkyEnergy Lithium cells in my EV. I like working on my EV. Gets my out of the office and I enjoy the mechanical side, especially welding. A pleasant way to spend a few hours a day.
There has been a cool side effect from working on my Electric Car. Since I started this EV project 2 years ago there has been a steady improvement in my mechanical skills. I feel capable of handling other mechanical jobs that previously would have intimidated me. When I hit a tough problem I say, “Well, if I can build an Electric Car, then how hard can this be……..”.
In this post I will talk about my Lithium battery rack construction, as this might be useful to other people contemplating their own conversion. I certainly found the battery racks a major challenge when I started my EV project, but of course it depends on your skill level.
The main reason for the Lithium conversion is the long life cycle of Lithium cells (3000 cycles plus, or perhaps 10 years). We have done about 10,000 km on lead-acid batteries, and they were a fine way to get started with EVs. However with the natural drop in new technology prices and the high Australian dollar Lithium packs have dropped in cost by 50% over the last two years. So it was time for us to take the plunge.
Out with the old:
In with the new:
Lie, Damn Lies, and the Range of EVs
I had a firm budget from my wife for the Lithium conversion that I was determined to stick to. So I decided to buy 36 100Ah cells. These have a nominal voltage of 3.2V so this gives us a capacity of 36(3.2)(100) = 11.5kWh. Cruising at 60 km/hr my EV uses about 5kW so this suggests a range of (11.5/5)(60) = 138 km. Hmmmm. One thing I have learned about EVs is that there are lies, damn lies, and EV ranges.
In the real world many factors reduce the practical range on an EV:
- In real world driving we need to stop and start a lot. Acceleration uses a lot more energy than cruising, for example normal acceleration to 60 km/hr requires 15kW in my car. Higher currents have higher resistive losses. I do recover some of this energy back, for example when I see a red light I coast the last couple of hundred meters, using 0 energy (try that in your carbon-burner that must idle all the time).
- Each of the cells in a battery pack are slightly different. Somewhere in there is the “weakest cell”. When any one cell in the pack is discharged you must stop driving. Passing current through a discharged cell will kill it. As all of cells are wired in series when this weakest cell is discharged thats the end of your driving. It doesn’t matter if 35/36 cells have 10% left – the weakest cell defines the range.
- People often ask me “what is the range of your EV”? I say “I don’t know – and I don’t want to find out!” Just like a petrol car it is bad news to empty the petrol tank completely. So we avoid fully discharging the pack, just like you avoid emptying your petrol tank.
- Range depends a lot on how you have been driving. For example lots of hard acceleration will drain your pack quickly. Power requirements increase sharply with speed. At 30 km/hr I use just 18A, and 60 km/hr 45A, at 80 km/hr 75A. This is probably due to drag increasing, it takes a lot more energy to push through the air at high speeds.
- Fully discharging your battery pack is bad on the lifespan. For example the Lithium cells that I am using have a 2000 cycle life at 80% Discharge, or 3000 cycles at 70%.
From driving my EV for a year I have an experimental figure of about 120Wh/km, about 50% higher than the 5 kW @ 60 km/hr cruise figure (5000/60 = 83 Wh/km). This figure was worked out from the range of my lead acid packs. So with a 11.5 kWh pack this gives an estimated range of 11.5/(0.12) = 95.8 km. Lets de-rate to 70% to be kind to the battery pack and allow for variation in traffic conditions and the “weak cell” lurking somewhere in my pack. So we arrive at a range of (0.7)(95.8) = 67 km.
Lithium versus Lead Acid Battery Racks
I decided to build a new battery rack for the rear of the car, and to modify the front rack as it also supports other items (such as the speed controller) and is more complicated and harder to rebuild.
There are a couple of cool things about Lithium compared to lead acid:
- Having a large number of smaller batteries makes it easier to fit into odd shaped compartments, such as those found in cars. Each of my Lithium cells is the size of a large paperback novel, 4 of them took up the same space as a 12V lead acid battery. I used this convenient fact to fit blocks of 4 cells in the front rack positions where lead acid batteries used to sit.
- Lithium packs take up less volume overall that lead acid, their volume/energy density is higher than lead acid. Put another way – I get my boot (trunk) back! The rear Lithium pack ended up fitting in the spare wheel bay at the bottom of my boot.
- Weight is the biggie. My previous lead acid packs weighed in at a hefty 280kg, the Lithium pack is around 112kg. This means I can now seat 4 people rather than 2, and stay beneath the legal GVM of my car. I estimate the curb weight at around 900kg, just 40kg over the weight of the same car in Internal Combustion Engine (ICE) configuration.
- Lighter weight has a knock on effect on battery rack construction. The rear rack only needs to restrain 70kg of batteries instead on 180kg, each of the blocks of 4 batteries in the front weigh just 13kg rather than 32kg – lighter than a regular car battery! So lighter battery racks can be built saving more weight and safely securing the racks is easier.
Building the Lithium Battery Racks
By trial fitting Lithium cells in various parts of my car I worked out a rough idea of where they would all go. I ended up putting 21 in the back, and 15 in the front. My welding skills dictated that the design would be welded up out of 20 x 20 mm right angle mild steel. I started with a simple base that held the bottom of 3 rows of 7 cells:
The curvature in the photo is due to my camera. The right angle pieces were all 500mm. Compared to lead acid Lithium cells are harder to secure. Instead of 8-10 batteries I now have 36 that must be kept from rattling around. The sides of Lithium cells should also be restrained otherwise they can bulge during heavy charge or discharge. I used plywood panels at either end of each row of batteries to restrain the ends.
To cut the metal to length I use an angle grinder with a thin cutting disk. Make sure you wear safety glasses and ear protection. My arc welder is just a low cost $200 model. Tip: don’t arc weld in shorts. I speak from experience.
I welded up a top section with right angle around the outside and 20mm flat across the top.
The whole thing gets clamped together using threaded rod at each corner:
OK so now I hit my first problem. Lithium batteries have a lot more exposed terminals than lead acid packs. The top section of my rack was just a few mm away from shorting out dozens of connections. What to do? I ended up raising the top section 20mm using wooden battens as spacers. You can see the wooden spacers in the photo below of a trial fit in the back of the car:
The wooden spacers raise the metal top section above the plane of the terminals. I painted the rack using grey pressure pack primer and black gloss paint, $3 a can from the auto shop. Quick and easy, and looks nice.
The rear rack is bolted to the car via 8 bolts through the thin sheet-metal of the spare wheel bay. On the underside of the car the bolts run through lengths of 20mm flat to spread the strain around the thin sheet metal:
The previous lead acid rack (supporting 200kg) was bolted through the chassis rails but with the low mounting location and light weight of the Lithium rack I feel comfortable with a lighter mounting method.
The guidelines we have for battery racks in Australia are hard to interpret and impossible to test. I spent a lot of time and energy worrying about battery racks on my first two EV conversions. My conclusion is that ultimately it’s up to the subjective opinion of the inspector on the day you get the car checked out. As described in my EV bugs post I found the whole inspection experience Kafkaesque. Some people breeze through it, others get a good a hammering by the inspectors.
In the absence of useful specifications I design battery racks by comparing the weight to people. My back rack weighs 70kg, which is about the mass of one person. A person is secured to the car at three points using a woven seat belt and 12mm bolts fitted to reinforced points on the pressed sheet-metal chassis. So I ask myself, “would this rack design hold a person in the case of an accident or roll over?” (In the case of the lead acid racks it was three people).
I am less concerned about the batteries in the front as I figure in an accident where are they going to go? At worst they might bounce around the engine bay, but will be unlikely to penetrate the firewall into the passenger cabin. My ICE car battery is secured by a plastic molding (!), and my Lithium cells weigh much less. I test them by shaking each rack as hard as I can and making the car bounce around.
The rear rack sits nice and low in the spare wheel bay, with the carpet replaced I have nearly a full boot for shopping. I don’t carry a spare tyre, as I reason the EV is always no more than 20km from home and a rescue mission can be mounted by our other car.
I used similar construction for the front rack (photo at the top of this post). I welded in extra rails to the existing racks so I could slide in linear blocks of Lithium cells. The top sections were the same as the rear rack, and the same wooden spacers were used. There is room for a few more cells should I want to upgrade.
If this was a new conversion I would have placed all of the front Lithium cells in one block, as interconnecting them with cables and threaded rod terminal posts is a pain compared to using the strap type interconnects.
After a few weeks of messing about in the shed I backed my Lithium-powered EV out for the first time. My first impression – I felt like I was in a 4WD! Removing a few 100 kg of lead made the car sit up about 50mm higher than before. But it felt like 2 feet at the time!
There was no big difference in acceleration, in fact the car is not quite as zippy as with the lead acid pack. This is surprising given the lighter weight. Also my previous lead acid pack was just 96V, compared to the Lithium pack which cruises at around 115-118V. Guess there is nothing like the low internal resistance of a fully charged lead acid pack. However if I put my foot down I can still get 400A out of the Lithiums off the line (around 45kW). Electric cars get maximum power at stall (0 rpm) which is enough to give many ICE cars a good run for their money off the line.
The current is nice and low, about 45A at 60 km/hr which is great. The car does feel lighter around corners and over bumps, but it’s not a huge difference. I have done one run of 50km on one charge (in light traffic) so far, but most of our driving is 15km round trips, and we tend to charge between trips. After 50km the voltmeter read 115V, or 3.2 V/cell. Unlike lead acid batteries, this doesn’t tell me much about the state of charge. Lithiums seem to sit at 3.2V all day long then fall over in a big hurry. I really need an amp-hour meter to know where I am at.
I have designed a BMS Controller for the Lithium pack that let me recycle my old lead acid chargers.
I measured the power to charge the EV after a couple of test drives and arrived at a figure of 170 Wh/km (measured at the wall). This seems high compared to the 120 Wh/km figure I used above but may be due to charge efficiency of the batteries and chargers. An amp-hour meter on the car would help reconcile these numbers.
EV Running Costs
At 18 cents/kWh a km costs us (0.17)(18) = 3.1 cents in “fuel”. If I used off peak electricity at 9 cents/kWh at km would cost 1.5 cents/km. We average 30 km/day or 5.1 kWh. This amount of electricity could be generated for free by a 1kW solar PV panel apart from the capital cost (around AUD$5,000 at the moment, and dropping).
A petrol version of this car that gets 14 km/litre would cost (130 cents/litre)/14 = 9.3 cents a km, plus much higher maintenance costs. There is no servicing in an electric vehicle, brakes and tyres are the only wearing parts. You do need to replace the batteries every 10 years but I imagine that will cost about $3,000 in 10 years time and far far less than accumulated ICE servicing and repair costs over the same period.
However I don’t like arguing for Electric Vehicles on the basis of cost. It’s funny, many EV conversations focus on running costs. As if that’s all that matters. The main reason I like EVs is that they don’t use fossil fuels. Rather than burning irreplaceable fossil fuels just so I can drive down to the DVD rental store I make the fuel for my car right here on my roof.
Other Lithium Rack Ideas
Thanks to Peter Campbell for sharing the details of the battery racks in his Lithium powered Charade (same model as my car). As described in these AEVA Forum posts he used slotted steel which could be bolted together first for trial fitting, then welded.
Here are a few photos of Peter’s rear battery racks, thanks Peter!
I hope this post is useful to anyone thinking about an EV conversion. It’s the sort of information I would have found very useful when starting my conversion!