Dead DC-DC converter in my EV

About a week ago I returned from LCA 2013 after being away from home for 1 month. My EV was parked at a house close to the airport. It burst into life and off I went. However about 1 km from home the front lights and dash went dim and the EV ground to halt. It looked like the 12V system was dead. With some kind help from my daughter and her friends we pushed the car home and the next morning I went to work.

As I suspected, the 12V battery was flat. I traced the problem to the DC-DC converter, which appeared to be dead. In an EV, the DC-DC converter works like the alternator in an internal combustion car. It converts the traction battery voltage (in my case about 120V) to 13.8V to power the 12V systems of the car and charge the 12V battery. In an EV only a small 12V battery is required. Just enough to power the 12V systems when the car is off and close the solid-state relay that switches power to the DC-DC converter when the car is “on”. Once the DC-DC converter is on it takes over, providing power to the 12V systems and charging the 12V battery.

The DC-DC converter must have died a few days before I left. The 12V battery by itself could supply the few amps required for a few days. Leaving the car for a month meant the 12V (lead acid) battery was further discharged. So there was just enough left in the 12V battery to run the car at night without the DC-DC converter for 10 minutes before it was completely discharged. No 12V power meant no power to close the contactor solenoid and the EV stopped – despite a nearly full traction battery.

I ordered a new DC-DC converter from EV-Works for $179 including shipping. To limp around until my new DC-DC converter arrived I manually charged the 12V battery each day. During day time driving the 12V system only draws a few amps to run the contactor, indicators, and brake lights. So I restricted my driving to day time to avoid the 14A load of the headlights.

A few days later the new DC-DC converter was installed in about 1 hour by crimping the connections to the existing wiring. The mounting holes fit the old DC-DC converter mounting holes under the passenger front seat. The new unit outputs 14.1V under light load, which delivered 13.8V to the 12V battery terminals. The 0.3V voltage drop is due to schottky diodes mounted on the battery terminals to prevent the battery discharging into the DC-DC converter when the car is off.

The DC-DC converter model I purchased was rated at 144V, with a minimum voltage of 115V. This was a concern, as my pack regularly drops beneath 115V under load. So I went for a driving test. With the headlights on high beam, and the car accelerating (bringing the traction pack voltage down to 110V) the DC-DC output voltage dropped to a minimum of 13.5V which is quite acceptable. Here is my clamp on ammeter next to teh new DC-DC converter measuring the “idle” current of the EV, I think it’s topping up the lead acid battery:

This new DC-DC converter doesn’t have a fan, the old one looked more like a PC power supply and had a fan that would stop and start as I drove. In fact it was noisiest thing inside my EV.

My EV is a one off prototype, so I expect occasional problems like this. Still 1 hours work and $179 in the 12 months since the last bug is not bad. My little EV has now done 33,000 electric km since conversion about 4 years ago.

Living Without a Petrol Car

About a year ago I sold my old ICE car, and have been experimenting with living with just the EV here at my home in Adelaide. It’s working out well.

The longest regular trip I make is to a friends country property, about 40km each way at 80-100 km/hr. I usually stay at his house for a few hours, charging while I am there to top up the pack. For an interstate road trip last year I rented an ICE car for 1 week, and some times I borrow my parents car when I need a 5 seater (my EV is limited to 4 seats).

I feel I am experimenting with different forms of car ownership. Rather than owning a long range ICE car and a short range EV, I am using an EV I own for 95% driving then temporarily using cars I don’t own for the rarer longer distance trips.

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