Sparky, The Electric Vehicle (EV)

Electric Vehicle Project: DC-DC Converter

In the test run debrief I mentioned that the DC-DC converter did not hold up well. First off, I must say it was not the fault of the DC-DC converter. The problem turned out that it was just undersized for the job at hand. The DC-DC converter is rated for 350 watts. I had it adjusted to provide 13.6 volts (for charging the auxillary battery and running the other 12 volt circuits). Given this setup it can only supply 25-26 amps. The S-10 requires much more than that.

During the course of my investigation I decided to measure exactly how much current Sparky requires to run the 12 volt circuits. I inserted a .02 ohm power resistor in series with the DC-DC converter and powered up the various circuits. I powered each circuit up individually to prevent any overloads. Just turning the vehicle on, with no other circuits running, Sparky draws around 5 amps. That includes the primary contactor, fans, and charging the aux battery. I would then, for example, turn on the headlights and note the difference. I repeated this for all of the other accessories. Summing up all of these values gives a worst case condition (i.e., heater on, wipers on, headlights, etc.) that calls for over 40 amps!!!! No wonder it shut down.

I also discovered that the ABS pump is a power hog! The ABS pump does not kick on until you are moving forward. I found that even with all non-essential circuits turned off (heat, lights, radio, blowers, etc.) the DC-DC converter would shut down as soon as I started moving forward. Once I isolated this problem to the ABS circuit and I disconnected it, the DC-DC converter held up under minimal conditions. This made the car driveable.

In the meantime I discussed this with Bob at ev-america and he convinced me to try adding a second dc-dc converter in parallel. We do this all the time with the power circuits on our boards at work, so why not? (I was ready to buy the tried and true model, taking the easy way out.) Anyway, after some discussions with Astrodyne and my friend Dean (another electrical engineer), I came up with a circuit to join the two DC-DC converters. Bob, was kind enough to buy the second converter for me as long as I shared my results. The circuit is simple enough, using two diodes in a summing network. I also added power resistors so that I could measure the current that each one was providing. They also help balance the circuit. The diodes also keep the aux battery from back charging the output of the converters when things are turn off.

Results: The two converters work great! I can power up everything and have plenty of headroom for surges (like the wiper motor). The lesson here is to find out exactly how much power the +12V circuits require. Then size your DC-DC conerter accordingly. Unfortunately, this information is not typically documented. The best thing to do is to measure your system using some in line resistors.