Jeff’s Zenn #1.

Posted: September 30th, 2008 under BSD General, On Road EV.

My friend Jeff drives a Zenn. It has a 72V pack of 6 Discovery batteries. The Zenns “fuel gauge” has about 4 segments, which doesn’t give a lot of Information. Dave Cloud installed a BatMan4-EV100, and battery heaters. The next day, Jeff called me to tell me the BatMan had quit.

It turned out that Jeff lives on a hill, and after charging overnight, he left home the steep way. His Zenn has been modified by MC-EV to have an AC drive system. When going down the hill, the BatMan said “OverLD”, then blinked and went out. I found that one of the power supplies died, so I repaired it. I modified a unit to be able to go to higher voltages, and sample faster. We popped Jeffs repaired unit out of the car, and plugged in this special unit. We recorded the data with my laptop, and here is some of the data, showing the first time we went down the hill.
Jeffs Voltage and Current Chart

Now remember that this is a 72V pack, and that it was already down by about 10% from a trip into Woodinville and back. Near the middle of the graph, you can see the pack voltage going over 100V as the controller put over 150A into a nearly full pack! Now the 100V BatMan can measure 100V, but the power supply is only designed to handle 90V. So even though this is a 72V system, which shouldnt go over 90V even under charge, we saw enough voltage to blow the power supply in the BatMan. What we did was add a zener diode between the measurement pin, and the power supply pin to drop some of the voltage. The problem with this, is that if Jeff takes the pack too low, the BatMan drops out, forgetting what it had collected since reset. He currently has a 27V diode, which we may change for an 18V one.

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