Tuesday, February 9, 2016

More Precharge Challenges

The precharge circuitry on the Porsche has been a challenge since day one. I'm appreciating more and more the well integrated design of the Soliton1 controller in my evTD. I never had to even think about precharge on that because it was built in. Sorry they've left the EV scene.

You may recall that the first time I powered up the Porsche, I fried the contactors. Then I did it again. Third time is the charm, you say? Well, after 200+ miles I came close to another wholesale contactor replacement. This time, after a jaunt up the MOPAC (that's a highway here in Austin, so named because it parallels the Missouri Pacific railway line), I parked it and came back a few minutes later to run an errand, turned the key to see only 6 volts register for the pack! That just can't be, so I spot checked some battery cells - all fine - then  pulled the contactors yet again to find they worked ok after cycling them a time or two, but the precharge resistor was dead - no reading on the multimeter in the ohm setting. So once again, even though it was in the circuit, no precharge was taking place. The contactors likely would have welded closed if I had put them through one more startup sequence.

I had wired the precharge resistor using this diagram from the GEVCU manual:

So my guess is that I either had a defective resistor, or it didn't like being in the circuit full time. After receiving a replacement resistor, I reworked the arrangement adding the yellow high voltage precharge relay to energize the resistor only on precharge and keep it out of play at all other times.

After a quick spin around the block, things seem to be back to normal. We'll give it a more extended test run tomorrow.

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