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Plug Bug: Fixed, and then broken again


The electric bug constantly has stuff going wrong with it. I got it back on the road and registered as a salvage vehicle. I could finally drive it to work again, but I’m a bit nervous about using the carpool lane since it only has one of the required three stickers showing.

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 A few days later I noticed it was pulling strongly to the left on the freeway when I was braking. So, I popped of the brakes, and the pads were pretty worn. A few days later I had some new pads and it was once again braking in a straight line.  This wasn’t too big a deal, and was fairly straightforward to do. 

But last week, I went to go home from work and discovered that the battery hadn’t charged at all! Checking on ChargePoint I could see that it charged for exactly one minute before stopping, even though it was somewhere in the 47% range for the state of charge. The charger has broken before, so I immediately suspected it and unplugged it from the battery pack to isolate it. This seemed to fix something, and allowed the car to start; previously, the “key on” wasn’t working to initiate the controller into driving mode. I did question my ability to make it home since the pack didn’t charge. I figured I could make it home on 47%, but it was risky. The distance usually takes around 20-25% to make it home, depending on how I drive and the outside temperature (colder takes more energy). However, I have some cells that are starting to age, and they might be completely discharged at around 20-25% state of charge; taking the pack any lower would mean I could potentially kill those cells and render them useless. Worse, the  battery management system was going haywire, and reporting a really low overall voltage, but seemed to accurately present the charge percentage. Something was messed up pretty badly, but I decided to drive home always and take a risk.  I slowly accelerated and drove 45-50 MPH on the freeway to avoid using an excessive amount of energy.

Once I got home I started looking for problems. I unplugged the Belktronix DC-DC converter and turned the key on. To my surprise and delight, the battery management system started working normal again. This meant the problem was in the DC-DC converter, and not the charger.

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At this moment I recalled a similar problem I had a few years ago, where something similar happened and I isolated it to the DC-DC converter. I had to mail it back to the guy who made it, and he took a look at it and tightened some things down, but said it seemed to be working normally.

This time I decided to try to figure out what was wrong myself. I dismantled the DC-DC converter, pouring out a bunch of sand (or silicon, or something) that was in the converter for additional cooling and vibration protection. I poked around at the parts and looked for potential problems. I finally noticed an issue; a small white wire was attached to a blue capacitor, but it was bent over slightly and touching the body of the capacitor. This small wire was connected to the negative side of the high voltage line, and having it touch the capacitor body meant it was making a circuit with the ground of the entire car! An EV will keep the high voltage system completely isolated from the car’s chassis, unlike the low voltage (12v) system which uses the chassis to transmit the ground. This leak was likely what caused the battery management system to go haywire, and in turn it didn’t allow the car to charge.  

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I moved the wire aside and protected it a bit with some electrical tape, and then assembled the converter back together. To my delight, it seemed to fix the problem!




(c) 2008-2017 Corbin Dunn

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