All is not well in the land of 12 VDC subsystems. I've always made a point of recording both the good and the bad on the car, and on occasion, the stupid things I've done. Get ready, cause this is going to be entertaining.
I need to give you a little back ground first. After I had taken the car apart, I found out that you're supposed to store these LiFePo4 batteries at a 50% charge if you expect to let them sit idle for over a couple weeks. Well when I started the dis-assembly, the batteries were at 90% capacity. Knowing that the repairs would take several weeks I needed to run them down, but how could I do that without the drive line in place. I hit upon the idea of simply running the heater system and burn it off that way. It worked beautifully. I was even able to predict within 5 minutes when the pack would be drawn down to 50%.
Before I started the heater, I had to connect the batteries I'd taken out of the car to the ones that were still mounted, hook the heater up to the batteries and the DC to DC converters up to the batteries as well. I carefully connected everything up, but I found something unexpected. I had hooked the positive lead to the DC to DC converters, and then put the multi-meter on the negative lead. I don't really know what I was expecting to find, but I can tell you it wasn't what I did find. I found that there was a positive charge on the negative lead. The positive side of the batteries was hooked to the converters, and I could read that positive signal on the negative terminal to the converters. The current was passing through the converters!
Maybe some of you reading this will say "Well of course it does." But I had no idea. That meant that when I hooked it up to the negative terminal, there was a nice big arc. No damage and all worked fine, but it still sucked.
Flash forward to this week. In deciding that I wanted to turn the DC to DC converters off when the car is off, I decided that I'd use two big relays; one on each side, positive and negative. I figured that way when they connected and disconnected at the same time, there'd be no time for a charge to build up in the converters and no arcing. Apparently that was wrong because in about 5 or 6 cycles, I managed to weld the contacts for both relays closed.
OK, so now my solution for shutting off the DC to DC converters isn't working. I'm thinking that some heavy diodes might solve that problem, but I'm not sure. But it turns out that all of the work I did yesterday was the result of an assumption and I dare say, a stupid one at that.
You see, ever since the car first hit the road, I noticed that sitting idle and off, the meter would show that 500 to 600 W/hours had been consumed in a 12 hour period. Now, there are 5 things hooked to the high voltage side on the car: the Zilla, charger, DC to DC converters, the ceramic heating element and last, the Link 10 meter. I had always assumed that the vampire in the car had to be the DC to DC converters. This past week I even disconnected them so that I could test whether the little 12V SLI battery could handle the car's load while the converters were disconnected. I dutifully measured the progress of the SLI battery on it's march to death (rescuing it well in time). But since I disconnected the converters by tripping the emergency disconnect switch, I got no data on the Link 10 to show if the pack had gone down. I just assumed that it hadn't! That was a mistake.
Last night, the car sat for 12 hours with the SLI battery powering the 12V system and the DC to DC converters off (a flick of one of the relays freed it and broke the connection). When I came out this morning, I found that 500 W/hours had vanished. Well, it clearly couldn't have been the converters, they were off! That's when I realized that I'd been guilty of some very poor troubleshooting practices.
OK, so what to do? Well, I've now tested one of the components that is hooked up to the high voltage system, and found (to my surprise) that it's not the energy vampire. Before I move any further, I have to explore all possibilities. As far as I can work out, the possibilities are either one of the other components is burning off that energy, or the Link 10 is reporting that the electricity has been used, when it really hasn't been. I think it's safe to rule out a short in any given battery as I tested them all after I assembled the car and found they were all the same. If it were a battery, that one would be dead.
So, what to do about it? I think the first thing is to test the Link 10. Right now, the high voltage pack sits in the car with the Link 10 being the only thing connected to it. I should know in 4 or 5 hours if it's going to report any draw down on the pack. If it does, I'm left with the task of determining whether the Link 10 is responsible for using up the energy (unlikely) or simply reporting it wrong. If it reports no change, I'll know that I can hook up one more item. I'll keep doing that until I find the culprit.
So, what have we learned today? Don't make assumptions when troubleshooting! Now if you'll excuse me, I'm going to go write that on a black board a few hundred times.
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