I've spent the last week and a half working with the batteries and charger, trying to fine tune and develop a charging plan. The more I've worked on this, the more I'm starting to think the problems I've encountered up to this point were simply a result of setting charger's cutoff voltage too high.
Initially I'd set it at 168 volts, because that's the target voltage I wanted (3.5 x 48). But of course, that was too high because the charger simply begins it's ramp down of current at that level. That lasted one charge, then I turned it down to 166.5. That worked out well as the voltage on the pack was at 168 when the charger finally cut off.
But clearly as the cells broke in, or started showing their individuality, some started reaching their target voltage before others. In terms of actual current pushed to the cell, the differences are so small. In the last post, I'd figured that the difference between the extremes was about 7.3 Watt hours. That may actually be a bit high. It may be closer to 5 Watt hours.
I lowered the chargers cutoff to 165 volts, and that seemed to be working quite well. That put the finishing voltage at around 166.8 volts. The batteries prone to runaway were hitting about 3.58 volts, by the time the charger kicked off. The rest of the cells were only just behind. The difference in the entire pack's charge state reduced by only 150 Watt hours or so. But on two occasions, I saw one of the cells climb to 3.60 volts when there was around a minute left on the charging cycle. Had I let it go, they would have climbed to somewhere close to 3.64 Clearly, I needed to back it off just a bit more.
This morning, I charged the pack with the charger starting it's ramp down at 164.5 volts. It finished at 166.3 volts, with the highest cell at 3.52 volts and the lowest at 3.43. That, I think, will be just fine if we get something like that each time. I'll continue monitoring over the next few weeks to see how things progress, but it's looking good at the moment. As best I can figure, that cost me another 100 Watt hours. So I'm down my a grand total of 250 Watt hours in the pack. Less than one mile.
I had a local shop look at the springs on the car Tuesday. Since the car was riding 3/4" lower than stock when I got it, I had always wondered if it had received aftermarket springs to lower it, or if the original springs were just tired. They invited me down to take a look. Turns out they are the originals, which means I have two options. I could get coil overs for around $1500. Or I could have some springs made. A place out of Kansas City will fabricate new springs for me which will take the proper weight, if I send them one of my springs, some geometry measurements off the suspension, the new weight of the car and $285. Sounds like a good deal to me, but I'm not in a position to take a spring off my car. I need to find one at a local salvage yard. Over all the car is riding 1.5" lower than stock, but just on the front end. The back end is perfect. I look like I'm going down hill all the time.
The good people at ADOT have seen fit to grant my request for a custom license plate. Check it out!
How sweet is that?!