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I'm confused, because C2 looks like it only has 600 maH of capacity remaining. Am I misunderstanding?
 

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Discussion Starter #22
Right, that's what I said a couple posts up. The pack sat for about a month, C2 self discharged and ultimately only ends up with 600mAh. But because of the faster self discharge, it was also cycled low, and cycling low keeps the voltage from becoming depressed. I'm using this as an example of the ameliorative impact of cycling low. C2 itself is not a good cell - because of the self discharge. Ideally all the cells would be equal, with even self discharge, and then you'd be able to cycle all the cells low and they'd end up with nice, lofty voltage curves, just like the C2 curve, only all the way up the charge state range.
 

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Discussion Starter #23 (Edited)
Figured I'd add a perfunctory update of sorts to the above ideas, as I've been mostly continuing with, well, testing of pretty much the whole 'low charge state usage' theory that started this thread. But most recently it's just focusing on what's going on with this one faster self discharge cell and taking low charge state usage to the ultimate extreme -- near perpetual rock-bottom pack usage... I have to say something because it just blows my mind every time -- just how different, and better, these cells work than you'd think if you had just used them as they're normally used in the car...

Most recently, I've been driving for at least a week or two with the actual state of charge for most usage no higher than 20%, but I estimate more like around 13%, with one cell, the fast self discharge cell, at zero when the pack neg recals. I've basically been resetting nominal state of charge with OBDIIC&C to 40% when pack neg recals, and pack neg recals when this fast self discharge cell is close to empty - while other cells are probably around 13%... Last night I did some tap discharging and subtracted probably about 450mAh from the 13% cells, so they're closer to the fast SD, 'zero' cell...

Suffice it to say that I've been operating the pack really low and trying to get it even lower. It should be around 6% now...

Power Output
The main thing I want to emphasize here is the raw performance capability of the OEM NiMH cells. Even at this super low charge state and at cool temps (say about 50F to 68F), I can easily pound out about 70 amps at about 120-130V. Voltage doesn't plummet, it stays pretty steady, I can get the full 4 seconds or so of full throttle assist. When I do let it charge up higher, 30 amp assist happens at no lower than 140V, I think usually around 140-144V; 20 amp assist is usually above 150V, I'm often seeing something like 150-156V. This is higher than I used to see; it used to be more like 20 amp assist at the greater than 140V level. I have to add that I don't have to charge it very high at all, though, to see this kind of performance. It takes maybe 10-15% above the neg recal point to see these metrics...

Temp
I pound on this pack and the temp change I see is no where close to what I've seen in the past with various packs. I used to think it was 'normal' for the pack to increase in temp quite noticeably after some full assist and regen events. But now, I do see a little temp increase, but it takes quite a bit of usage, for example, probably something like half a dozen full assist and braking regen events and I might see a temp increase of a couple degrees F. In the past I've seen single 'full assist' events make temp increase right away. I never see that now...

Gets rid of Voltage Depression
Another thing I noticed is that, when I do let the pack charge up, pack voltage goes up to about 168-170V fairly quickly and stays around there during most of the charge. I'm pretty sure this low charge state usage does indeed eradicate voltage depression. So, if you go back a couple posts and look at the black and yellow discharge graph I posted, one cell is depressed (black) while the other isn't (yellow). I'm pretty sure that now the depressed cell, which is in this pack, can't be depressed any longer. If I discharged it its curve would look more like the yellow curve, lofty until the end...


Anyway, the performance I'm seeing is... fascinating. I wish I had a better handle on the electro-chemistry. What's interesting is that I just never would have expected these cells could put out so much power at such low charge state. There's this weird thing going on, this incongruity or something, with the difference between charging and discharging: charging is harder, discharging is easier; charging heats the cells more than discharging. Not all chemistries are like that; it's the opposite with my LTO cells and I think my boiler-plate lipo cells, too... I'm thinking that something with this incongruity might have to do with why perpetual high charge state usage causes problems, and on the flip side, why low charge state usage would end up, well, 'fixing' things in the first instance, but then performing better and preserving the performance... It just seems really weird that the cells would have such an easy-er time pulling protons from the negative electrode and pushing them into the positive electrode (discharging) on the one hand, versus pulling them from the positive electrode and pushing them into the negative (charging)... If anyone knows why that'd be the case I'm all ears...
 

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Discussion Starter #24
FYI, the different behavior of the A03 BCM I talked about a while back, excerpt pasted below, turned out to be the 'cell difference' thing I describe at the end. Sources say that later Insights use Civic cells, and that this A03 BCM is programmed for those cells. But I was using it with early model, 2002 Insight cells, which need to be charged to a higher voltage to hit the pos recal point. Hence, the A03 BCM was inadvertently under-charging my early model Insight cells, pos recal-ing early. This is consistent with the graph I mention below: with the A03 BCM I was seeing a pos recal at something like 168-170V at ~6 amps. At the stick level that's about 8.45V. When I identify 8.45V on my graph, that corresponds to about 75% charge state for the Civic stick, but only about 40-50% for my Insight stick...

I guess I'll post that graph:
84195




I've been using/testing a later model BCM for the past few weeks, discussed a bit here: IMA Battery will not charge on level road at constant speed

Thought I'd mention here that this BCM - an A03 from something like 2005 and/or 2006 - seems to implement a lot of what I describe in this thread: it doesn't charge nearly as high and appears to concentrate usage below 50%.... I don't know, it's really weird how drastically different this BCM is compared to those others. I'm not sure what to think of the top-end threshold, for instance. In some sense it's too low... I also wonder if there's something different about my cells that causes a lower top-end. I don't think there is, but maybe there were slight differences in the cells used on later Insights that called for different programming, kind of like the purported differences between Insight cells/BCMs and Civic cells/BCMs... I did try to make graphs for Civic and Insight cells at one time, and those graphs do show that the Insight cells need to reach a higher voltage to reach a given high state of charge relative to the Civic cells. But I can't be sure that my test cells were truly representative... Plus, we're talking Civic vs. Insight here, not early Insight vs. later Insight...
 
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