I only read the introductory material and the chapters that include information on NiMh cells, didn't see anything on cell reversal. No mention of cell reversal in the index...
.... measuring voltage on the entire pack and using MIMA, a good pack will drop to a certain voltage, let's say 144V under load, and pretty much stay there after the load is removed. Yes it will pop up a little bit, but not much.....
Interesting... In general I'd expect good NiMh batteries to:
-'sag' less under given discharge load
-hold steady during discharge (or more steady than weak ones)
-'rebound' higher and quicker after discharge
-have a lower voltage upon charge
Any of this incorrect? Sounds like you're saying the third point isn't true...
Interesting... In general I'd expect good NiMh batteries to:
-'sag' less under given discharge load
-hold steady during discharge (or more steady than weak ones)
-'rebound' higher and quicker after discharge
-have a lower voltage upon charge
Any of this incorrect? Sounds like you're saying the third point isn't true...
eq1,
In my experience with the pack, the better cells tend to sag less when discharging and climb less when charging. So yes, actual cell data tends to disprove your 3rd point.
Another way to look at it:
If we have a "huge" 12 volt car battery, and only pulled let's say 1 ma current from it, the voltage before and after the current draw would be exactly the same. No drop during the current draw and no rebound afterwords.
Same is true of really good NiMH cells. Little drop under load, and little rise after current draw stoppage.
....The other effect that seems to be clear, is that the topping voltage at a 350ma current is a good indicator of the cells condition. the higher the voltage that the cells top off at, the weaker the cell and the higher the Internal resistance.....
Maybe our/my terminology and descriptions aren't clear enough. What Mike says in that post is my 4th point: voltage under charge will be lower for good cells versus bad.
The 3rd point, that I called "rebound," is just after a discharge, but not while being charged. So you discharge and then the cells are at rest. My understanding is that voltage for the good cells will jump back up closer to what was the initial voltage upon discharge and do that more quickly than bad cells. For example:
-good pack: start V=160, discharge V=140, V ~10 seconds after discharge ends, at rest=153V, after ~1 minute =155V.
-Bad/weak pack: start V=160, discharge V=130, V ~10 seconds after discharge ends, at rest=147V, after ~1 minute=150V...
My battery does the rebound thing. On a full pack showing 167v, 15a assist will drop it to 145-150v or so, and then it goes back to 155-160v. I tried deep discharging today, resetting SOC high every time the car started a forced charge, and discharging further. I got as far as 139v unloaded before I decided to just give up and try grid charging. It's on the battery smoker right now.
My battery does the rebound thing. On a full pack showing 167v, 15a assist will drop it to 145-150v or so, and then it goes back to 155-160v. I tried deep discharging today, resetting SOC high every time the car started a forced charge, and discharging further. I got as far as 139v unloaded before I decided to just give up and try grid charging. It's on the battery smoker right now.
Bit of a follow-up on the previous, because I hate it when I see people not follow through on their "to be continued" posts.
My battery seemed stronger today. With the battery discharged nicely, it's pretty cool to regen at 50a rather than 10-15a. I will continue to do the OBDIIC&C pack smack a couple of times and then re-evaluate, but so far, so good.
That's one reason I want to avoid the top now - because the batteries are supposed to work more efficiently in the middle... And they seem to, too, such as with higher discharge and charge rates and cooler temps...
....I tried deep discharging today, resetting SOC high every time the car started a forced charge, and discharging further. I got as far as 139v unloaded before I decided to just give up and try grid charging. It's on the battery smoker right now.
For reference-sake, when I did this the first time, it was after a negative recal at about 56.3% SOC, and I got it down to 47.1% SOC. The next time I did it was after a negative recal at about 54.2% SOC, and I took it down to 41% SOC... Can't confirm that this documents an improvement from the first to second time, as I wasn't following any strict protocols, like maybe the second time I happened to reset more times. But there could be some improvement reflected in these numbers... In any event, sounds like maybe our packs could be similar - yours going down to 139V, mine to 141V...
...The car by itself pumps in something like an extra 2% capacity daily; I think part of the programming accounts for self-discharge, likely a set level, not a dynamic sort of thing (i.e. the programmers perhaps believed NiMH batteries had a self-discharge rate equal to say 2% - so they programmed the car to put in an extra 2% per 24 hours, on average)....
Another thing I noticed along these lines is perhaps surplus charge/capacity relative to the number of cycles.
In my spreadsheet I can keep track of this surplus charge - the charge that pushes my accounting of net amp-hours above 65mAh per 1% capacity - and in many instances, the short trips I take with few charge-discharge cycles end up with little 'surplus charge', whereas the trips I take with multiple charge-discharge cycles have greater surplus charge. I don't keep close enough track to tell for sure; for example, I don't count the cycles while I'm driving and write them down. But lately for obvious trips where I know I drove with many or few charge-discharge cycles there's a clear difference.
I'm thinking the car might account for charge-discharge efficiency, where the more cycles of charge and discharge you do, the more amp-hours that are lost due to inefficiency, and thus the more the car wants to put back in in excess of the strict 6500 mAh net amps accounting...
It's just hard to tell at this point whether there's multiple management schemes happening here or whether I'm seeing just one scheme at different time scales. For example, over the long term, the car puts back in about 2% capacity above and beyond the 65mAh per 1% capacity level. This includes hanging positive recals at 72%, where the car dumps in about 200-300 mAh above that 65mAh per 1%. But if you keep away from the 72% positive recal threshold, keep under or over it for instance and document those trips, the car still puts more amps back in then it takes out (if you keep track of amps-in and amps-out and calculate state of charge based on that alone, the result will slowly creep up, say, instead of calculating 75% SOC as the OBDIIC&C reads, you'll calculate 85% in a few weeks or so)... But anyway, under these short-trip, few charge-discharge cycles, there seems to be little surplus... I'd have to be more judicious with my trips and data recording, as well as look at the data in some different ways to see if there's correlations...
Here's a screen shot of the data and calculations I do. These document everything since my last grid charge. The column at the far right is the 'surplus mAh' I'm talking about. The "pr72" rows are positive recals at 72%, where the car usually puts more in then it took out. Note how 'surplus' values for some of the upper rows shortly after the grid charge are small or negative, when the car actually takes more out then it puts in. Nearly every row, however, is positive; there's no arithmetic reasons why that should be so...
I tortured my battery today. I discharged the battery as much as possible last night, and then discharged some more today while constantly setting SOC to maximum to avoid getting any regen in. After a couple of long uphills with the battery below depleted, about 115v under a 15a load, I let it finally start to recharge. What's interesting is that before, the car would recharge only about 7-8% SoC before it recalibrated to 80%. This time it's been sucking up the charge all the way from 25 to 60 percent, and I haven't actually driven it enough to get to 80 percent. I could have rammed it to 80 by resetting the SoC low but I wanted to see how far up I'll charge before it'll do a positive recal. So far, so good - my battery feels stronger than at any point in time that I've owned the car before, and yes, I'm getting more amps and higher voltages than before under load. The best part is, my assist doesn't dial back to 8-10 amps after a couple seconds of sustained load anymore. I went up a nasty hill today and the car handled it fine, even doling out 20a sustained for the second half of the hill, once I worked up the nerve to try getting even more assist.
The pack smack has done more to revive my battery than grid charging ever has. I think that people with bad battery packs should try running them down, and I mean down, and then recharge.
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