Pack capacity, SOC and grid balancing

[aisbell]

Originally Posted by mikemo

Negative recal happens at cell reversal, which means at least one cell is completely empty.

I hope that this is not true. If cell reversal occurred, wouldn't that cell be permanently damaged?

I'm assuming that a negative recal occurs when the indicated charge level is above 0 bars and either the voltage of the entire pack drops below the lower limit voltage of the pack or the voltage of one 6-cell stick drops below the lower limit voltage of one stick, whichever occurs first. This lower voltage limit for one stick must be higher than if one cell were at 0 volts to prevent cell reversal. This must be tricky to determine because voltage is dependent on current.

[Eli]

That is correct, the car only allows discharging to 20% and charging to 80%. Cell reversal should never happen, but it probably does for a split second in very weak packs.

The condition of the pack is determined by the 10 voltage tap monitors. A negative recal is when one of these voltage taps hits the "low" voltage point. A positive recal is when one of these taps hits the "high" voltage point. It's very difficult to determine NiMH state of charge by voltage alone, except at the upper and lower limits.

The BCM does count amp hours, but it's not hard coded. For example, with the BetterBattery, which has an 8 amp hour capacity(4.8Ah usable - 60% usable range), the BCM does not artificially limit you once you hit 3.9Ah, which is the 60% usable range of a healthy, 6.5Ah Honda pack. It waits until a voltage tap actually hits the low voltage point before the negative recal happens and the pack is considered empty.




Here is a graph showing this. Notice that the State of Charge hangs at 28% while it waits for the pack to actually become empty:

BetterBattery Discharge Test

The small drop from 28% to 25% at about 5:34 is the negative recal.

Here is a graph showing this with a "healthy", 1.5 year old(at the time) stock pack. Note that the negative recal happens relatively high overall, at about 48% at the 3 minute mark, leading me to believe that this stock pack is actually slightly imbalanced.

Stock Battery Discharge Test

Finally, here's what the discharge of a very weak, IMA light throwing pack looks like:

Weak Stock Pack Discharge Test

So, if you grid charge, it doesn't matter what your actual capacity is. The pack won't be considered empty and do the negative recal until a voltage tap actually hits the low voltage point. Note that pack capacity is actually determined during the charge cycle after a negative recal.

So if you grid charge your 2.5Ah pack, and are now able to pull 3.0Ah out of it - IF you can put 3.0Ah back into it before the positive recal, that is your new capacity.

[Cobb]

I think Mike DB2K has a good graph showing on his site what happens and why either occur. I doubt a cell reversal happens in that nature.


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