Yes, figure 11 and the explanation about it is confusing.
Why is it confusing?
The way they specify their cell capacities is for X mAh at a certain discharge rate to 0.9 volts. This is what they call "100%"capacity. They show you in Figure 10 that the faster you discharge, the less total capacity you get. At slower discharge rates (they say up to about 1C) , the 0.9 volts cutoff is great and means the cell is essentially "fully discharged". At
higher discharge rates, like 4C, for example, terminating your discharge at 0.9 volts may be premature, though, and instead of having used up almost all of the available capacity since they drop like a rock once they really hit that end of charge "knee", there may still be significant usable capacity left due to the depressed voltage caused by the aggressive discharge rate.
In these cases, they are essentially saying to do a load test (or use published high-discharge curves) at your high discharge rate to come up with a graph like Figure 6. Once you have your data, you calculate where that
Midpoint of Discharge is, and see what the voltage is
at this point which will be
lower than the 1.2 volts shown in figure 6. You multiply that voltage by 0.75 and get some number
lower than 0.9 volts and you use
that value as your cutoff voltage.
As to discharging to 0 volts vs. 0.9 volts in a typical application to try to get more than 100% of their rated capacity, that extra "10% or less" of the total chemical charge that is
greater than their rated capacity (because they rate them for a 0.9v cutoff, as the knee is just starting rather than 0 volts) by extracting that last little tiny area under the curve... Well, you
CAN do that in a single cell application if you want, but it is pretty pointless because your available voltage is dropping like a rock... You will never be able to go beyond the start of that "One Electrode Reversed" plateau because you only have one cell, so you shouldn't actually cause any
damage to the cell as long as it is recharged fairly soon afterwards. You're not supposed to store NiMH in a totally discharged state for extended periods, unlike best practice for NiCd.
As soon as you put cells in series to form a battery, however, then you really want to stop discharging as soon as your output starts to drop because you need to avoid reverse-charging as this will permanently damage the cell. The battery will hold for some time on that first plateau without actually going negative by eating up some of the extra reserve positive electrode... Thus as long as the cells in your whole battery pack are capacity matched well enough to always stay in that first plateau when discharging, you are OK... Which is why they recommend always using the 0.9v cutoff... You are unlikely to actually reverse anything until one or more of your cells has significantly less capacity that the best ones.
Obviously, the more cells you put in series, the more you need to watch this effect. This is why things like power tools they tell you to recharge as soon as the tool begins to slow... If you waited until you get more than a perceptible voltage drop, your weakest cells in a well-used pack may be pushed into reverse by the time you actually hit 0.9v per cell. In a 15-cell pack (eg. an 18v cordless power tool w/ NiMH), for example, when you hit the 13.5 volt "75%" mark, you
might have only discharged all the cells to 0.9v if the cells are new and all fairly equal. In an older pack with worn cells, you are more likely to have many cells that are still at 1.1-1.2 volts while you're using your remaining capacity to smooshy-mooshy-mash angry pixies back into the weakest couple cells in reverse... That will rapidly destroy those cells, where you may well otherwise have been able to get dozens or hundreds more usable cycles out of the pack.
The phase described as "over discharge" includes the part we normally called "reversal charging" that requires an external power source, while I have always regarded "over discharging" means the part of discharging that take the battery to 0V (without reversal).
You do realize that your "External Power Source" can be the other cells in a series-connected array of cells, right?
A question here is: will (over) discharging a NiMH battery to 0V damage the battery?
ONE cell? No.
A
battery, with multiple cells in series? It certainly can, unless you do sophisticated individual cell monitoring and control like you would with a good Li-ion pack.
My understanding used to be that it will not have a negative impact and only reversal charging will do the damage.
Now, by the description of figure 11, actually, only the 2nd stage/plateau of the reversal charging causes the real damage. In other words, a NiMH may tolerate a moderate level (1st stage) of reversal charging without being damaged or having conpacity reduced.
Yes, they can tolerate that time with the cell holding at 0v (the first plateau) alright, though it is hard on the positive electrode, it won't actually start liberating hydrogen and venting until you hit the end of that first plateau. You're
NOT actually reverse charging it yet until the cell goes below 0v.
This "moderate level" reversal charge tolerance is upto about 40% of the battery conpacity. This is good to know. It means several batteries in series with upto 40% conpacity difference can run down to 0V w/o bad effects to the batteries.
Right, which is why they don't normally put fancy monitoring on each cell in a typical series NiMH pack but rather just stick them in series and hope for the best.
They are reasonably tolerant for that first bit.
Remember, though, that the more cells you put in series, the more potential there is for damage. This is why it is pretty much totally retarded to try to discharge a multi-cell battery beyond 0.9v per cell. Very little capacity left and larger and larger chance of damaging cells prematurely the lower you go and the more cells in series.
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