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Suppose a pack of 8 AA NiMH cells are serially attached and its known NiMH cells should not go below 900mV per cell.

Does this really mean the pack should not be discharged past 7.2v?

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  • \$\begingroup\$ What do you mean by "max discharge voltage?" Do you mean max charge voltage, which is an important value to know? The minimum usable voltage depends on the device they are running. There is also (for rechargeable batteries) a minimum discharge voltage below which the batteries tend to decay, especially if left dead. \$\endgroup\$ – Sparky256 Aug 28 '18 at 2:59
  • \$\begingroup\$ @Sparky256 - the question is explicitly about the discharge cutoff voltage. Though 900 mV is probably too low, most would say 1.1 v under light load. \$\endgroup\$ – Chris Stratton Aug 28 '18 at 3:13
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    \$\begingroup\$ 900mV per cell is a commonly used value. Or 800mV per cell. You have to be fairly sure that none of the NiMH cells get reversed. Consider if one cell has only 1500 mAh capacity, and all other cells have 2000 mAh capacity. If you discharge 2000 mAh from pack, the low-capacity cell will continue to discharge even after voltage = 0. This is referred to as "reversing" the cell. The cell will be permanently damaged (probably have zero capacity after that). So, it is best to stop discharge at 0.9 or 0.8V per cell. You can check the cells at end of discharge to see what the lowest cell voltage is. \$\endgroup\$ – mkeith Aug 28 '18 at 3:16
  • \$\begingroup\$ It does depend somewhat on how fast you are discharging them. The faster you discharge, the lower the end voltage. For light loads, you should cut off at a higher voltage. \$\endgroup\$ – mkeith Aug 28 '18 at 3:30
  • \$\begingroup\$ I meant the pack should not be discharged past 7.2v \$\endgroup\$ – user33915 Aug 28 '18 at 4:31
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Your sums are right, 900 mV * 8 = 7.2v.

However, what happens if the pack is unbalanced, has 7 cells at 1.0 v, and one cell at 200 mV? That measures 7.2 V at the pack terminals as well.

The cells may not start out this unbalanced, and of course they get charge balanced on each full charge. However, as they wear out, they may wear at different rates, and their capacities may drift apart a little.

Although the 200 mV cell is completely exhausted, it's likely not yet permanently damaged. That will happen if you use the pack for just a moment longer, and the cell goes down through 0 V, and keeps on going, getting 'charged' in reverse by the other cells. Note that's applicable to nickel chemistries only, lead and lithium have different damage voltages. Once you've reverse biassed a cell, the battery pack becomes more or less unusable.

When you're using a battery of series cells, without access to the internal cell voltages for monitoring, you need to have some extra cushion voltage to allow for cell mismatch. The endpoint voltage you choose will result in a tradeoff. A higher endpoint results in less usable capacity per cycle, but more expected cycles lifetime before you damage the pack through this mechanism.

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  • \$\begingroup\$ A very good answer. If I were building my own pack, and I knew I would remember to check the cells at end of discharge occasionally, or measure capacity of individual cells, then I might do this. But for a production design, it might be pushing the limits. On the other hand, if the cells are very well balanced maybe it would still last for 500 or 1000 cycles, and maybe that is good enough. \$\endgroup\$ – mkeith Aug 28 '18 at 5:02
  • \$\begingroup\$ This was very informative and this is for a hobby project and will not have significant charge cycles before I recommission the cells for other things so it seems ok to just discharge it up to the 7.3v threshold voltage. \$\endgroup\$ – user33915 Aug 28 '18 at 6:48

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