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I have an 8s7p Li-ion battery with a fully charged voltage of 29 V, a nominal voltage of 25.9V and capacity of roughly 20.5 Ah. This battery was used to run a brushed DC motor (rated no-load current 0.8 A) using a motor driver circuit(IBT-2) (which I assume helps prevent back EMF charging of the motor), and then stored (not at storage voltage owing to the fact that the battery is to be used occasionally every 3-4 days or so). The battery is now showing an open circuit voltage of 30.3 V. Is this possible? What steps should I take to amend this and prevent this in the future? Thank you.

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    \$\begingroup\$ I wouldn't rule out that it's being regeneratively charged by the motor. Is the motor ever spun by external forces? Protection diodes and the like may allow current to flow back and charge the battery. \$\endgroup\$ – Drew Nov 21 '18 at 6:54
  • \$\begingroup\$ But isn't a motor driver, here an IBT-2 helping it not get regeneratively charged? And no, no external forces were used to turn the motor shaft. \$\endgroup\$ – Abhinav Nov 21 '18 at 6:55
  • \$\begingroup\$ The problem is, while storing it was at 26.5 volts, we didn't think much of it, but 30.3 is a bit too much. \$\endgroup\$ – Abhinav Nov 21 '18 at 6:56
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    \$\begingroup\$ LiPo has a max voltage about the same as li-ion, 30 3V across 8 cells is around 3.8V. Seems fine to me \$\endgroup\$ – Linkyyy Nov 21 '18 at 7:04
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    \$\begingroup\$ Not knowing the chemistry of your battery is somewhat similar to not knowing whether to put petrol or diesel in you car's fuel tank. Doing it wrong leads to bad consequences in both cases. If you cells are LiFePO4 (which seems likely) then 25.6v/8 = 3.2V = nominal LiFePO4 MEAN voltage. Max charge is about 3.65*8 = 29.2V BUT significantly higher voltages will not overly hurt LiFePO4. LiIon or LiPO = 3.6V MEAN and 4.2V full charge - which seems not to match your cells. BUT how are you charging them - if you have no spec sheet and don't know the chemistry "you are in trouble". \$\endgroup\$ – Russell McMahon Nov 21 '18 at 11:09
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Not knowing the chemistry of your battery is somewhat similar to not knowing whether to put petrol or diesel in you car's fuel tank. Doing it wrong leads to bad consequences in both cases.

If your cells are LiFePO4 (which seems likely) then 25.6v/8 = 3.2V = nominal LiFePO4 MEAN voltage.

Max charge is about 3.65*8 = 29.2V BUT significantly higher voltages will not overly hurt LiFePO4.

LiIon or LiPO = 3.6V MEAN and 4.2V full charge - which seems not to match your cells. BUT how are you charging them - if you have no spec sheet and don't know the chemistry "you are in trouble".

Cell capacity:

25 Ah / 8p = 3.125 Ah/cell.
What size are the cells? (14500, 18650, ...?) If not known, please measure and advise OD (if tubular) and length. 18650 = 18mm dia and 65 mm long.

3125 mAh is near the top end for "genuine" 18650 LiIon cells and above what 18650 LiFePO4 cells will give (closer to 2000 mAh).

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When you remove the load off a battery, the voltage will slowly rise and recover a little, depending on the load size that it was exposed to. There is some good information and analogies in This question.

Normally the battery voltage given is the nominal voltage, and as you mentioned, this is 25.9V (across 8 cells), which makes it around 3.25V, normally a characteristic of a LiFePo4 cell.

Assuming LiFePo4, the maximum charge voltage is around 3.65V(really need a datasheet to be sure), the maximum pack voltage should be 29.2V. Your measurements being slighty on the high side.

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