Charging nimh batteries

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Nov 22, 2003
Ithaca, NY, USA
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This article could bring you some of the facts of charging/cycling nimh batteries.

When Nickel based cells are used, they can develop large crystal growth. This large crystal growth is enhanced by slow charge rates, self discharge of the cell, extended trickle charging, and low current draws.

This large crystal growth affects both the Nickel and Cadmium chemistries, but does not affect the Metal Hydride chemistry. The result of large crystal growth is increased internal resistance. This manifests itself as lower voltages under load, more commonly known as voltage depression. If your circuit cuts off at a minimum voltage, it sometimes seems that the cell remembers that it was only partially discharged several times before, thus came the widely known memory effect term.

To quickly produce voltage depression you have to follow an exact recipe. Sanyo has an example with NiMh cells. Charge at 1C, discharge at 1C to 1.2 volts, and repeat 20 times. On the 21st discharge, try to take the cell down to 1.0 volt, and it will show very little capacity under 1.2 volts, as if it has remembered that it was only going down to 1.2 volts previously.

To erase this voltage depression, you need to do a couple of full charge/discharge cycles, discharging all the way down to 1.0 volt.

This exercise works even better with NiCd cells.

When cells are new, they benefit from a standard charge and standard discharge. The standard charge is a 0.1C charge for 16 hours. The standard discharge involves discharging in 5 hours. This standard charge/discharge cycle evenly distributes the electrolyte within the cell, saturates the separator with electrolyte, and lays down an even crystal formation within the cell. This is referred to as forming the cell.

The 16 hour standard charge also slightly overcharges the cell, so if you have several cells in a battery pack, it is a good way to balance the cells within the pack. Even though there is a slight overcharge, the charge rate is low enough that it does not damage the cells.

In use, cells may develop large crystal growth. This hampers their performance, and the usual recommendation is to do a standard charge/discharge cycle. If your standard use involves shallow cycles, your cells would benefit from a standard charge/discharge cycle every 20 – 40 shallow cycles.

The standard charge/discharge also helps revive cells that have been in long term storage (more than 30 days).

Discharging a NiCd cell to 0 volts does not seem to hurt it at all, and it tends to eliminate large crystal growth, so there is some benefit from this. You can also store a NiCd cell at 0 volts, and can usually bring it back to normal capacity after a couple of charge/discharge cycles.

NiMh cells are different. They can handle a discharge to 0 volts, but if you store them at 0 volts, the electrolyte oxidizes. This process takes some time, so the longer you keep them at a very low voltage, the more damage is done to the electrolyte. Large crystal formation can be broken down by discharging down to 1.0 volts, or if you want to deeply discharge them you can go all the way down to 0.9 volts, but beyond that, you run some risks.

Finally, my comments on GhostRiders question is that there should be no problems discharging your pack at 1C down to 0.9 volts per cell. As others have indicated, there is probably no benefit from it, but it should not be a problem, unless your pack is severely imbalanced. I would suggest doing a standard charge first in an effort to balance the pack.

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