Charging questions

[777qqq]

When should NiMh and NiCd batteries be completely discharged? Some believe that they should be completely discharged for the first cycle when priming (cycling several times) new batteries to measure the maximum capacity or for the first cycle when priming old batteries stored too long when trying to break up crystalline formation and once for every couple dozen charges for batteries used regularly. Since NiMh is not as robust as NiCd I believe NiMh should only be fully discharged infrequently. Also, I don't have a battery tester right now and want to discharge my charged cells down to around one volt per cell for storage. About how long should I discharge them based on the time required to fully discharge them e.g. if my charger manual says it takes ten hours to completely discharge a certain capacity should I discharge the cell for e.g eight hours to get down to around one volt? Sorry, but I want to store these cells now. Thanks for any replies.

 

[TooManyGizmos]

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[tylernt]

I'm not sure why you would discharge them for storage, but here is a relevant thread.

To discharge based on time is hard. You can use a resistor and a resistor calculator, but discharge current slopes down as voltage slopes down, increasing the time required, so it's not a simple linear equation. You'd need some fancy math or a circuit simulator to calculate the time. Or just watch it and disconnect when voltage hits 1.0. Or see the above linked circuit to do it hands-free.

 

[45/70]

First off, you should never fully discharge a NiMH cell, as it will damage the cell to some extent. A discharge to 0.8-0.9 Volts under load is acceptable, occasionally. The OC voltage when this is done, should return to around 1.20 Volt. NiCd cells are a different story. Individual NiCd cells can be discharged to 0 Volts.

The ideal voltage to store NiMH's at is 1.20 Volt OC. I store mine at slightly above that, and then check them once in a while. If the voltage has fallen below 1.20 Volt, I give them a brief charge to no more than 1.25 Volt.

Dave

 

[777qqq]

Thanks. That's what I needed to know.

 

[marksun]

Time is a difficult method for discharging reliably. In my nicad and nimh days of RC car racing, we would discharge individual cells of a battery pack with a resistor and diode in series. For a six cell battery, six separate discharge circuits were used. The diode turns off at about .7V stopping the discharge, and the resistor is there for current limiting. Generally we'd do this when the battery pack had been discharged close to the point of depletion but before any single cell in the battery was fully discharged.