This is what I call, "notes to myself". I gathered this from Duracell and Energizer web sites, for my own use. You are allowed to check their web sites, too.
Caution: Most measurements cited by the manufacturers assume a charge rate of 1C.
1C is unhealthy for NiMh batteries. It causes too much heat. This makes designing a good charger rather difficult because the specifications are only valid for a poor quality charger. Maybe a way around this is to use a C/2 or C/10 rate until the battery is mostly full (SC is approaching 1.45V) and then use a 1C charge rate for a few minutes while measuring dt/dT (d temperature / d Time).
The usual way to detect full charge is dt of 1º- 2º C / minute (1.8º-3.6º F) @ rate = 1C
There is a peculiar characteristic during charging, in that a voltage drop of -.02 V per cell happens at about 94%. It is possible to use this as a method to detect 94% full. The last 6% is delivered at C/10 (3.6 minutes).
A trickle charge is defined as .05C to .03C.
The “nominal” rating of NiMH batteries is 1.2 volts. They really deliver 1.25 volts at ½ amp from 90% to 20% full. A 2.5Ω, 2 watt resistor is an important tool for measurements on single cells, like AA or AAA batteries.
Zbatt =.05 Ω This is why NiMH is better than alkaline for current surges, like recharging a photo flash.
If a charging event achieves 1.78 volts per cell, the cell is overcharged!
If a charging event achieves 140ºF (60ºC), the cell is overheated!
More likely numbers for a full charge are 1.41V to 1.45 V, NL.
If a memory effect happens, exercising the battery down to 1.00 volts per cell and then recharging will correct it in 3 or 4 cycles.
Voltage reversal can happen, but it is very bad. Do not allow loaded voltage to go below .8V
Self discharge might be as high as 5% to 10% in the first 24 hours. After that, ½% to 1% per day.
The recharge cost in amp-hours is 50%. (A 1 amp-hour cell will require 1.5 amp-hours to recharge.)
The chemical result of overcharging is the creation of free oxygen gas. The cells can not survive undamaged if O2 is generated at greater rate than C/10. They might even explode.