Battery measurement after charge

Assuming your batter has zero capacity right now and your charger has a constant current output of exactly 300mA (no trickle chagrining), an ideal NiMH 3000mAHr battery with no heat losses will take exactly 10 hours to charge. However, taking into consideration that you have losses, the time can go up by 20% or more.

The reality of it is, is that quality chargers don't charge NiMH batteries based on time (only cheap chargers do that blindly at very low charge rates). There are at least two criterion quality chargers look at:

1. The rate of which temperature is increasing (if the battery gets too hot too fast, this tells that the battery is near full capacity)
2. The voltage peak and roll-off (depending on the NiMH battery chemistry, the voltage will peak and hold OR start roiling down after hitting a local maxima)

Then there is the issue of trickle charging... so its not as simple as you might think. Its really depends on the chemistry of the battery as well as how complex is your charger.

^{simulate this circuit – Schematic created using CircuitLab} Here is a simple example. From zero crossing of the ac waveform, the car battery attached at right can cause the zener diode to conduct. This removes the bias from r3 that would normally turn on the scr. So when the battery is not charged to the level preset by the pot, the scr will tend to fire and charge the battery. More sophisticated chargers than this will still use the same basic idea of driving a pulsed DC into the battery and doing measurements at or near zero crossing. Several chipsets have been developed to simplify making a battery charger that keeps track of the energy stored in the battery during charging. TI and Linear technologies both have lots of literature to describe this further.