I have a battery charger and the charging time for a 2300 mah NiMh AA battery is 18.5 hrs at a charging rate of 170 mA.

My math skills are rusty.

What would the charging time be if the charging rate is
150 mA ?

Homework?

Basic calculation is
Time = Capacity maH / rate ma × loss

We can determine loss based upon your charge time.
Loss = 18.5 / (2300/170) = 1.37

Time = 2300 / 150 × 1.37 = 21.0

My best guess.

And the answer is...

No it's not homework, Andy is into RC helicopters.

I think michaelk has it correct, in short my thinking was "a bit longer than 17.5 hours" since the mAH rate is lesser than your 17 hour example. This is all assuming the battery is discharged to a uniform point, the battery age is a factor, etc, etc. I'm sort of agog at these charge times. To me it would seem that you should be able charge a battery within 4-5 hours.

It depends on the charger. Simple trickle chargers by design use rates <= c/10. You can charge faster if you monitor temperature and voltage etc.

It does, indeed. 'Traditional' chargers depend on a series resistor, and the charging rate goes down as the cell voltage rises. 'Modern', 'high rate' and multi-chemistry chargers don't work like that, though.

These use genuine electronics and it is hard to predict what they do as cell voltage rises, short of actually measuring it.

And then you have to do something like integrating the varying current over time (simpler if the current doesn't really change much until the end point).

The modern ones tend to charge at a constant-ish rate, higher than nominal c/10, until the voltage kicks up towards the end. You can get chips specifically for this and they have the advantage that work reasonably well with cells with different chemistries and hence cell voltages. OTOH, the battery capacity does tend to go off a bit faster, over life, than you think it should.

If that really were the end of the story, the time ought to be (roughly) 170/150 times the original (the charge/discharge discharge efficiency varies with charge rate, but usually not much if the current doesn't vary much), but I suspect that the current varies enough to preclude an exact calculation. Why not measure it?

Thanks gentlemen.

I looked under Thread Tools but could not find out how to mark threads as solved.

How do I do so ?

You can't do that for threads in the Non-NIX General forum.

Might want to add that to your signature.

To expand on what a couple others said about charging. If you are running a simple trickle charger (simple resistor) you can wear out your batteries early by overcharging. heat is the enemy of batteries and trickle charging slowly cooks them.
This is why a lot of cordless phone batteries never last more than a year or two, they are using simple trickle chargers. I managed to find one with a proper charger and its over 5 years old.
You would be wise to invest in a MAHA C9000, it might seem expensive for a charger but if you have a lot of batteries you can keep them working for a decade, mine has paid for itsself already.