Charging NiMh battery pack

[soldar]

I have rebuilt a NiCd drill battery pack with NiMh cells and need to decide on how I am going to recharge it.

After reading a bit on this topic I am inclined that the charger should have a faster charging, say C/5, a slower charging, say C/20, and then shut off, zero charging.

I can push a button and start on low charge directly and the charger will shut off when the voltage reaches 22V

I can push a different button and the charger will start on fast charge until it reaches 22 V and then it switches to slow charge until it again reaches 22 V and it switches off.

This seems easy enough to implement with a very simple electronic circuit and I think it would be a good charging policy. If I am working with the drill I can fast charge. If I have finished for the day I can slow charge.

I welcome comments and ideas.

[mariush]

I wouldn't have bothered... that's what ... 18 cells?

Ideally, you would add some kind of fuel gauge IC to know how much energy is still left in the cells, and for example if the amount is below some threshold, run the cells down, discharge them, and only then charge them up.

Not sure how I'd do it .. maybe I'd have the battery arranged in 3 groups of 6 cells, or 6 groups of 3 cells that could be charged in parallel (have some solid state relays or some way of separating the groups when you plug battery in charger)

This way you'd be able to charge the groups with 5v (for the 3 cells in series) or 9v or higher for the groups of 6 cells

You'd probably be better off with a pack of LifePo4 cells ... with 3.6v charge voltage, you could have 6  in series and charge them directly with a 20-21v power adapter (charge voltage would be up to 3.6v per cell, 6 cells x 3.6v = 21.6v ... charging them to lower maximum voltage will increase their life)