Solar NiMH trickle charger

[Dunngeon]

Originally Posted by asid61

If you don't use some kind of cutoff, the batteries are liable to explode.
If it's just a trickle charger for smaller cells, you could look into some of the commonly available charging ICs. Some of them come in DIP format, although the ones that need fewer components are limited to 300ma/hr charge. You should use one of the buck-boost chargers or use some kind of voltage regulator such as an LM317 with the charger in boost mode.
It's usually easier and cheaper to just buy a charger and hook it up to some kind of regulator circuit coming from the solar panel.

If he keeps the amount of energy flowing to the battery small enough ( i don't know the range for NiMH batteries) it could be done easily with a simple resistor circuit. When I did my paper on recharging Alkaline batteries, I found that if they received .003 mA of current they could be kept in that state without any negative side effects. Although loss of charge is more of a problem with NiMH batteries, I'm sure there is an effective range for developing a trickle charger. The hardest part is finding it, I found Alkaline's by accident, but it looks like OP has some actual math backing up what he has found.

Ninja edit:

You could use a variant of pulse charging to do this I think. Just make the positive area of the wave slightly bigger than your depolarizing load area. This should help prevent polarization of the cell and limit the potential for overcharging (which is polarization in itself)