Solar NiMH trickle charger

[New User Matt]

Does anyone have any experience doing a trickle or maintenace charge to NiMH batteries with a solar panel?

If you do, how did you do it and how has it worked out?

BTW Energizer website mentions a maintenance charge of C/40 is acceptable.

[Mk.32]

I have used chargers powered by a solar panel before... I would NEVER plug any type of battery into a direct power source. You want some kind of voltage monitor/shutoff/etc for charging batteries.

[New User Matt]

Yes, but what if for 4 AA nimh batteries you kept it between 6 - 9 volts and a charge of c/100 and less (when cloudier)?

Keep in mind this could only be for about 8 hours a day - so it would not be a continous charge.

Or if you think that would not be acceptable then what about using a shunt voltage regulator or LM317 as a current regulator - but at what current?

[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.

[FrankJ]

They make solar panel / battery packs with USB outputs. I have several friends that use them to keep their devices charged while camping.

[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)

[DonRotolo]

Originally Posted by Dunngeon

received .003 mA of current

Wouldn't that be 3 uA?

Anyway:
The bottom line here is that direct connection of NiMh batteries to a solar panel could get exciting, in a bad way. The absolute gold standard is to use the panel to power some kind of charging circuit.

An LM317 as a constant current source would work, albeit not very efficiently. There are many single-chip solutions available, for example from Maxim, who has a policy of handing out samples quite freely, so if you need just one....