Charging NiMH 1.2v batteries from 12v

[1stmate]

Hiya all-

This is probably absurdly simple, but as I get on in years the simple things sometimes elude me.
I'm setting up a solar 12v charger to use for small scale LED lighting & such in power outages. I'd also like to arrange a charger for AAs & the like from the 12v battery.
Is a step down transformer necessary? Or can I do it with resistance in the wiring? I don't want to weld the little batteries into the charging box! :o

Thanks in advance...

[kawalekm]

The AA charger that I have has the charging unit itself that is powered by a plug-on AC power supply. *The output of the power supply is 1A at 12 volts, which suggests to me that you could just supply 12V power directly to the unit rather than pluging it into the wall.

I also have an automotive power supply/transformer with a banana type plug that switches from 3 to 4.5, to 6, to 9, to 12volts. *It has plug-in adapters to switch to any sort of plug you need to plug in. *You could charge a deep-cell battery with the solar panel, then charge individual units with the transformer, or maybe run the transformer directly off the solar panel. *I bought a couple of these for 5$ each at the flea market.

[12vman]

"Is a step down transformer necessary?
Can't use a transformer with D.C.

You can control the current with the size of panel you choose. NiMH's will handle over voltage conditions somewhat. It's the current (amps) that heats things up..

Example..
A standard 5 watt 12 volt solar "trickle charger" for an automobile will only produce around 1/4 of an amp. (.250 mA.) during full sunlight conditions. (dead shorted into an amp meter) That's the maxium current that it will produce.. (with no 12 volt battery in circuit)

Let's say that you have a fixture that will hold 6 NiMH batteries connected in series @ 2000 mAh. each. Using the above 5 watt panel @ .250 mA. and dividing that into the batteries mAh. rating will figure to about an 8 hour charge. (1 good sunny day) The internal resistance of the batteries will hold the voltage down to reasonable levels. Charge less batteries, less charge time needed. 250 mills isn't going to blow anything up. Just monitor the voltage of the fixture. When it reaches around 3-4 volts, the cells are pretty much charged..

When charging from a high current device such as a battery, "regulation" of the charge is a must! I would suggest finding a charger that has an input voltage of 12 V.D.C. either from a wall wart or a cigarette lighter in a car. I use this (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47620) charger from my battery. Been using it for years now with no issue. The wall wart puts out 12 D.C. @ 1 amp. I cut the plug from the wall wart and connected to my battery. (with proper polarity)

Hope this helps..
~Don

[1stmate]

Thanks for the input guys- I'll see what I can come up with based on your suggestions.

1stmate

[turboRC]

Charger you need right here http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMAJ0&P=ML