Recharging 12V NIMH Batteries from a Solar Array

Re: Recharging 12V NIMH Batteries from a Solar Array

If your charge rate is C/10 or less, generally, you can simply connect the solar panels directly to the battery pack without problem... If did this 24 hours per day x 7 days a week, the battery pack may only last something like a year +/- vs several years with a charge controller.

But, you are only charging it with a solar panel (4-6 hours of full sun equivalent per day)... So I would not worry. Just use the correct AC battery charger when at home (and even then, for better life, only connect the charger once a month for a day to keep the batteries charged if you do not have a "float" stage on the charger--many AC chargers tend to overcharge the batteries too if left connected 24x7).

Power wise, it is not unreasonable to derate the panel + charge controller by 0.77 (panels have "marketing" ratings--which tend to be over optimistic in hot weather and also to account for other losses).

Recharging NiMH battery packs tends not to be very efficient--Perhaps 60% efficient--but it is pretty variable. A lot depends on the exact current you supply to charge the batteries vs their AH capacity.

You could use a $60 DC Amp*Hour meter like one of these to measure how much it takes to charge the batteries after each use.

Also--Solar panels need to be free of shade and pointing at the sun--so during your ride (assuming this is when you charge), you will need to mount it so that panel is pointing relatively at the sun.

If you charge while camping for a few days--the panel should be well anchored to prevent it blowing over in the wind--The panels are tempered glass. Pretty tough, but once they get hit wrong the panel will shatter like safety glass.

So, to estimate panel size lacking any more data:

• 5 amp*hour per night * 17.5 volt panel rating * 1/0.77 derating * 1/0.60 batt charge eff * 4 hours min sun per day = 47 Watts minimum

The above is a guess at what wattage rating you would need for a successful system.

If you are cycling only in summer in the US southwest (or other "sunny" region, you might want to assume 6 hours of sun per day). The battery pack you have may be more efficient than I have guessed.

And do you need a backup (say you have a few days of clouds--what happens if there is no AC backup power). Should you bring a DC clip on charger and ask if you can use somebody's car to recharge?

Regarding charging without a battery charge controller--You could use a solar charge controller to limit voltage and/or a temperature sensor in the pack to cut the power/alert you if the pack is getting too hot to help reduce the chance of overcharging. Everything adds weight and complexity.

-Bill

Re: Recharging 12V NIMH Batteries from a Solar Array

I think those side of tire alternators were ~6 watts at 6 volts or so or about 1/2 amp at 12 volts and 10 hours of peddling (or 60% more) to recharge after a night of use. And it would be like peddling up hill the whole time. And the sidewall generator (probably alternator) would only last 100's or a 1,000 miles or so...

There are generator hubs--If they were 2x as efficient--that would still be a lot of peddling/load for a night's use. I guess the new ones are rare and expensive... You may find some older ones (late 60's or later?) that work.

-Bill