Charging batteries with Solar energy

Posted by Matthew Mon, 10 Apr 2006 15:25:00 GMT

Depending on the application, charging batteries can be complex process. Charging methods range from constant voltage to pulsed and random charging. Once power is being delivered back into the battery, you have to know when to stop charging!

Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done.

Typically, common household batteries are charged with a current that is kept constant and relieved when the batteries reach a predetermined potential. However, solar cells typically generate a constant voltage of 0.5V and a varying current that depends on the amount of collected light. As such, a consant voltage charging model is easier to implement. I found two respectable tutorials on building your own charger:

AA Battery Solar Charger
Clean Power’s Solar battery project charger

Solar cell array

The first solution uses a diode to stop the batteries from discharging when there is no sunlight. I highly reccomend including this protection. Unfortunately, neither project implements a charge limit. You have to remove the batteries and test their charge with a multimeter. A shunt regulator is the simplest way to regulate the upper limit.

Solar battery charger schematic

Comments

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ShadyCraig said 8 days later:

Hi, Interesting article! Do you think this method would be suitable for lead-acid batteries?
Matthew said 10 days later:

Lead-acid batteries are generally charged using the constant voltage method. In a car, the current delivered by the alternator varies much like the current of the solar cells. A regulator makes sure the voltage output by the alternator stays relatively flat around 12volts.

Therefore, you could use this method to charge lead-acid batteries. Generally lead-acid batteries are very high capacity, though. You will need a large solar array or a lot of patience.
ShadyCraig said 10 days later:

Matthew, thanks for the reply!

I’ll tell you about the idea…

I have a long back garden and I don’t want to lay a cable all the way down there for a garden light. The idea was to get a high capacity battery that can slowly charge it up and by the weekend then there would be enough charge to power the light on Saturday nights when I’ve got friends round. I might use it for a radio too. I’m also considering wind power for a more reliable charge year round. Same voltage variation problem.

The voltage delivery for both these sources would be lower (I imagine) than the 12v battery, so I’d need a way of stepping the voltage up or a lot of solar cells!
Stilgar said 3 months later:

Thanks for highlighting our solar battery charger, the url has changed we dropped the cleanpower subdomain in favor of a more stream line site. the new url is http://www.thesietch.org/projects/solarcharger/index.htm

Thanks. Great site by the way full of great stuff.
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