I have a 12v, 5w solar panel that I would like to use to charge 2-4 AA or AAA batteries. The current rate I get from the panel, depending on conditions ranges from 30-200ma. And the voltage ranges from 3 to19. So my current needs will be way less than 1 amp! And the voltage needed would be 1.2 to 1.25.
(1) If the current rate is 10% or less than the battery's rating (AA = 2100-2600mah, AAA=900 to 1000ma), is a charge controller even necessary? My thinking is no because the current flow is not enough to overcharge. But perhaps I'm wrong here.
(2) I would also like to use the 5w panel to charge a 12v, 1.3ah lead-acid, deep-cycle battery for powering another device. Would I need a charge controller here?
NOTE: I will not be connecting the AA / AAA batteries and the 12v together. They would be done separately. So only one will be charging at a time.
Ideally a charge controller will maximise the amount of useful power you can collect from your panel and convert it to the voltage you need. Without one you would be wasting over 90% of the energy available if the panel is supplying more than 12 volts when you only need 1.2. Conversely, when used to charge your 12 volt battery, you panel will be totally useless if the amount of light it is receiving is not enough to give you an output voltage that is over 12 volts. Look into a curcuit called a buck-boost converter.
Your batteries will have a much longer service life if they are charged properly without being overcharged. NiMH cells are nowhere near as forgiving of overcharging as the old NiCD cells were. Your charging circuitry should incorporate some means to sense when they are full so you don't cook the life out of them.
In your situation I would try to find a charge controller that outputs a constant 13.5 volts (at varying currents) regardless of whether your panel is delivering 3 volts or 19. That should be about right to float charge your SLA battery which you can then leave connected. Then look for a NiMH charger that will work on a nominal 12 volt input. One possibility is the Duracell CEF23 (if you can still find one) which will charge 1-4 AA or AAA cells while individually monitoring the state of charge on each. I personally own 3 of them and they're worth searching for.
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