can 2 9-volt in parallel be recharged from a 12 volt solar panel, or would they only charge up to 6-volts each?
I know they can not be charge in series, the 12-volt is to much for the 9-volt, so they have to be in parallel. but will a 9-volt recharch to 9-volts if the input is only 6-volts.
I know a capacitor can save more then it receives, but can a battery?
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You have it backwards. In series, each battery would only be charged to 6v per battery. In parallel, the voltage remains the same across all components, so technically, you could charge as many batteries in parallel as the charger could supply current to all of them.
With two 9v batteries in parallel, a charging voltage of 12v would be equally applied to both batteries, which would definitely charge them. Problem is, without careful monitoring, those batteries could be overcharged and subsequently damaged.
It works well to charge any voltage “below” the voltage of the photo cell. It is best to either build of buy a charge controller for the voltage you want to charge – that will keep you from overcharging your 9V batteries. (The other answer is correct – two 9V batteries in parallel = 9V) That is how I would set them up, in parallel. If you put them in series they will be 18V and IF the solar cell puts out 20V (which most do – even those marked “12V” unless they have internal regulation which most don’t! ) you could still use the solar cell to charge them – just build a controller for 18V instead of 9.
You can just put a 9V – 10V voltage regulator in the circuit to assure you don’t overvoltage them if you charge in parallel but building a charge controller is really a better solution as it will not overcharge them!