How to convert solar car trickle charger to usb/battery charger?
I would like to know how I could convert one of these solar car battery trickle chargers to a charger that could charge ideally AA and/or AAA batteries but have a female usb port to charge usb powered devices. The point of storing the power in portable batteries would be that it would then double up to charge those batteries if I ever needed to charge them. I do want to spend as little as possible and buy as few parts to do this as possible. I understand that larger batteries would be able to store more power but it would then mean it would not charge AA or AAA batteries and make the whole thing a lot bulkier and expensive.
I have a choice of solar trickle chargers being 12V/12W, 12V/4W, 12V/2.4W and 12V/1.5W. They come with crocodile clips and a cigarette lighter plugin. I also have an unrelated usb charger (with female usb port) that can plug into a cars cigarette lighter so Im not sure if it would be possible to attach the usb section to the solar panels cigarette lighter plugin. I also have a usb charger that plugs into the mains. I know almost nothing about circuits. The general point is to be able to make the charger as multi-purpose as possible, like being able to charge batteries or usb powered devices but without making it awkward to use or un-portable. I have looked at the related instructables but im unsure how you could change the voltage to make it safe for the batteries/usb port etc.