Modding an 'AA' Solar Charger

Okay, I go on long back country trips and want something to charge 'AA' batteries while I'm out there. I bought this commercial solar charger for AA batteries and unfortunately it's a little disappointing. The charging circuit is just a diode to prevent discharge. It does have a nice little amp meter, though. It's disappointing because the box could have been a lot more compact given the lack of internals. I want to strip it apart to it's basics to make it more packable/lighter. I also want to add more solar power.

I have 2 solar panels that were salvaged off of broken solar lights. I just want to make sure that I wont break anything if I add one or both of these panels to the circuit. Also, should I add them parallel or in-line? If the stats on the panels will help, I can measure the outputs.

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You would want to put the added panels in series (in line) not in parallel. They will increase the open circuit voltage some and should also increase the charging current slightly. You may have to measure the output voltage polarity to make sure you wire them in properly.

The panel you have probably does a good job for the four batteries, if you can keep it pointing directly at the sun. Unfortunately, it takes hours to charge batteries properly, so you would need to keep moving the panel. A better way would be to use a much larger panel, 2-3 times the voltage you need to charge the batteries and regulate it down.

The panels from garden lights are usually 3-4 volts, 20-50 mA, and charge low capacity batteries (280 mAh), and take a full day of sun. You would need to add quite a few.

According to the Ammeter I get about 140mA charging current in direct sunlight. That would mean that a single 1200mAh AA battery will take about 9 hours to charge. I'm going to cut this thing down to it's basics and add two of the small solar cells in series. I want to be able to charge 2 1200mAh batteries within 2 days of direct sunlight.

We'll see how it goes on this trip. Otherwise there is a more professional fold out solar panel option, but it's pretty expensive. Thanks for the input.

You measured a current of 140mA from the solar panel at noon. At other times it is less. When it is cloudy then it is much less. At night then ....
Your calculation forgets loss. A 1200mAh battery with 140mA needs 12 hours to be fully charged from being discharged. There is 40% extra time to allow for losses (heating).

I bought a 12V/150mA solar panel for 10 bucks on sale. I have never seen more than 100mA because I don't live in the Sahara dasert. Behind glass its current is only 50mA at noon in summer

I have many solar garden lights. My government gave them away for free. I got 10 free ones. The rest were at half price.
Some are on my window ledge and I need to manually charge the AA Ni-Cad battery cell fairly often. The ones outside glow all night in summer following a sunny day. They are not very bright. They don't work in winter.

If your panels already have sufficient open circuit voltage to charge, wiring the additional solar cells in parallel will increase the charging current, which may give you better results. If you're changing the setup to charge more batteries in series, then you'd need the higher charging voltage of adding the additional solar cells in series.

You might also see if you can replace the discharge diode with a MOSFET in a type of synchronous rectifier setup, to reduce the losses from the diode drop-