Need help making a solar panel AA battery charger

[vandel212]

Hi,
I'm working on a project to build a solar powered AA battery charger. This is definitely not my profession, I only have a small understanding of electrical engineering. Mostly from what I learned in high school electrical class. Here is what I'm looking to do:

I want to charge 16 AA batteries. There will be 2 sets of batteries. 8 (series) AA, connected in parallel. If your are wondering why they are in parallel, ultimately this will be used to charge a tablet, so I want to up the capacity. If you are wondering why I don't buy one... They don't make one.

So I have a few questions:
1) To charge a battery, I need to hook it up backwards, that is I hook the positive of the power source to the negative of the battery, and the Negative of the power source to the positive of the battery. Right?

2) How much voltage do I need to put through this to charge the batteries?

3) How difficult would it be to make this charger prevent over charging?

 

[NascentOxygen]

Hi vandel212,

So I have a few questions:
1) To charge a battery, I need to hook it up backwards, that is I hook the positive of the power source to the negative of the battery, and the Negative of the power source to the
positive of the battery. Right?

2) How much voltage do I need to put through this to charge the batteries?

Nominally around a volt more than 8 x the fully charged voltage of a single AA cell should suffice, but with smaller solar cells you can go to a higher open circuit voltage.

3) How difficult would it be to make this charger prevent over charging?

Not so easy to implement. A bit of overcharging of NiMH cells is tolerable. You didn't indicate what chemistry cells you plan to use. NiMH would be safest.

I have reservations about charging two sets of cells in parallel unless you take steps to equalize the charging currents. Would be better to charge each set independently (or in series).

Is there any restriction on the area of solar panels you are able to use?

 

[vandel212]

Yes, they are NiMH batteries. No real restrictions on the solar panels.

 

[NascentOxygen]

Yes, they are NiMH batteries. No real restrictions on the solar panels.

Larger panels will accomplish the charging faster, but then be more likely to overcharge the cells. It's a compromise you will have to ponder.

NiMH cells are typically 2500mAH. If you charge at 250mA, you'll need around 15 to 25 hours of bright sunlight (or more hours of duller sunlight) to fully charge them from near flat. By that stage, the solar panels will be having a struggle to squeeze out a lot of current, so you could probably allow another 10 hours of gentle over charge without worrying. So that's at least 5 days before you'd need to remember to switch off the sun. Is that do-able? (I'm figuring on 7 good hours of sun per day.)

I think solar panel voltage goes up in steps of 0.6V, but you may not be able to buy them in such small increments. I think a panel of at least 11V is needed, so I guess that means buying 12V, and perhaps later put a diode in series to drop the voltage a bit, if needed. There is room to experiment, especially where you don't wish to do much electronics construction.

Can you give a link to the data sheet for some panels you're looking at?

I do think you should use C cells, to avoid the problems of running strings of AA's in parallel.

16 AA cells are not going to fully charge a typical tablet (say, 7000mAH).

 

[NascentOxygen]

There are modules available which convert a DC voltage to a higher DC voltage. Cheap, straightforward, low-loss, and easy to hookup. You could halve your total NiMH's then use such a module to double the voltage for charging the tablet. Use the bigger cells, C or D, perhaps.