A DIY Solar Cell Battery Charger
Instructions to build your own solar battery charger.
You don't need very many items to construct a solar power battery charger:
First of all, you need to decide which type of batteries (AA or AAA) that you want to recharge and also how many. Their size and amount will determine the size of solar panel needed.
A rechargeable AA or AAA battery is rated at 1.2 volts and needs a charging voltage of around 1.3-1.4 volts to reach full capacity. Therefore if you want to charge 4 x AA batteries you would need a 6 or 9 volt solar panel. Another thing to remember is that you should not charge a battery with more than 10% of its capacity as current. For example, if you have a battery with a 2,000mah capacity, it should be charged with a maximum current of 200ma to avoid the risk of overcharging.
Now, once you have decided the size of your charger and have acquired the right solar panel, it is time to put your battery charger together.
First of all the batteries are put into the battery holders and then the holders are wired together in series. To do this the positive lead from one battery holder is connected to the negative from the other. The remaining free wires - one positive and one negative - are our charging inputs.
In order to prevent stored power in the batteries being released through the solar panel during the night, a blocking diode is used. Placed in the positive line from the solar panel this only allows electricity to flow from the solar panel to the batteries and not from the batteries to the solar panel. So, to mount the diode correctly, the ringed end of the blocking diode is connected to the positive battery input (red), and the other end to the positive output from the solar panel (red).
The negative output from the solar panel (blue) is then connected directly to the negative battery input (black).
And then - tada - you're done! Easy, huh?
There is particularly one easy to fix improvement you might want to consider. First of all, there is no way to control that the batteries will not be overcharged. It is not a big problem if the solar panel and battery capacity is well matched, as in our above project, but if you for example would want to charge other types of batteries, overcharging could be a risk unless you change to another solar panel.
To solve this, you can mount a voltage regulator chip together with a suitable resistor to regulate current. A constant current circuit is created, that then is mounted to the solar cell battery charger. (See the picture below)
In this case, you would connect the negative lead from the solar panel to the free negative input of the battery holders, just like before. But the positive lead from the solar panel should now be connected to the positive input of the current limiting circuit, and the output from that circuit connected to the free positive lead of the battery holders.
Now you have a solar cell battery charger that can't overcharge your batteries.
If you want to continue develop your battery charger, you could also add a battery status monitor to be able to see how far the charging has come.
But if you are into solar DIY projects, you can both find more of them here at Solar For Energy, and you can get yourself some really good guides to continue your venture. See below for some ideas.