I'm starting to take more, and more, flying trips away from home. Some of these trips can last for several days. Recharging from the plane's battery is not a good idea since having a dead airplane battery would be a major bummer.
As an example, a friend of mine and I flew to Oshkosh, WI this year to attend the week long monster airshow. We were away from home for four days and three nights - tent camping all the while. Before our big trip, I knew I was going to need a way to recharge all our electronic stuff each day. The problem is, we're using our electronic junk during the day and need to recharge it all at night. The next problem is, if we recharge off a battery pack, the battery pack probably won't last.
The solution is to use a solar panel to recharge a main battery pack during the day and then use the main battery pack to recharge our stuff at night. This solution seems to work very well and it's dead simple.
Step 1: Building the battery pack.
The 5 watt solar panel ( 8.5 x 12 inch ) has a built in blocking diode. Therefore, when the sun goes down, no juice will get wasted by flowing back into the solar panel. The solar panel only charges the main battery pack if the voltage coming from the solar cell is greater than the main battery pack voltage. That is very handy.
Also, since the solar cell is only 5 watts, the cell can only produce about 250mA of current (peak at full sun). The 250mA charging current is well below the 1C current the batteries can handle. Plus, the peak output never last for long. The average output is more like 150 to 200mA. However, if the batteries are fully charged, the 250mA will get converted to waste heat instead. I've tested this, the batteries get only mildly warm if 250mA charge current is applied to a fully charge battery. No harm is done. There is one thing to know, NiMH batteries don't like to be trickle charged for days and days. For my application, it's not worth worrying about since my trips never last that long.
The battery pack is made up of 12 AA NiMH batteries, rated at 2500mA*Hr, all wired together in series. I found a plastic clip that the batteries snap into. One clip holds four batteries and the other clip holds eight. With the two clips wired together I get a nominal voltage of 14.4 volts. This is very similar to a vehicle cigarette adapter - with the engine running, the vehicle cigarette adapter output is 13.8 volts. Therefore, anything designed to plug into a vehicle cigarette adapter will work very well with this system.
Lastly, I wired an inline fuse just inside the PVC cap of the battery pack. I used a 2 Amp fuse and have not had any problem so far. The fuse protects both the batteries and the pack wires from over current.