the bblog

replacing the battery in a cheap solar charger

So I've got a very cheap solar charger, whose internal li-poly battery has been slowly dying, and is now mostly useless. Popping it open, I discover it's a 800mAh unit.

Why, robotmesh.com, my place of employment, just happens to sell 850mAh li-poly batteries! How convenient.

So let's replace this sucker.

While I've got it open, though, I want to see how much current the little solar panel can actually source. All you need to do this is a single multimeter, though it's more convenient to use two, one bridging the positive and negative rails, to measure voltage, and one inserted in the current flow, to measure amperage. (If you get it backwards, the ammeter will look like a dead short to the voltage source, and either blow a fuse or melt your test leads.)

(Diagram made with Circuitlab, which can apparently do all sorts of fancy simulation stuff, none of which I actually used.)

This looks very neat and clean on paper, and becomes a horrifying tangle of wires when implemented with multimeter leads and alligator clips. (Mostly hidden off-frame)

I set everything up, ready to finally, at last, read off the current...

And it's way off the low end of the scale. Switching to the digital meter, I discover that even in direct sunlight, the charging circuit can only manage a thoroughly unexciting 7mA. Assuming perfect charging efficiency, (which ain't gonna happen) it'd take 121 hours of direct, face-on sunlight to recharge a flat battery.

I wrote a gloomy analysis of a fictional solar charger on the other blog, and even with my worst case assumptions, this real-world solar charger is more than 26 times worse. Time, and sunlight, has not been kind to its panel.

So, now that I know how much it sucks, it probably wasn't worth spending 7 bucks to put a new battery in it, but oh well, let's close it up again.

Today's stars are my old Weller P2K butane soldering iron, here used just for the heat-shrink, a crappy Tenma unregulated soldering iron, a Panavise PCB holder that they apparently don't make anymore, and some Radio Shack "helping hands". A depressing amount of the work that goes into doing electronics stuff is fixturing-- getting things to stay in the right position while you do things to them.

I took six photos here, all of them in varying degrees of out-of-focus.

Someone more professional than I would probably have done a lapped splice here, in which case it's very important to slip on the heatstrink before you solder the joint, but I did a twist splice because I'm lazy and the joint wasn't going to take any mechanical stress anyway.

And we're done! Now I have a charger which should last several more years, at which point I will throw it right into the trash.