I never like to make projects permanent, because then I don’t feel as able to make changes when I want to. But the solar charger system has just been working so well, and with the addition of my new “poor man’s li-ion” charging scheme, I decided to tack it down. I put it on a basic perf-board. It’s all contained inside an altoids tin that I had originally set up with my voltage panel meter. I had always planned on putting the entire circuit inside, but until today, it was just a glorified volt meter.
I left a lot of space on the board since I’ll be adding two other features to the charger once I acquire two key components. First, the over-voltage cutoff circuit. This is a simple circuit that will simply drain off most of the current once the voltage on the out line is greater than 5.1. To do this properly, I need to get another switch, since I want to be able to disable this feature under certain circumstances. For instance, under situations other than battery charging, it’s not very useful.
The second extra feature is a constant current out. For now, I’m accomplishing this with another LM317, but I want to be able to control the level of current (since various devices may call for more or less). To do this, I need a variable resistor rated for at least 1 watt, but preferably 5. That way I’ll have more than ample breathing room, and power could be scaled later.
I’ve also dropped off my capacitor buffer because I don’t have a lot of capacitors on hand, and to get the capacitance I wanted, I would’ve needed to use 4 of my biggest 150uFs. That also takes up a lot of space, so when I get the variable resistor, I’ll probably just get a few 1000uF capacitors.
I’ve also abandoned the heat sink I had been using on the first LM317. First, it took up too much space. Second, I had only really needed when the array was configured for 12 volts. Those were experiments only and I’ve found I’m using the array at 6 volts most of the time. I’m sure I could find a heat sink that would fit, but I’m not going to bother until I come across a situation that requires it.
Oh, and my poor man’s li-ion batter is basically an old cell phone that I pulled the battery from. This particular battery setup was fortunate enough to contain some raw power points on a small circuit board attached to the battery. So I soldered on two extra wires to the main battery connector points, re-assembled the battery into the cell phone, and added two holes for the new wires to come out. So now I can take the normal cell phone charger plug, and hook it right into my charger system and then charge the phone. To draw power out, I just use the two wires I soldered on. The phone never has to come apart. Li-ion charging made easy (for now). As an added bonus, the phone obviously still works, so I can just turn it on to see the state of the battery.