|To work with solar panels, you have to sit outside.|
What I have are a hundred pieces of six volt, hundred milliampere panels. I am using them to charge two varta three-packs of nickel metal hydride cells, totalling a normative voltage of seven point two. So, of course I made a switching boost circuit, comprising an n-channel mosfet that momentarily pulls an inductor to ground, but when current develops through the inductor to positive, a capacitive coupling to pnp turns the mosfet off. Thus the inductor circuit oscillates at its ideal, between 8000 to 320000, depending on load. I initially wasted a lot of time with so-called "joule thieves," but these were the least efficient, compared to a snappy mosfet action plus tightly factory bundled inductor.
So, my only modification to check outside with the panel was the hypothesis that, when the battery was completely discharged, the switcher should be inhibited, yielding pure DC through the inductor and diode. So I made a simple PNP switch comparing panel voltage to battery voltage, to control the switcher bias. I discharged a battery pack completely, and attached it to the circuit. DC flowed and switcher was off, success. However, I noted that the efficiency was only about 150% better, and most importantly, the NiMH only stayed below 6 volts for a very short period of time. Thus, I'm not going to add this mod, because it really doesn't help. I've found that a NiMH acts like a capacitor when discharged, so it charges up really fast. It is only when it is up at its voltage, that injecting more current is what re-alchemizes its chemistry. Also there is various literature pointing to the aggressive pulsing of a switcher actually helps keep the chemistry good in a battery. The most important thing about this switcher is that the panel can be small; it is matched to the wattage dimensions of the battery, and that it works even in room light, so it is always adding charge.
I'll talk about this later, but there is a deep thesis in solar powered electronic music. The idea is that you should be playing during the day while it charges, and use the battery powered sounder at night, around the campfire, to scare away ghosts, or in a cave, to hear the galleries and depths.