I'm trying to build a treadle-powered general purpose USB charger.
Internally, the treadle spins a motor which feeds through a diode bridge to charge up a storage battery. The battery then discharges through a voltage regulator to power a USB port (with some capacitors scattered around to smooth out noise).
As far as I understand, I need to worry about three potential modes of failure:
- Battery overcharge
- Battery undercharge
- Regulator overheating.
The latter two are easily handled in a loose way by a couple of LEDs and a thermistor, the former is more complicated.
My immediate thought is that it might not be necessary. As the battery becomes overcharged, it seems to me that the current necessary to charge it farther increases, so the treadle would quickly become very difficult to move.
Failing that, it seems as though the simplest solution would be to put a Zener diode in series with an LED or some other indicator, so that when the voltage of the battery increases past a certain point the Zener diode turns on.
The problem with this is that the range the Zener diode should act over is pretty small, and I don't think that I can know what it will be before putting everything together. Also, it may just mean that the Zener explodes before the battery does.
Third and most complicated solution is to have the Zener diode turn on a transistor or relay shorting out the motor. This probably avoids the problem of the Zener exploding, but leaves in the one about deciding what range to use and makes for some very noisy behaviour when the battery is on the edge of being overcharged.
The motor is a simple DC motor. There are a couple of options for the battery, though the best one at the moment is a ~4.5 Ah 6-V Lead Acid. Do I need to implement one of the latter two solutions, or was my initial thought correct?