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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:

  1. Battery overcharge
  2. Battery undercharge
  3. 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?

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  • \$\begingroup\$ How will having a thermistor prevent regulator overheating? \$\endgroup\$ – WhatRoughBeast Mar 8 '16 at 19:42
  • \$\begingroup\$ Either by disconnecting the regulator when it gets too hot or just by letting me know so I can turn it off. Part of the point of this thing is that when it's in use I'm always sitting there when I'm using it, so I can be more lax about this stuff than I otherwise would be. \$\endgroup\$ – P... Mar 8 '16 at 20:03
  • \$\begingroup\$ "The motor is a simple DC motor." - a brushed DC motor (used as a generator) or BLDC motor (used as an alternator)? How long do you expect the treadle will be operated for to charge the battery? \$\endgroup\$ – Bruce Abbott Mar 8 '16 at 23:40
  • \$\begingroup\$ If "...part of the point of this thing is that when it's in use I'm always sitting there...", just hook up a volt meter, keep stare at it and turn off whole thing once it indicates what you wanted to see.... \$\endgroup\$ – soosai steven Mar 9 '16 at 1:07
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Just do it right!

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Adjustable voltage regulator.

Use a proper voltage regulator. The LM317 is an adjustable voltage regulator. Adjust R1 so that the output is 7.2 V before connecting the battery. Pedal like mad.

A 1 A charge rate at 12 V from the generator will be 12 W. A passably fit cyclist could generate 100 W for a sustained period.

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  • \$\begingroup\$ Use heatsink for the LM317. A 10 ohm current limiting resistor will be a good idea... Put it in series with D1. \$\endgroup\$ – soosai steven Mar 9 '16 at 1:10
  • \$\begingroup\$ What @soosai said, and add an LED that comes on when V1 reaches above, say 10-12V (or whatever value you prefer, so long as 1: It's below the AMR value for your LM317's Vin, and 2: It's not likely to force the LM317 to dissipate more heat than its heatsink can remove) \$\endgroup\$ – Robherc KV5ROB Mar 9 '16 at 6:12

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