Hi, I'm running a 6V, 3W light setup (Union brand). What I'm interested in is adding battery backup for illumination at slow/no speed, as well as a redundant system in case of gen. failure, plus if I just want to be lazy and let Duquesne light do all the illumination work, lol.

I happen to have 10, Ni-mh batteries that output ~1.35V each. I was thinking that I'd wire 4 of them in parallel with the gen, and at lower speeds, the batteries would "win" the voltage war and drive the lights. At higher volts, the gen would win.

Three Q's:
1. Do I need to put a diode coming off the batteries? (At high speeds, the gen would otherwise attempt to "charge" the batteries, and I'm not aware if there'd be an overcharge concern.)
2. Do I need to put a diode coming off the gen?
3. Does this sound otherwise workable?

Just hooking the batteries directly might work but in a very limited way.
1. Batteries may overcharge depending on the generator characteristics and lead to premature battery failure.
2. When stopped the batteries would discharge into the generator as well as the light so not good.

You do need some sort of isolation between battery and generator to properly control charging and discharging. A diode is a simple solution to discharging into the generator.

Another important question to answer is: Does the generator put out AC or DC? AC is OK for the light but not so for the battery.

Why not just isolate one from the other with a couple of cheap switches from Radio Shack?

Thanks for the responses.

Didi a bit of looking on the 'net and Sheldon Brown covered this already here: https://www.sheldonbrown.com/dynohubs.html

Sounds like he just hooked them up in parallel and let the gen charge the batteries. The only thing I wondered was if an unregulated charge of the batteries might harm them (recall from my flying days that Ni-Cds are subject to a peculiar little phenomenon known as "thermal runaway.")

Pretty sure it's DC because I already tested output w/ a voltmeter and got good results...imagine it'd have a hard time making heads or tails out of AC current in DC setting.

You could try these:

Altronix AL624 6v/1.2a power supply
Altronix SMP3 6v/2.5a power supply
Altronix SMP5 6v/4a power supply

Just keep in mind that they are designed for charging SLA batteries, not nicd or nimh. I think a hub dynamo could replace the transformer input to these units and charge the batteries during riding time with lights off.

Here's how you do it.

4 diodes form a bridge rectifier converting AC to DC current.
S2 disconnects the batteries from the bridge rectifier and dynamo.
S1 turns the LEDs on and off.
The 4 Ni-MH batteries must be solder tab type with no possibility of poor or open connection.
Paralleled resistors are used to obtain the required values.
350mA LED is used in the headlight and the 36mA LED is used in the taillight.
The LEDs must not use more current than the dynamo can produce. The small excess current recharges the batteries as you ride compensating for stops when the dynamo is not producing power. Closing S2 and opening S1 allows rapid recharging of the batteries at 500mA while riding.

The batteries act as there own voltage regulators. As output voltage from the dynamo increases there internal resistance also increases. The resistance reaches a point where the dynamo is so loaded down that it can't produce more voltage. 4 series connected rechargeable Ni-MH batteries will limit voltage to 5.2 volts.