In my bicycle, I have a hub generator which provides alternated current, at nominal 6V / 3W.

I have built a regulator/filter/rectifier, using a rectifier bridge, a 6800 uF electrolytic capacitor, and a 6V/5W zener diode, and it is working great.

Now I am considering, as commonly seen in vehicles, to add a battery in the system, but I wonder how should a battery be wired / specified so that:

  • The battery doesn't discharge through the regulator;
  • Every excess power produced by the system while in use is sent to the batteries;
  • Running devices take the power from the battery automatically when the generator is not running (that would mean the devices would "blindly" use the energy from the system, disregarding if it is provided online by the generator, or drained from the battery).

I guess this is a fairly simple and common setup, but I am looking for the theory involved, instead of just taking a ready circuit design for a random blog, without even knowing what I am doing.

EDIT for more context:

I am using a 6V regulated generator, so I am thinking about using 4 x 1.2V (4.8V total) rechargeables. That would be enough to keep my stuff (led lights mostly) running while the generator is not active. All the lights have series resistors, so their brightness is not severely affected by 1,2 voltage drop.

One concern is that, besides not discharging the battery through the generator (I don't think that would happen since there is a rectivier bridge there), I shouldn't damage the batteries through over-charging them if I keep riding for hours and hours...

  • \$\begingroup\$ What type of battery are you looking to use? While your generator is up and spinning, if it's outputting a pre-regulated 6V, that's good. You might need just a battery charging circuit. I imagine your current setup wastes a lot since the zener is probably passing some \$\endgroup\$
    – krb686
    Oct 19, 2013 at 15:20
  • \$\begingroup\$ I'll update my post with more context \$\endgroup\$ Oct 19, 2013 at 15:43
  • \$\begingroup\$ Perhaps you should tell us a bit more about your setup. What does your generator put out without the bridge rectifier? If you want to charge 6V batteries, you've already got the means to if your generator puts out more than that. I would say just adjust your rectifier to put out higher than 6, and then buck regulate down to whatever level is necessary for your batteries into a charge circuit. It also depends on the batteries you use. Some are very sensitive to charge levels, like lithium, and others aren't \$\endgroup\$
    – krb686
    Oct 19, 2013 at 22:06
  • \$\begingroup\$ Pololu has a bunch of nice regulators you can use for this task. You can rectify at a higher voltage and just buck regulate down, or even use a buck boost. This one has an efficiency of 80-95% pololu.com/catalog/product/2118 \$\endgroup\$
    – krb686
    Oct 19, 2013 at 22:12

1 Answer 1


If you are using a 6volt battery then for most of the time you need a boost switching regulator except for those periods when the generator might be producing voltages at about 6volts. The problem here is that the generator is about the same voltage as the battery and this effectively means using a boost switching regulator generating maybe 8volts followed by a buck regulator that takes the 8volts and charges the battery thereon.

The alternative is to use a 12volt battery and avoid the buck regulator. In effect you just have a boost regulator and it will be about 90% efficient.

Ensuring the battery does not lose energy back into the charger when the generator is stopped is trivial so you have to decide on your battery voltage and your battery technology. This, might warrant a fresh question should you struggle to decide.

  • \$\begingroup\$ I'll edit my post with more context... \$\endgroup\$ Oct 19, 2013 at 15:44
  • \$\begingroup\$ if 4.8 is your battery voltage then you'll need a boost regulator producing greater than 5v followed by a current regulating buck regulator. It would be simpler and more efficient with a battery voltage above about 7volts. \$\endgroup\$
    – Andy aka
    Oct 19, 2013 at 17:20
  • \$\begingroup\$ do you only want to charge the battery when the generator is at max volts or would you like it being charged when it may only be producing 2volts. This could make a big difference to my answer. \$\endgroup\$
    – Andy aka
    Oct 19, 2013 at 17:57
  • \$\begingroup\$ Why would he need a boost converter? His generator already puts out more than 6V, unless he wants it to still run when the battery drops below 6. \$\endgroup\$
    – krb686
    Oct 19, 2013 at 22:05
  • \$\begingroup\$ before his edit he didn't specify a battery voltage plus, the generator at slow speeds may not produce greater than a couple of volts - what happens then? \$\endgroup\$
    – Andy aka
    Oct 19, 2013 at 22:26

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