Ok, general electronics question here.

I want to build a mini mini wind turbine using a 6-9V motor as the turbine and a 9V rechargeable battery to store the electricity…this is purely as a demonstration of processes rather then a real application.

Now in my very limited experience, i guess i could get away with just using a diode if i wanted to go barebones and ignore the overcharging issue.

But thought it may be nice to include something like a led as a dump load?

Any chance of some help with what would make a good simple charge controller circuit for this tiny 9V set-up?

this is purely as a demonstration of processes

Well, unless one of those “processes” you want to demonstrate is “cooking batteries”, some sort of load dump or disconnection would be a good idea ;D

The general rule of thumb is that you can safely overcharge nicads or NiMH at .1C with no damage: the battery will dissipate the extra energy as heat with no harm. Since most 9V rechargeables are rated at about 150mAH, you can skip doing active charge control if the source only supplies about 15mA. This is not true of Lithium-based batteries: you have to be very extremely careful with those.

Most “9V” nominal batteries are actually 6-cell packs (you have to shop carefully to find the few 8-cell ones available), so your cutoff will probably be around 8.4V (some good info in this Wikipedia article and this page about how to decide when to stop the charging).

The LED load dump sounds like a neat idea: not only does it protect the battery, it gives you a “charged” indicator and some debugging info as a freebie.


The further Ran’s point, add a zener diode in series with the LED to obtain the voltage cutoff you’re looking for. Match the total voltage of the LED and zener and dump current to about what you think your charge conditions might be. Use a low value current limit resistor on the LED, like about 10-68 ohms to buffer the surges.

If the LED draws 20mA at 2.7 volts, then put a zener around 5.7 volts in there so the LED is at full brightness right around the full charge voltage on the cells. When charging stops, the LED will bring the cells down a little and then shut off. Put a few LEDs in parallel if you think your charge current could be over 20 or 40 mA. Or use a mid-high power LED. :wink:

Watch the wattage on the zener diode. It will be burning off a bit of energy. 5.7 Volts at 50 mA → .285 watts, so you’ll need a 1/2 or 1/3 watt zener diode.

any chance of a quick little hand drawn diagram? :-*

Here you go:

These numbers are based on the assumption you have an 8.4V nominal battery and won’t exceed 20mA charge rate. You’ll need to modify accordingly for your real values.

Thanks koyaanisqatsi, your an absolute star!

Just wanted to check that I was visualizing it right!!