# Wind-powered fairy lights + battery charger: preliminary help

I'm new here and asking for help designing a yard-art project. I would like to use a small DC motor as a wind-powered generator to charge a battery (with overcharge prevention) and run some battery operated fairy lights. I'm looking for autonomous function (no manual input). I would like it to be lit any time the wind is blowing or the battery is charged, and continue to be lit from the battery when there is no wind and the battery has remaining charge (obviously not lit when the battery is drained and there is no wind).

I was thinking along the lines of the following. Once I know the basic design, I will look for the correct components, so appreciate any preliminary advice you may have!

simulate this circuit – Schematic created using CircuitLab

My thinking is as follows:
V1 is the DC motor used as a generator
D1 and D2 are the fairy lights (2 small strands if I can)
D3 is meant to prevent battery drain when the wind is not blowing
D4 and D5 are meant to prevent overcharging of the battery
Bat1 is the battery

I probably also need a resistor to regulate current charging the battery, and a whole lot of calculations to 'right-size' the components, but at this stage I am looking for help with the general concept so I can start the process of looking at the components.

Thank you in advance, and apologies if anything (or everything) above doesn't make sense.

• My answer is a "starter" - it can be extended as required depending on your requirements. Adding extra information in your question is useful. Aug 24 '20 at 1:13
• Please ask a specific question, edit your question, get it reopened Aug 24 '20 at 20:15
• Any progress to report ? Sep 12 '20 at 12:09

The trick to integrating this is to understand impedances of all sources and voltages under all conditions,

The generator had a DCR value and open circuit voltage Voc and short circuit current Isc if driven by a motor. Yet wind coupled to blades is more of a current source so the maximal power will vary from 72 to 85% of Voc. Since generator voltage is proportional to RPM minus conduction losses from DCR, you need to determine avg min wind speed to capture most of the time, or have a current to constant voltage converter that always matches wind impedance and thus generator impedance. Since this varies widely the challenge is to capture low speed power most of the time and deal with excess energy at higher winds.

This demands the use of an algorithm to match source to load.

The load is a nearly constant battery load Vc with ESR that defines the current limit from a voltage source but in series with the high impedance current sourcing generator makes the regulation of the generator and design of blades and RPM critical.

 The analogy is choosing a motor boat prop for speed or pulling 2 water skiers.
In either case you want to match impedances for maximum power transfer.


You can use a suitable tungsten lamp to dump excess wind energy with a voltage comparator to Vmax with a FET switch so that it regulates Vmax.

The LED impedance becomes a linear series incremental resistance above when then become dim. say at 9V if a 12V string and increases power with the square of the current rise *R.

So you see none of the impedances match, which demands some smart control system to match impedances. DCDC converters do not do this. But MPPT converters do on the input side but not necessarily on the output side unless designed for a certain chemistry.

Your first challenge is to define these power, voltage and impedance variables with a wind profile then choose the generator. a high RPM turbine works well into some while high power types use fixed RPM with variable blade angles and rotor azimuth.

A small buck converter with a >3:1 input range with a matched generator voltage at mean range of winds might be desirable. Consider an aeroplane rooftop with a wind direction rotor/generator that runs at high RPM to a matched power kV/RPM motor to generate Vbat to >3x Vbat with a buck regulated CV float battery charger. You want to work at average daytime windspeed and use a light diode sensor to activate the LED driver at twilight and switch off at low voltage 11.5 for lead acid 12V.

• Thanks Tony! I appreciate the rapid response. I will need to educate myself on some of the considerations you mention. I am not so worried about efficiency but rather that it simply lights up if there is a decent wind (I will be making a small, 3 cup anemometer to drive the generator) and probably use NiMH rather than lead acid battery for this small application Aug 24 '20 at 17:46
• You must prevent battery under/ overvoltage yet have sufficient, so test Voc/Ioc briefly at threshold (decent wind and high )speeds ... as it will stop on short circuit , so it must be measured quickly then report back Aug 24 '20 at 20:45
• Thanks Tony. Seems like average wind speed is around 5 mph with monthly average peak Around 10 mph, which seems quite low compared to our not-infrequent afternoon winds of 25+ mph. For measuring Voc/Ioc I will need to partly assemble,so will post after that is done and tested Aug 27 '20 at 14:30
• sorry, mis-spoke about wind speeds. Monthly average: 8.4-10 mph, monthly max average 14.2-18 mph Aug 27 '20 at 16:23

The concept is good - the details need work.
You know that :-).

Maximum charge voltage:

If using NimH batteries, which are rated at a nominal 1.2V, charge voltage for full charge is in the 1.4V - 1.45V range at charge rates of up to around C/10 (mA = mAh/10).
Using two diodes as shown as a 'clamp' will reduce capacity stored to a small total of full capacity. Better is to use a well defined clamp such as a TL431 "adjustable zener" - plus an extra clamp transistor if power levels warrant. Details of this and other options can be provided once the other details are sorted out.

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Here is a TL431 + PFET clamp regulator circuit - from my SE EE answer here.
R2 can be a wire link.
For currents below TL:431 rated Imax remove Q1 and short R2 & R1.
To use a small pnp bipolar transistor in place of Q1 use R1 = 10k, R2 = 1K.

For LiIon adjuist Vclamp to 4.0V/cell or slightly LESS.

Or

from here

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LEDs require a minimum forward voltage (Vf) to operate. Red requires about 2V,

This table is 'somewhat generalised' but gives an idea.

Modern white LEDs typically need 3V or more. If you have control electronics then you also need some "headroom voltage". If you are operating white LEDs directly from battery then 3 x NimH will work acceptably and 4 is probably better.

An alternative is to use a single cell Lithium Ion (LiIon) battery - such as are used in almost all cell phones. These are readily available and even a well used one may meet your need. The key thing about LiIon is that they MUST be charged in such a way that they are not rapidly destroyed. This is easily enough achieved but must be done.

Alternatior / Generator:

A small brushed DC motor rated at 6V or more will probably meet the need.
Small stepper motors also can work well - but may have somewhat more startup torque.

Small brushless DC motors can work very well. Surplus low voltage DC fans form PCs etc are a good source - but the internal electronics need to be removed or bypassed. That's not hard and there is much on web re doing this. More on that if required.

The stepper motor and brushless motor solutions will usually have longer lifetimes as there is no brush wear.

DC-DC inverter?

If you want to use a single NimH cell then a DC-DC step up inverter will allow LEDs to be driven from a 1v - 1.2V source. These can be built 'from scratch' but salvaging one from a single cell lawn light is a low to no cost option. Lawn lights may die for a range of reason so a dead one may be the source of a suitable inverter.

Wind turbine mechanical aspects are not hard when power levels are this low and efficiency is not very important. Ask if needed.

See what you think of the above and ask questions and the answer can be expanded to suit.