# Boosting very low voltages for battery charging.

#### Dodgy Geezer

Joined Nov 30, 2009
177
This is another example of 'not wanting to start from here' - treat it as a theoretical exercise if you will, but the fixed doll's house application I'm starting with can't change, so there is little point in suggesting that...

We are looking at a (very) small wind turbine generator which will be spinning a brushless motor from a disk drive. Such a set-up will probably spin at 300rpm for much of the time, and this will generate a voltage of perhaps 0.5v at perhaps 5mA. So...0.0025VA. And I would like to make that do some useful work - perhaps charge a battery (which is 12v).

I had thought of taking A/C off the motor into a transformer (probably small audio) to bring it up to 12v, with a rectifier on the output which would give DC, and stop the battery charge leaking away. But I see various clever circuits around on the Web - which I suspect won't work with such a low input voltage. Has anyone got any better ideas? Apart from giving up, that is....

P.S.
I've just spun one of the motors, giving 0.7v at 100mA. A bit better than I thought, but still pretty minimal for Power Electronics...

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#### LowQCab

Joined Nov 6, 2012
4,061
Since Diode Drops are everywhere,
the Transformer would appear to be the only viable answer at first glance.

You will need at least 14-Volts to actually "charge" a 12-V SLA Battery.
Assuming zero losses, the Transformer will have to convert the
0.7-V into 14-V,
14 / 0.7 = 20, = 1 to 20 Transformer-Turns-Ratio,
of course you must also divide the available Current by 20 now,
which would be,
0.100 / 20 = 0.005 Amps, or, 5-ma .
Which means .......
Starting out with a small SLA Battery with "zero" Internal-Leakage-Resistance,
taking it from Dead, to Fully Charged, would probably take around a full Month,
that is, if it didn't sulfate all the Lead-Plates in the first week of being Dead.

You might have better luck trying to charge a string of "Super-Capacitors",
that would give you the best chance of actually collecting a little bit of useful energy,
which might be enough to light a small LED, maybe even 2 or 4 !!!
The LEDs can also perform the job of Rectification after the Transformer.

#### Papabravo

Joined Feb 24, 2006
21,225
The endless supply of sisyphean tasks boggles the Western imagination. This might be the closest approach to futility that I have seen in a very long tome. Knock yourself out with this one.

#### Irving

Joined Jan 30, 2016
3,878
If it's a brushless motor presumably it has 3 phases? Were you measuring across 2 of the 3? You may get more output using all 3 phases, but the losses are likely to overwhelm any useful output!

#### BobaMosfet

Joined Jul 1, 2009
2,113
This is another example of 'not wanting to start from here' - treat it as a theoretical exercise if you will, but the fixed doll's house application I'm starting with can't change, so there is little point in suggesting that...

We are looking at a (very) small wind turbine generator which will be spinning a brushless motor from a disk drive. Such a set-up will probably spin at 300rpm for much of the time, and this will generate a voltage of perhaps 0.5v at perhaps 5mA. So...0.0025VA. And I would like to make that do some useful work - perhaps charge a battery (which is 12v).

I had thought of taking A/C off the motor into a transformer (probably small audio) to bring it up to 12v, with a rectifier on the output which would give DC, and stop the battery charge leaking away. But I see various clever circuits around on the Web - which I suspect won't work with such a low input voltage. Has anyone got any better ideas? Apart from giving up, that is....

P.S.
I've just spun one of the motors, giving 0.7v at 100mA. A bit better than I thought, but still pretty minimal for Power Electronics...
Voltage and Current are inversely proportional- Meaning, they have a ratiometric relationship. You can't get both at the same time, you get one from the other. Since both you have are low, you're more or less stuck.

#### Dodgy Geezer

Joined Nov 30, 2009
177
Ah... I should have said that it's a 1.3Ah battery - but even so that would be 10 days to charge. I was hoping that it would simply keep the battery topped up after it had been charged, assuming that it would not be drained too often during play. Even so, this assumes 100% efficiency and ignores the diode losses in the rectifier.

Irving is right about the phases - that may give me a little more current to play with. But that would be dependent on a decent low-loss 3-phase rectifier. and there won't be many of them about!

Nevertheless, it seems on the edge of possibility. If I can get a low-loss rectifier of some kind, ditto for the transformer, i may be in with a chance. Though finding small 100mA 20:1 transformers is a bit tricky - I had thought of going for audio output ones, but they all seem to be 1:1....

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#### Dodgy Geezer

Joined Nov 30, 2009
177
The endless supply of sisyphean tasks boggles the Western imagination. This might be the closest approach to futility that I have seen in a very long tome. Knock yourself out with this one.
I'm guessing that you don't have a small daughter who is into renewable energy...?

#### Papabravo

Joined Feb 24, 2006
21,225
I'm guessing that you don't have a small daughter who is into renewable energy...?
That would be accurate. I do have two grown up daughters who are doing fine. I did teach the younger one to drive my car in a parking lot as part of a science experiment when she was 13. Life is good.

#### LowQCab

Joined Nov 6, 2012
4,061
You're a brave man !!!

#### MrChips

Joined Oct 2, 2009
30,794
My 6-month old granddaughter getting an early start!

#### Papabravo

Joined Feb 24, 2006
21,225
You're a brave man !!!
She'll be 39 in October, and she is an excellent driver despite nearly giving me an MI during the science project.

#### Dodgy Geezer

Joined Nov 30, 2009
177
Schottky diodes for the rectifier, but has anyone any recommendations for a suitable transformer?

#### Irving

Joined Jan 30, 2016
3,878
If you can use all 3 phases and can get to the common connection, you may be able to use 3 very low forward voltage schottky diodes to create a 3-phase rectifier with a smoothhing capacitor from the junction of the three cathodes to the common line. If you can get >0.5v then a TI TPS61200 low input boost converter will give you 1.8v or more output.

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#### Deleted member 115935

Joined Dec 31, 1969
0
This is another example of 'not wanting to start from here' - treat it as a theoretical exercise if you will, but the fixed doll's house application I'm starting with can't change, so there is little point in suggesting that...

We are looking at a (very) small wind turbine generator which will be spinning a brushless motor from a disk drive. Such a set-up will probably spin at 300rpm for much of the time, and this will generate a voltage of perhaps 0.5v at perhaps 5mA. So...0.0025VA. And I would like to make that do some useful work - perhaps charge a battery (which is 12v).

I had thought of taking A/C off the motor into a transformer (probably small audio) to bring it up to 12v, with a rectifier on the output which would give DC, and stop the battery charge leaking away. But I see various clever circuits around on the Web - which I suspect won't work with such a low input voltage. Has anyone got any better ideas? Apart from giving up, that is....

P.S.
I've just spun one of the motors, giving 0.7v at 100mA. A bit better than I thought, but still pretty minimal for Power Electronics...

what a fantastic project

#### Dodgy Geezer

Joined Nov 30, 2009
177

#### Dodgy Geezer

Joined Nov 30, 2009
177
Since I need three - 1 per phase - I was hoping to get some rather cheaply...

#### BobaMosfet

Joined Jul 1, 2009
2,113
Ah... I should have said that it's a 1.3Ah battery - but even so that would be 10 days to charge. I was hoping that it would simply keep the battery topped up after it had been charged, assuming that it would not be drained too often during play. Even so, this assumes 100% efficiency and ignores the diode losses in the rectifier.

Irving is right about the phases - that may give me a little more current to play with. But that would be dependent on a decent low-loss 3-phase rectifier. and there won't be many of them about!

Nevertheless, it seems on the edge of possibility. If I can get a low-loss rectifier of some kind, ditto for the transformer, i may be in with a chance. Though finding small 100mA 20:1 transformers is a bit tricky - I had thought of going for audio output ones, but they all seem to be 1:1....
A 1.3Ah battery means it's rated to deliver up to 316uA per second.

#### KeithWalker

Joined Jul 10, 2017
3,090
Have you considered rewinding small audio transformers to get the turns ratio you want? It's not as difficult as it first appears. The tricky bit is getting the interleaved core laminations out without damaging anything, You know the original turns ratio, so count the turns as you take off the outer winding and rewind it using thinner wire, with the number of turns you need.
If you use three transformers with one connected across each phase, you can wire the secondaries in delta to a three phase Schottky bridge rectifier. That way, you will get the most available energy from the generator.

NOTE: When I was a project engineer, working for Hewlette Packard, the sign on my desk stated: "Miracles performed while you wait. The impossible takes a little longer." I never turned down a project. They were all successful and completed on time and within budget, So don't let people tell you your project is not possible, or a waste of time. Take their comments as a challenge, and prove them wrong.