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Forum Index : Windmills : how much voltage is too much?

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adric22
Regular Member

Joined: 06/08/2008
Location: United States
Posts: 47

Posted: 02:27pm 17 Aug 2008

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As far as charging a 12V battery goes, is it possible to give too much voltage? In designing my wind turbine I've realized the most challanging part is going to be to get enough voltage out of the thing. But what if I end up with too much? Lets say I've got 20 volts or more coming out of it? Will it charge less efficiently than with say 15 volts at a higher amperage?

martinjsto

Senior Member

Joined: 09/10/2007
Location: Australia
Posts: 149

Posted: 03:30pm 17 Aug 2008

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hi adric22, the more volts you produce the more charge you will get.
the F&P can produce high voltage or low voltage depending on your wiring configuration you choose.
see
http://www.thebackshed.com/windmill/FPRewire.asp

if for example your wind charger produces 48 volts at 200 rpm connected to a 12v battery the battery will only see 12v but at 4 amps.
that's how i understand it anyway, i am by far no expert on electronics, only a novice learning lots as i go,
obviously if your batteries are fully charged then any volts above 12v will be trying to charge the batteries and therefore a potential to boil them rendering them useless. that's why we use a dump charge controller to dump excess power into a load IE a 12v spotlight not s aolar controler which would just switch off the charge leaving your mill to spin unloaded untill destruction.
hope this helps
martin

free power for all
McAlinden WA

KiwiJohn
Guru

Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 07:10pm 17 Aug 2008

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What is needed for charging a battery is current, a higher voltage will push a higher current but as martinjsto points out pushing current through a charged battery will damage the battery.

The total power that the mill can produce for a given wind condition can be expressed in watts, which is current multiplied by volts. So you can see that to make best use of the power available you want the voltage that produces the highest current but is still above the present voltage of the battery.

It is always a compromise, as far as I can see.

Gill

Senior Member

Joined: 11/11/2006
Location: Australia
Posts: 669

Posted: 10:51am 18 Aug 2008

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G'day adrick22,
Yes, you are using good reasoning there and are asking good questions. The answers to your questions are the foundation of cutting edge development in the DIY Wind gen area today.

Firstly you ask

adric22 said is it possible to give too much voltage?

. So we'll have to say yes to this as a battery could not sustain the voltage of say a lightning strike. It would arc and melt everything obviously. We don't really worry too much with this max upper voltage level because of the reasons raised in your more pertinent second question.

adric22 said Lets say I've got 20 volts or more coming out of it? Will it charge less efficiently than with say 15 volts at a higher amperage?

Now here you have implied a fixed power as I note the comparison of a high voltage compared to a lower voltage with the corresponding higher current. Good question.

A little exercise disregarding diode losses, line losses etc. etc.

Into a pure resistive load:
Generator output = 200 Watts = 20 Volts @ 10 Amps
or
Generator output = 200 Watts = 15 Volts @ 13.3 Amps

Now when this is used to charge a battery, the battery clamps the voltage low. So lets use a typical battery under charge voltage of 13.8 Volts and see the actual power the battery receives from a 200 Watt generator.

13.8 Volts @ 10 Amps = 138 Watts into Battery
or
13.8 Volts @ 13.3 Amps = 184 Watts into Battery

Clearly the second puts more power into the battery.

"So what happens to the 62 Watts or 16 Watts that disappear between the generator and battery" you may ask?
A. It is lost as heat in the stator and rectifier.

For you the DIY wind gen designer, need seek a Voltage high enough to cut-in and charge the battery at low rpm and balance this with the need for higher current(at lower Voltage) once past cut-in.
This compromise is influenced by the available wind conditions at your site.

For advanced designers there are options such as Star Delta switching and current developments in MPPT(maximum power point tracking)to manipulate the voltage lower in exchange for higher current as applicable to the gens rpm. MPPT has additional features to Star Delta switching, but that's another story.

So I hope that's added to Martin and John's good replies.

was working fine... til the smoke got out.
Cheers Gill _Cairns, FNQ

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