What size battery bank?

[alibro]

Hi Guys

I am finally getting to the stage of erecting my wind turbine. It is a 24V 10 footer built from old Volvo bits just like the early DanB turbines but I havn't got the batteries yet.

Can anyone tell me is there an optimum size of battery bank for this turbine? I am hoping for around 500W - 600W in a strong wind and 100W - 200W in lighter winds but haven't tested it yet. I know the correct way to work out battery bank size is to first calculate power usage and get sufficient batteries to match, but at this stage I don't know how much power I will be using. I intend to power part of my house from it through an inverter so I will probably use all the power available.

Is it possible to connect too large a battery bank so that the wind turbine cannot charge it properly, or too small so that it will be damaged by the turbine?

I don't want to spend hundreds of pounds on batteries only to find I have too many.

Thanks in advance

Alister

[Flux]

If you go too small you restrict the power that you can use without cycling the batteries too deeply. If you restrict the discharge and use a charge control you should not actually damage them.

It is possible to have too much capacity and in periods of low wind you may not be able to keep them charged enough to avoid sulphating.

Nearly impossible to give much guidance without knowing your power in and power out.

I would imagine that a reasonable starting point may be about 500Ah. Initial cost generally tends to prevent most people having too much capacity.

Flux

[AbyssUnderground]

Is it possible to connect too large a battery bank so that the wind turbine cannot charge it properly, or too small so that it will be damaged by the turbine?

Yes this is possible, but if you use the correct charge controllers etc it won't overcharge if its too small. However it is possible for it to be too big so it won't charge properly, something which no charge controller can avoid.

Until you know for definate what you will be using on your system its hard to say what size battery bank to go for. Other factors also come into effect such as how far down do you want to run it? Most would say 50% as an absolute maximum. Also how much backup do you want incase there is no wind for days on end? 2days, 3 days, a week?

We can't read your memory so we need much more information before we can help you any further.

[alibro]

Thanks guys

I know I am being vague about power usage but I have to admit I still have not yet worked out how much of the house I will be running off the inverter. Initially I will be using a 1500VA UPS as I got it free but will upgrade to a purpose made inverter when funds allow (yes I know it is not the most efficient but as I have one I will use it). I intend to wire it into the house in such a way that if we have a period of calm weather I can throw a switch to put the circuit back onto grid mains. If all goes well I can then set it up to do this automatically but it means I am more concerned with matching the battery bank to the power output of the turbine than how long the batteries will provide power.

I guess I was wondering if there is a way to mathmatically calculate the ideal size of battery bank for a given power out from a wind turbine.

If you say 500Ah is a good starting point Flux then I will go with that.

Cheers Guys

Alister

[fungus]

A way I have heard to size a battery bank is to have a charging source at around C/20; the input power in A should be around 1/20th of the battery capacity. This should really only be a starting point however, it depends on how much it will be used. 500ah is a good figure, Hugh Piggott says for his 8' gen to use a minimum of 400ah IIRC.

[dinges]

Correctly sizing a battery bank takes a little more care than that, Angus.

Consider the battery to be a vat of water that gets filled at occasions, and gets drained at other occasions or continuously.

The trick now is to have such a size vat that it never goes below 50% full. If it gets constantly filled up you can use a small vat, if it gets constantly drained and only once a week filled up you need a larger vat. Etc.

The water analogy is a simple but illustrative one.

The correct vat size (battery bank size) is also influenced by backup charging measures, like a gasoline powered genny when there's no sun or wind.

So, in short, one needs to make an analysis of the situation. Normal usage, the situation it will be used in, etcetera. General rule of thumbs like C/20 have only one advantage; that they're simple...

[Bushwhacker]

Not meaning to hijack the thread but I'm leaning towards a couple (one to start and add on) of used forklift batteries. They are cheap if you can find a warehouse getting rid of them and the capacity is huge. When the wind blows fill the batteries up and when it's calm drain them down.



Please feel free to shoot holes in my theory, I'm am newbie and have much to learn.



Cheers,BW

[Jason Wilkinson]

"They are cheap if you can find a warehouse getting rid of them and the capacity is huge. "

Two things there i love  ie "cheap"  and "getting rid of" is anything good if ur getting rid of it?