voltage reduction

[tailwind]

I have  tried to post 5 times with no results. My computer skills are very limited so please bear with me. How can I get 12v off my 24v battery bank? Can anyone tell me how to build a dump load for my Xantrex C40 that is one order? I have 2 wind turbines up and running but find my mechanic skills far exceed my electrical and computer skills Tailwind

[joestue]

edit: will comment later.

[Flux]

If your 12v requirement is modest the best way is a 24/12v dc/dc converter. These get expensive if you need much over 200W.

The old trick of tapping the battery is not a good one and best avoided except in an emergency.

For the dump load the simplest way is to buy power resistors, either wire wound of the metal clad ones that need a heat sink.

If you are skilful you can build power resistors from various heating elements but joining nicrome wire is not easy.

How much you need to dump depends on your input supply but you will not need above 40A with that controller so at 24v you will need at most resistors that will make 0.75 ohms in parallel. Worst dissipation is about 1200W total. With small wind turbines you may not need to cater for the full 40A dump.

Flux

[tecker]

If you need amperage get two 12 volt batteries and tap one for your 12 volts .12 and -12 is usefull in computer applications also

[tecker]

If you have this down ignore this followup

[SparWeb]

Tecker,
Without knowing too much about the OP's needs, that may be OK or it may be a bad thing to do on a long-term basis or for high-power.

You essentially have two parallel strings in a 24V bank in your drawing.  Tapping half-way into the left side, as you show it, gets him the 12V he needs, but discharging that cell alone imbalances it against the others in the set.  That would reduce the life of the entire bank.  You've also missed a "sneak circuit" through the 24V cells which would charge the lower 12V cell.  Total imbalance.

I would do it in an emergency, as Flux puts it.  Have done it, in fact.  But I'd never make permanent wiring like that.


Tailwind,
If you really want to switch between 24V and 12V, often but not permanently, then it is possible to set up battery switches in a way that disconnects 12V batteries from being in series (where 12+12=24) and then switches them into parallel (all + terminals are together and all - terminals are together).  This takes some "electrickery" to accomplish, and there are some "thou shalt not" rules to mind.  Not for the faint of heart, so let's only talk further about it if you are prepared to take the crash-course in power wiring.

Perhaps a better starting point would be to ask: What system voltage do you really want to set up, 12 or 24?  What are the turbines wired for, or are they adjustable?  Will you be using an inverter?

[Mary B]

DC to DC converters are readily available. I use a 30 amp one to power my ham radio gear. Samlex makes high quality ones that are used in emergency response vehicles quite a bit.

[tailwind]

Here we go again. I had this message all typed and it erased when I tried to post. 'Thank you for the quick reply. First, my situation has changed since my last posting. Went out this AM and found one of my towers leaning about25 degrees. Too much rain, not enough concrete. Lowered tower, no damage. My friend is coming this AM with his backhoe. I have a 10ft and an 8ft. Homebrew design wind generators. I'm wired for 24v but want to use some 12v to run a small refrigerator that I don't need just to use some of this juice I'm creating. I think I'll use Mary Alana's suggestion for v reduction. I do use a 24v inverter but really don't need it since I'm on the grid. I always seem to be over or under v on my battery bank. Where can I buy resisters of the type Flux suggest?  Thanks,Tailwind

[Flux]

If your refrigerator is the type that runs on mains, gas or 12v it will be 12v only. If it is the compressor type most of these will run on 12 or 24v so it could be worth checking.

I am not in the best position to advise on suppliers in your country, others should be able to help more but I believe RS have an outlet in some form. Farnell also do ( I think it may be Newark Components). You should also have Mouser and Digikey. Almost certainly renewable energy supply people should be able to get you something.

If you choose the metal clad type it usually works out cheaper to use several smaller ones than go for the big wattage ones. Here the 150 W ones usually work out the cheapest and you can heat sink them easier than the big expensive ones.

Flux

[tanner0441]

Hi

Flux is right never tap your 24V bank at 12V or any other voltage for that matter.  If you need occasional heavy loads as in motor starting etc, a 24/12 inverter floating a separate 12 V battery, I used to use that system on boats to run fridges, radios, nav equipment etc. Even a 10 or 20 Ah battery close to the point of use will take the sudden load from your inverter.

Here in the UK 24/12V inverters are available from most good truck and bus ancillary suppliers. Or the internet but a supplier will most likely only sell equipment they use and found to be reliable.

Brian

[kitestrings]

never tap your 24V bank at 12V or any other voltage for that matter

You can, however, do this with an equalizer.  It's not ideal, but quite common when you have 12V needs on a higher nominal voltage bank.  We've done this for years because we started with 12V and later upgraded to a higher voltage.  You can tap the center of a 24V bank, then have one of these balance between the two halves:

http://www.solarconverters.com/index.php/products/82-battery-equalizer/158-eq-12-24-20

Wired 24V in, and 12V out, which is possible, you are limited to the output of the converter, but with a "tap" the battery can cover larger momentary loads (starting a motor), and the equalizer takes a little longer but works to balance the two "halves".

Here's a couple possible sources for resistors.  I go to Allied or Newark Electronics, but you have to know what you need.

http://www.backwoodssolar.com/
http://www.altestore.com/store/Charge-Controllers/Dump-Loads-Dump-Load-Controllers/c439/


~kitestrings

[tecker]

12 volt batteries have lower impedance than the 24 volt battery (he dosn't say much about the 24 volt battery) . I have always tapped my 48 volt bank with no problems 24 volt for fans and 12 volts for lighting . No charging problems.  I always have a marine battery or two on the system no problems in 8 yr life of the Duracell Procell 6 volt( Batterys Plus ) brand

[tecker]

It is a good thing to tap with another storage that equals the voltage of your load to maintain the charge distribution . It forces a regular  visit to the  bank to  make checks and watch your loads.
Dc to dc and other electronics are just like wallworts a constant drain and more low voltage connections to monitor for me .Also a static hazard and more fuses in the system .