12vdc and 24vdc at the Same Time . . . Is this Possible ?

[fabian1956]

Hi All,

I have a question which I haven't been able to find an answer to.

Currently I have two separate banks of 6x2v batteries plus an engine start battery. They are all AGM, 12 Superstore 275amp 2v and a Fiamm 12v 100amp as an engine start battery.

Everything is organized so the engine starting battery is isolated except when a paralleling switch is evoked (for alternator charging only). The two house banks are paralleled into a main system distribution buzz bar and are independently switchable.

Charging is by 2 x 120 watt Kyocera panels and a Blue Sky 3024i controller. Everything works well at 12v except that when the weather is not solar friendly we have to run a genset to boost our batteries. A 2kva Kippor is fine but noisy and obtrusive. It is also an overkill for our Xantrex 40amp charger and an inefficient charging solution.

Enter a new Ampair 300 24v Wind generator capable of beefing up our power when the solar isn’t working (mostly bad weather cloudy and windy and at the time we want the most power). The Solar controller has an upgrade available for it to add a wind generator to the system at the controller level. Me thinks a very good thing!

As power cable losses are lower at 24v and everything at the charging level can work at 24v in particular the Ampair 300 is it viable to series the house battery banks for charging only and then draw out the supply power at 12v from each of the banks as is happening now? Is this likely to cause any problems or can it dangerous for any reason? Is there any extra equipment needed to effect this set? Does anyone have a setup like this?

Thanks in advance for the replys,

Fabian

[htraudes]

"two separate banks of 6x2v batteries plus an engine start battery. They are all AGM, 12 Superstore 275amp 2v and a Fiamm 12v 100amp as an engine start battery."

6x2V=12V, but 12x2=24V. diagram would help to clarify setup of 12V arrangement.
BTW: where you write "amp" you mean "amp.hour"?

Challenge 1: Charge 12V arrangement with 24V generator in combination with two other existing generators that instead are 12V.
Challenge 2: To uninterruptedly be able to draw 12V from this arrangement while charging, including from the wind generator.

Not simple. Wind generator produces 24V DC? If AC it could be transformed down. Can maybe rectifier circuit inside generator be bypassed and then the output of 24V AC transformed to 12V AC be rectified to 12V DC? Most wind generator do not produce that many Watts, thus only a few amps, so cable loss is negligable and 2:1 transformer can be small.
Bringing 24V DC down to 12V DC with better than 90% efficiency is pain.

[djmarchand]

Fabian:

How about this approach:

Install a double throw switch on the input side to your Morningstar controller, so you can switch from the wind generator to the solar panels. The controller, if it is a MPPT type, will "transform" the 24 V of the wind generator to the 12 V required for the batteries.

It might be possible to wire both the wind generator and the solar panels to the controller without the switch. When the wind generator is putting out at night for example, the controller will adjust to the 24V inut to maintain maximum power. During the day if the wind generator isn't putting out anything, the controller will adjust to the solar panel's lower voltage. This is all theoretical and I would be much happier with the switch.

But check your controller spec. Make sure that it is an MPPT and that its maximum input voltage is above your wind generator's output.

David

[Andina Marie]

When charging at 24 volts it becomes very important that all the cells are matched for capacity. The charging current has to pass through EVERY cell so the one with least capacity will be fully charged before the others and from then on it is being overcharged as the others finish charging. This is OK for lead acid batteries, less advisable for AGM and destructive for gel style batteries. If they are (more or less) matched they will all reach full charge about the same time and won't destroy each other.

[centralyachts]

What you don't say Fabian is whether you are running a 12v or 24v (or combination of both) system on your boat. I assume 12v, hence your question.
My advice is to sell the 24v wind gen on Ebay and buy a 12v.
You will never be able successfully to 'split' the 24v down to 2 x 12v supplies to charge your system, although you could try using a Redarc charge splitter or similar.
As Adnina outlines, you run the risk of one half of the 12v batteries getting completely 'out of sync' with the other. With flooded cell batteries this can be fixed with an equalisation cycle from your AC charger. With gel / AGm it's goodbye to your batteries.

[fabian1956]

Hi All,

Thank you all for the input.

An electrical engineer finally put me straight on how this should be set up and this is what we will be doing shortly.

The wind generator is an Ampair 300 tuned to 24v and will be connected directly to our main Buzz Bars for input charging The load will draw the voltage back to what the batteries can handle until they achieve a high state of charge. This buzz bar set connects the solar, 240v and wind generator charging to our batteries. Our solar panels will be reset in series to output 24v into a Blue Sky 3024i controller with a Duo upgrade fitted to it. This effectively adds a monitoring circuit to the solar controller to sense the state of charge of the batteries. When the batteries achieve a fully charged status a switch automatically engages a power dump from the batteries to something useful like a water heater or a watermaker or we can as we closely monitor our state of charge turn on our 240v inverter and bake some bread or something that will chew up the excess power being generated. Or we can simply shut down the wind generator until the batteries are in need of another boost. If we are not aboard for some time we will choose this option as part of our lock the boat up process.

When the sun is shining with clear skies we usually are quite fine power wise it is only when the weather is overcast or rainy that we suffer from a lack of power right at the time when we would use our laptops or watch a movie or the like... then we struggle with power. At present the gen-set goes on and the tranquility of the setting is destroyed. The Ampair 300 is quiet as in no noise and will solve the problems unless there is no sun and no wind which in our part of the world is rare.

I'll post the results when the job is finished and I've had a chance to assess its success.

In the meantime the correct answer to the original question about running the two 12v battery banks in series for charging while simultaneously drawing 12v from each bank was a definite "don't do it" from the engineer as this has all sorts of potential problems associated with it.

My next post will be a while as it will take a bit of time to make the genny riser pole and install everything between the important jobs like fishing and sailing in our summer season.

Cheers all,

Fabian

[Andina Marie]

Why on earth would you want to switch all your charging to 24 volts on a 12 volt boat? At best you are going to save 1% of power lost in the resistance of the wiring. To save that 1% you are going to use a 24 volt wind generator at 12 volt currents which will be very inefficient. The windings in a 24 volt generator are only half the gauge of a 12 volt one so it will be a mis-match to the load. The output current capacity will still be only the rated current for a 24 volt load and the rest will be lost in heating in the generator windings or the regulator. A 12 volt generator for the same size will put out TWICE as much charging current and you don't have to mess with the problems introduced by 24 volt charging such as detroying your batteries the first time you forget to dump excess power.

Take the earlier advise and sell the 24 volt generator on Ebay and buy a 12 volt one. Leave the Solar in parallel, they will also be more efficient at 12 volts since again the load wil match their design operating voltage. Loading each 12 volt panel down to 6 volts and running the current through BOTH panels will seriously reduce your output power.

Keep it simple. You are inviting all sorts of problems, wasting power, risking destroying batteries in an effort to fit one square peg in a bunch of round holes. Don't change the holes, get rid of the peg.