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  #1  
Old 21 Jun 2011
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Series and Paralell Batteries

Hi Guys,

Getting to the stage of fitting internal electrics of our truck project, and wanted to know if anybody can see a problem with the following.

Truck cranking batteries are of course wired in series. 24V.

I want to have a 12v system for leisure system, mainly because 12v components are slightly cheaper and easier to come by.

I will buy a 24v input/12v output split charge (Durite do a good one, no need to fiddle with alternater circuit, its just connects from one set of batteries to the next).

Question is, could I maintain 2 leisure batteries at the same time by wiring them in paralell?

Would there be any problems with that? This way not only can I have 12v leisure electrics, but also, if needed, jump the 24v starting batteries off the 2 leisure ones.

Main thought is, if the split charger churns out 12v, but I have 2x12v batteries hungry for juice, will it just take longer for them to get charged up as both will be drawing current? Is there anything obvious I am not thinking of by doing this?

Any thoughts much appreciated, thanks in advance.
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  #2  
Old 21 Jun 2011
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It comes down to amps - ie. how many does the durite device kick out

Sterling do a 24 volt - 12 volt battery to battery charger rated at only 25 amps

So if you have bank of 2 100 a/h batteries each 50% full then you have to supply 100 amps so the sterling would take 4 hours I think

The durite I found was only 20 amps

It will do 2 batteries cos a battery is actually a battery of cells in series, you are just adding another in parallel

Of course you must buy 2 identical batteries and if one fails then replace both as they need to be matched well

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  #3  
Old 22 Jun 2011
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Thanks for the thoughts Rich.

Will be running 6 internal lights, a waeco fridge, and an inverter off the leisure batteries.

I will seldom be laid up (ie engine not running) for more than 3 days at a time, so they should always get a regular charge up.

But take your point, maybe better to get a 24v to 24v split charger, and wire the leisure batteries in series as well.

Food for thought, thanks again!
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Old 23 Jun 2011
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I think it is a bad idea to run them in parallel.

If one deteriorates slightly it will absorb power from the other, plus a fault anywhere will flatten both. I would run the fridge off one and the lights off the other but have some means of easily changing them in case it becomes necessary to re power the fridge.
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Old 27 Jun 2011
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So lets get this right - so you'd charge them as a 24 volt bank (ie. in series)

But when parked up you'd use them as parallel (as a 12v bank)

You'd have to have an isolator switch so they wern't both at the same time i think

I'd be inclined to run everything off 24 volts to keep it simple and then step down for the odd 12 volt accessory you want to run (ie. get a step down device - don't just hook up to 1 battery otherwise you'll get weird flows of current between the 2 and probabily horten the life of both)
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  #6  
Old 27 Jun 2011
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I think the Keep It Simple rules apply: As the truck is providing 24v, minimising electronic gadgets and making the "house" 24v as well makes sense to me. I know where you're coming from with the price of 12 vs 24 stuff is concerned, and I'm currently (no pun intended) experimenting with a 24 to 12 dropper on a mate's truck but I'm not a fan - constant current draw even under no load and a flimsy looking (but still expensive) bit of hardware being the main focus of my anxiety.
As far as battery banks go, I believe you can have as many as you want in parallel, the voltage across the "final" terminals being the main factor, and the more batteries, the more capacity. (Takes a lot of cable bridging but a 24v bank is obviously possible by joining a group of series wired pairs of batteries in parallel) According to Charlie Sterling, if you want to run a 240v inverter 300W is as much as you'd want on a single 12v battery, but the bigger the bank, the higher the power. Have a look at some of the canal boat systems that have huge banks of cells charged with a combination of photovoltaic cells and the engine (which of course doesn't run for long 'cause everyone's in the pub most of the time in my experience, ahem) to see what's feasible.
Certainly use a dropper rather than just wiring off one battery - it's amazing how even just a car stereo wired like this can destroy the balance and kill one of the two.
Waeco, Dometic and Engel all do 24v fridges, or you could run a domestic fridge off an inverter - certainly the cheapest option but the losses in the inverter probly kill that idea? (I'll get my coat!)
Just reread the first post. No prob wiring the 2 leisure batt's in parallel for 12v, and surely although they might theoretically take longer to charge, they won't have discharged as much in the first place.
OK, now tying my brain in knots. Taxi...?
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Old 28 Jun 2011
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Thanks for all your thoughts.

I've decided to go for 24v leisure batteries in series, so will buy 24v components (Except stereo, which Ive already wired off a 10amp durite voltage dropper)....24v stereos are crazy expensive.

My idea originally was to have 2x12v leisure batteries, wired in paralell, so I can have 12v components, but also in an emergency switch them from paralell to series if I needed to jump start the cranking batteries.

But it's all too much faffing, I'll just have 2 x 12v leisure batteries in series.

Thanks for all your advice again.
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Old 28 Jun 2011
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Not a bad decision as you can travel lighter as you've only got 2 batteries, but your loosing the advantages you've mentioned above.

Another option, is to mount a 12v alternator on your engine, and have it run both alternators. That gives a ton of redundancy as you can then jump start yourself when in deep trouble with a bit of battery swapping.

Merv.
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Old 28 Jun 2011
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Quote:
Originally Posted by Dave The Hat View Post
I've decided to go for 24v leisure batteries in series, so will buy 24v components (Except stereo, which Ive already wired off a 10amp durite voltage dropper)....24v stereos are crazy expensive.
A wise descision IMHO.
It's what I've done on my 24v Landcruiser and it just works.

As for the constant draw from the 24-12v dropper. I've wired a properly rated dash mounted switch on the input to the dropper, so if you're leaving the truck laid up for any length of time, just flick the switch to stop the current draw.

Sam
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