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24v start up with a 12v leisure bank

3760 Views 8 Replies 8 Participants Last post by  CliveMott
Hi all.
I'm having a problem finding the solution to my problem.

My van runs on 24volt. Alternator to 2x 12v batteries wired as 24v.
In the back, I have 2x 200ah sealed gel leisure batteries.

What I am trying to work out is how to send a 12v charge from the 24v batteries whilst I am driving, so that the leisures are fully charged upon reaching my destination.

I do have some solar that I am going to connect to the leisures but that doesn't answer my question. I think it is a waste of good energy to not charge the leisures from the alternator whilst driving as the weather in Britain is very un-reliable, as everybody is well aware.

I can't seem to find the part nor the solution.

I feel the equation is something like:
alternator-> 24v batteries-> (?)-> 12v leisure batteries

can anybody put an answer to my question mark!

Thanks in advance!

1 - 9 of 9 Posts
You can get 24V to 12V downconverters.

Lorry drivers use them.

Would have to be a chunky one to charge two batteries though.

Too late tonight to do the research for you.
im not a auto-electrician, but if you take a feed from the center of the two van batterys that should be twelve volts

or look here24volt to 12volt
I seem to remember that Mercs that size had 24 v starters but 12 v electrics. Strange I know but worth checking. :?
For normal leisure use your two 12v batteries are connected in parallel. I think by use of a suitably rated double pole switch you could alter the wiring for them to be in series i.e. a 24v battery which could then be charge from the vehicle alternator. The switch would also isolate the 24v feed from the vehicle alternator whilst in leisure mode, and isolate the 12v wiring for the leisure circuits when in charging mode.
Hi iamere

Try searching for DC-DC converter on Google. You need a 24V input (capable of taking up to 32V maximum) and 13.8V output to charge the leisure batteries. It also must have electronic current limiting for use as a battery charger (some converters are only suitable for use as power supplies, i.e. powering a load such as a CB radio).

A typical suitable one is here:

I should declare my interest in this converter in that I designed it (I don't have any financial interest though).

You have got 400Ah of leisure batteries, so if you charge them at 15A whilst travelling for, say, 6 hours then it will still only put 90Ah maximum into the batteries. It won't be cheap to get a useful size of converter.

Don't tap off the mid-point of the vehicle's 24V battery as this would unbalance it - the top 12V would over-charge and the bottom 12V would under-charge. Could be very dangerous.

Hope this helps

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The older mercs have the batterys under the drivers seat, you simply take a feed from one batteries + & - pole to give yourself 12 volts. I would get it done properly from an experieced electrician though.

Kees suggestion is a good one but you need to add a split charge relay to disconnect the input to the converter so that you don,t continue running the converter charging your 12 volt bank when the engine is not running.

Alternatively fit a 12 volt alternator to the engine bay. If your van does not have aircon then there is probably a place where it fits and this is normally ideal for a second alternator.

I have 5 X 14 volt 90 amp brand new Bosch alternators lurking in my garage if you are tempted to do some engineering.

One word of warning, you should NOT take a tapped 12 volt feed off the 24 volt starter battery bank because this will unbalance them and one will end up undercharged and the other overcharged.

1 - 9 of 9 Posts
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