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535i Andrew

Charging another 12V battery from your car battery

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In theory, if you wanted to charge up say a leisure battery from your car battery, is it just as simple as wiring a split charge relay between the car battery and the one you are charging up.  That way the leisure battery only gets a charge when the relay senses the increase in voltage from the alternator in the car and is separated from the car battery so it doesn't contribute during starting the car engine.

 

Essentially how is a charging circuit wired up in a camper van with an engine or car battery and leisure battery for operating lights etc in the 'living area'.

 

Didn't some E34's have two batteries in them?

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You can charge several batteries at the same time just by wiring them up in parallel. I've been doing it a lot on an electric bike project i'm playing around with that needs 4 x 12v batteries. 
 

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That's only ok if the batteries are of a similar discharge, which they would be in your case as they all see the same load. The problem with doing that with a leisure battery is if you discharge the leisure battery doing whatever you want with it and then put it on the car unless you immediately go for a drive and charge it back up to 12v it will drag the charge out of the cars battery so in the morning you have two half discharged batterys.

   

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1 hour ago, 711jrp said:

That's only ok if the batteries are of a similar discharge, which they would be in your case as they all see the same load. The problem with doing that with a leisure battery is if you discharge the leisure battery doing whatever you want with it and then put it on the car unless you immediately go for a drive and charge it back up to 12v it will drag the charge out of the cars battery so in the morning you have two half discharged batterys.

   


Valid

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