# charging two identical batteries from alternator or mains



## stevegos (Jun 23, 2013)

Hi

I'm looking at adding an extra battery to my existing leisure batt.

I know it's best if they are same capacity and type and age etc.

But charging two when the alternator and onboard mains charger is designed for one leisure battery... will this place additional load on the charging system and thus extra strain or is it normally not a problem.

All help greatly appreciated.

Steve


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## rayc (Jun 3, 2008)

*Re: charging two identical batteries from alternator or main*



stevegos said:


> Hi
> 
> I'm looking at adding an extra battery to my existing leisure batt.
> 
> ...


The alternator or on board charger will have a maximum charge rate. All that happens by doubling up on the battery is that they take twice as long to charge as a single one. Obviously you can not just keep on adding more batteries without eventually having to uprate the charger and its wiring to cope.

A lot of MH owners have added a second battery. My Rapido had a second position allocated when it was built. The multi stage charger has a maximum current of 16amps during the constant current phase and it can easily cope with two 110AH batteries. The alternator and the solar panel also appear to cope with the second battery without any perceivable strain.


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## Mrplodd (Mar 4, 2008)

No problem at all, just connect them in parallel with decent sized cable. Many MH's are prewired for two leisure batteries anyway, so it's worth checking first. If it has a Sargent electrical unit it's all prewired, you just need to buy a small wiring harness from the. Dead easy to fit. Go for it!


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## stevegos (Jun 23, 2013)

Thanks for the advice.

My old MH only has space for one battery, but there is space in a rear locker where I'm planning on installing the 2 new batteries and an inverter.


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## Mrplodd (Mar 4, 2008)

Just make sure you use decent sized cables otherwise you will get a large voltage drop over that sort of distance. 

Also keep them as near to the rear axle as possible as they will weigh about 25kg each and you don't want that weight too far back as it will have an impact on your rear axle loading!!!


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## Spacerunner (Mar 18, 2006)

The charger can't tell the difference between one or multiple batteries. It just treats them as a single larger battery.
BTW Halfords have some made up battery leads at various sizes complete with terminal at reasonable prices.


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## Mrplodd (Mar 4, 2008)

Stevegos 

DONT think you will need huge great starter feed sized cables!! You will not be using your leisure batteries for starting your beast will you?

I am not sure what sized cables you will need (someone will come along with that info shortly) but something the size of domestic mains flex should be more than adequate (I think :roll: )


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## crusader (Jul 5, 2012)

if as read he is joining batteries to an inverter he WILL need heavy duty linking wires or when drawing from the inverter the wires will have a huge load on them ( i am summising the inverter will be 1500 to 2000 watt ?) if its a toy inverter then yes thinner wire would be ok , jim


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## spykal (May 9, 2005)

Hi Steve

Here is a handy table of cable sizes from Sterling Power for battery wiring to chargers and inverters.

http://www.sterling-power.com/images/downloads/WHAT CABLE TO USE IN mm sq.pdf

Mike


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## Mrplodd (Mar 4, 2008)

Jim

Good point !! I forgot he was going to connect an inverter to them :roll:


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## melfay (Apr 6, 2014)

*inverters*

powering a domestic fridge with an inverter that is higher wattage than the fridge can it be done


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## listerdiesel (Aug 3, 2012)

Yes, as long as the inverter can handle high inductive loads, many cannot.

As far as battery interconnect cables go, 35mm or 50mm welding cable is as good as it gets.

25mm is as small as I'd want to see, but it does depend on the battery size and AH.

Peter


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