New Battery cables in parallel help!!

Discussion in 'Motorhome Accessories' started by gazza280, Jan 11, 2014.

  1. gazza280


    Sep 26, 2012

    I have just bought 2 new 110amh batteries to replace the one 5 year old battery that was in the van when I bought it, because I am going to France in a few weeks so want to make sure the van is tiptop.
    My understanding is not to use the new batteries with the old battery because the new 110amh batteries will be affected by the old battery, is that right?
    I researched to see if it was a straight forward swap. Take old battery out put new battery in, then put 2nd battery in parallel to give me more 12v power. I know positive to positive negative to negative is parallel but I have just been told by my ex trucker brother in law that although the cables are rated as 110amh I should replace them with cables rated above 220amh to accommodate 2x 110amh = 220amh.
    Now I don't think thats correct, but if it is will I have to replace all the cables and the split charger in the van to 220amh.
    Also I want to use the old battery in the van too with a separate charger thats plugged into a separate 240v plug so it only gets charged when on hookup. For charging laptop and phones and using a 300w inverter. The thinking behind this is that the old battery is still ok to use so use it without affecting the new batteries.
    Does this sound too complicated , is there a better use for the old battery?

    It goes without saying I am a novice with electrics. So any help is appreciated.

    Many thanks Gary
  2. funflair

    funflair Funster Life Member

    Dec 11, 2013
    Hi Gary

    Your cables off the battery/batteries should be fused for protection. As long as you are not using any more power than before there should be no need to upgrade cables, the extra ah will just mean that your batteries last longer. If you were running a big as in 2000 watt inverter you might need to look at the cable sizes. I would leave the old battery at home if you are going to have 2x110ah.

  3. Serendipitous

    Serendipitous Funster

    Aug 4, 2012
    Came across this info that may be useful.

    Would however recommend that you look at the current fuse size between your old battery and if it is say 50 amps then connect the 2 batteries together and to your system using 2 off 25 amp fuses.

    Sorry reading that it sounds a bit confused in practice each battery has a 25 amp fuse that connects it to the same place as your old battery fuse did.

    The advantage is that fault current between batteries is limited. Similarly fault current available to the old system is the same as the original.

    There are several posts on the subject if you google something like "connecting leisure batteries in parallel"
  4. SomeoneElse

    SomeoneElse Funster

    Apr 22, 2013
    No need to change the cable size as you will be using the electricity at the same rate. Your battery power will last twice as long.

    Its the same principal as water storage, if you doubled the size of your fresh water tank you wouldn't need to double the size of your pump pipes and taps. You would however be able to go twice as long between fill ups.

    If you were going to add a larger inverter then thats a different matter entirely.

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  5. peterc10

    peterc10 Funster

    Jul 5, 2013
    Tunbridge Wells
    Your brother in law is confusing the measurement of the total amount of energy the battery can store with the actual energy that flows from the battery. The former (110Ah) is a function of the type and size of battery you buy. The latter is a function of the number and type of loads you put on the circuit that joins the positive and negative terminals. If all you have is one 3W LED bulb connected then the current will only be 0.25A and you can use thin wiring (depending upon the length of course). But if you have a lot of equipment running off it and especially if you have an inverter, then you will have a lot more current potentially and therefore will need much thicker wire.
  6. hilldweller

    hilldweller Funster Life Member

    Dec 5, 2008
    Battery 3.

    Seems a lot of hassle for an old battery, tying it down, wiring a new charger. Extra weight.

    Sod's law says it will fail as soon as you start using it.

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