Battery bank in my motorhome

Discussion in 'The Projects Forum' started by Jonat127, Apr 27, 2015.

  1. Jonat127

    Thread Starter New Member

    Apr 27, 2015
    3
    0
    Hello,
    I want to install a bank of batteries in my motorhome to provide about 300-350Ah of capacity. I intend to use it to power a 350W aircon unit via an inverter. I want to be able to run the cooler for up to four hours so by my calculations, the above should be ok.
    AC unit - 350W @ 240V
    Inverter @ say 75% efficiency = 467W
    Therefore draw from battery = 38.9A say 40A
    40A for 4hours = 160A which is about 50% of the total 350Ah capacity.

    So, does that look right?

    Next, I have a question about the battery bank.
    The motorhome has installed as standard an 80Ah leisure battery all wired in to the vehicle alternator. In order to get to my 350Ah target would it be best to install a single 250Ah leisure battery (wired in parallel with the original of course) or three 80Ah leisure batteries again in parallel with the original? (I know this gives 320Ah total not 350 but that is double the 160A requirement.)

    Hope that all makes sense.
    Thanks in advance for your advice.
    Jon
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    One thing to keep in mind is the lead-acid batteries have a normal discharge rate at 20 hours, so if you take the capacity(350/20)=17.5 amps. If your discharge them quicker than the 20 hours they will not have the full capacity. If you were to dischare the 350AH at 350amps it would last about thirty minutes. There is a website that has much more information about batteries, search for battery university.
     
  3. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Your math is good.
    This forum does not allow discussions about automotive modifications, so I can't tell you to explore the ways in which the, "leisure" battery is protected or kept separate from the engine starter battery. I can say that playing mix & match with batteries causes difficulties, so if you can't get the size you want in one battery, buy them all the same size and same age. Really, age does matter about getting them to charge and discharge in an orderly manner.
     
  4. Jonat127

    Thread Starter New Member

    Apr 27, 2015
    3
    0
    The leisure battery circuit is good and separate from starter. I know this because current leisure battery has been flattened more than once, oops!
    So, regarding age and condition of existing leisure, it is three years old and, as I say, has had an "active" life. Therefore, it should definitely be replaced right? Even tho it still works ok.
     
  5. Jonat127

    Thread Starter New Member

    Apr 27, 2015
    3
    0
    Regarding capacity, thanks for the lead k7elp60. The max space I have will limit me to 5 batteries. I have found some that quote capacity at 5hrs and 20hrs so now I know what that means and what I am aiming for. Thanks again
    Jon
     
  6. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    It certainly won't make a matched pair with a new battery, no matter what size you buy!
    Maybe you can get a couple of bucks for the old one from somebody with a trolling motor on their tiny boat...
     
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