Habitation battery changeover switch

Discussion in 'Motorhome Chat' started by Touchwood, Aug 10, 2015.

  1. Touchwood

    Touchwood Funster

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    When I bought my MH there were two habitation batteries fitted, a large 110 Ah in the external locker and a smaller one under the side bench. The smaller one was not connected; the connections are there but not hooked on to the battery terminals. There is also a switch fitted next to the smaller battery and wired in somehow, but it looks to me like an ordinary household light switch. I asked the previous owner what the intended setup was, but he was pretty vague about it, and given that some of his "modifications" have proved a bit iffy I'm planning to ignore what he had in mind.

    What I want to achieve seems quite simple to me; I want the option to switch to the smaller battery if the larger one is drained by extended use. This isn't going to happen much, if at all, as we are always on hookup, but it would be nice to have the backup just in case.

    As far as I can see all I need is to wire both batteries in, with a simple switch to isolate one and put the other in line, and vice versa. Am I right in this? and what about the switch itself? I feel an ordinary household light switch isn't the right choice! I have searched this forum for advice but the only threads I can find get far too complicated with talk of split charging, relays etc., I'm just a simple mechanical engineer!
     
  2. Flamenca

    Flamenca Funster Life Member

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    A lot of boats use a 3 position rotary switch labelled Battery 1/Off/Battery 2 which would fulfil the purpose of selecting the battery you are using. However, how do you propose connecting them when charging off the engine alternator?
     
  3. Flamenca

    Flamenca Funster Life Member

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  4. TheBig1

    TheBig1 Funster Life Member

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    the switching currents and a domestic light switch are not compatible, unless of course you use a relay. you also need to consider how the normally disconnected battery will charge
     
  5. Doctor Dave

    Doctor Dave Funster

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    You would be better off having the two batteries permanently connected in parallel.

    Dave
     
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  6. Touchwood

    Touchwood Funster

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    I thought this wasn't a good idea if the batteries are different capacities?
     
  7. Derbyshire wanderer

    Derbyshire wanderer Funster Life Member

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    They must be matched the same though or one will ruin the other. This would be far easier for charging and will double the capacity so much less chance of actually running out if the single battery now used normally lasts you.
     
  8. Flamenca

    Flamenca Funster Life Member

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    Not a problem in practice. I had twelve standard 110Ah lead acid batteries connected in parallel on a solar powered system and the batteries lasted 10 years.
     
  9. TheBig1

    TheBig1 Funster Life Member

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    different capacities would be fine connected in Parallel, but they must both be same age and condition as if either fails it will kill the other
     
  10. Doctor Dave

    Doctor Dave Funster

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  11. Touchwood

    Touchwood Funster

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    A three position switch sounds ideal. Presumably when charging off the engine alternator whichever battery is "on line" will charge - this may seem less than ideal as of course the other won't, but bearing in mind this is only an emergency backup set up I don't see a problem - if I have to switch batteries whilst off hook up I'll just have to remember to switch back again when I'm running the engine to charge the primary battery, then at some stage switch to the secondary to re-charge that, either from engine or EHU.

    Dave - I really don't want to wire the batteries in parallel, as the smaller battery is quite old and I'm worried it could fail with the possibility of draining the principal battery. If I didn't already have the smaller battery installed I wouldn't bother.

    Anyone any ideas on where to purchase a three position switch? Preferably cheap and small so as to be unobtrusive when fitted.
     
  12. Derbyshire wanderer

    Derbyshire wanderer Funster Life Member

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    Doh, not thinking before typing. They only need to match if connected in series. Parallel connection only needs the voltage to match.
    You could just wire it permanently to the bigger battery and not need a switch or need to swop over for charging it.
     
  13. icantremember

    icantremember Funster

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    I have two identical batteries with a four way changeover switch, bat1 , bat2, both and off.
    The only reason I have the switch is so either battery can be isolated without losing 12v habitation supply but is normally on both.
     
  14. Techno

    Techno Funster Life Member

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    Last edited: Aug 10, 2015
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  15. mick noe

    mick noe Funster

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    use a marine battery changover switch as @icantremember is using then choose which battery to use/charge and then no problems with miss matched batts. Bought mine on ebay about £25. (don't forget to use heavy interconnecting cables)
     
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  16. Teuchter

    Teuchter Funster

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    Use a 4 position s/w as described by Flamenca - switch to "both" position when driving or on hook uyps (this way both batteries will charge in parallel, this way you always start with 2 fully charged batteries. When pitched without hook ups then switch to 1 and have 2 fully charged in reserve. This is how I do it on my boat :)
     
  17. Techno

    Techno Funster Life Member

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    As per my link earlier
     
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  18. icantremember

    icantremember Funster

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