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Thread: Trailer power - one BIG battery or two small ones?

  1. #1

    Default Trailer power - one BIG battery or two small ones?

    I have one of the Sears Platinum (Odessy) group 31 batteries in my 80 as the aux battery. It's a real monster. I now am ready to buy the battery for my new trailer build and can't decide:

    Group 31 - $265, 75lbs, 205 min reserve capacity

    Group 34 - $205, 53lbs, 135 min reserve capacity

    The trailer will have the typical assortment of lights plus a Norcold 60 fridge and probably a TV/DVD player and stereo. I don't want to worry about running out of power in a couple of days though I do have a Honda 1000 generator if needed to top off batteries. I was thinking either two of the Group 34 size or one of the Group 31 big ones.

    Your thoughts?
    97 FZJ80. Lifted, locked, and ready to explore.

    Offroad teardrop build here.

  2. #2
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    If the battery has a reserve capacity rating instead of an amp hours rating, it's almost certainly a thin plate starting battery rather than a thick plate deep cycle battery. If it's a thin plate, then draining it deeply will cause it to lose storage capacity (sulfated) faster than a thick plate battery and it won't last as long (less total discharge/recharge cycles).

    With lead-acid batteries, the life of the battery is directly related to the depth of discharge. For example you can see in this graph from Panasonic, that if you take their lead-acid batteries down to only 30% DoD, you'll get around 1200 cycles, but if you take them down 100%, you'll only get 200 cycles:





    So, to supply a given load of X, a bigger battery will be drained by a lower percentage, and would last for more cycles. I.e., To supply a 50ah load, a 100ah battery would be drained 50%, whereas a 200ah battery would only be drained 25%. The bigger battery will last longer (more cycles before it's toast).


    Of course, if you use a thin plate starting battery in a thick plate deep cycle application - you aren't gonna get nearly as many cycles anyway.
    Last edited by dwh; 03-29-2011 at 12:33 PM.
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    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
    Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker

  3. #3
    That makes sense about the discharge or two vs one. The battery in question is an AGM though not a lead acid, so I don't know if that matters.

    Also, I've heard that AGM batteries can be mounted on their sides - anyone know if that's true?
    97 FZJ80. Lifted, locked, and ready to explore.

    Offroad teardrop build here.

  4. #4
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    From what I've read about AGM is due to how they are constructed, they don't vent, and they are air tight, and even if the casing is cracked, they won't leak.
    I'm no expert, so take this with a grain of salt, but I'd say they could be mounted in any configuration you want.
    I wish I could remember where I left my keys...

  5. #5
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    Two batteries is better than one. Try to never use more than 50% of the available power or you'll shorten the battery's life.

    Yes, AGM batteries can be stored in any orientation - they are sealed.

    With the TV/DVD in your mix of appliances, I would say you want two of the big Group 31's if you can afford it and handle the weight/space. That would be the best of all worlds - lots of capacity and cheaper per ah cost.

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    Quote Originally Posted by adventurebuddies View Post
    The battery in question is an AGM though not a lead acid, so I don't know if that matters.
    It doesn't have any effect on the dicharge depth/life cycle equation. That is common to all batteries. The type of battery has more effect though. Thick plate deep cycles have higher numbers verse starting batteries. A starting battery can die off after a surprisingly low number of deep cycles.

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    Quote Originally Posted by adventurebuddies View Post
    That makes sense about the discharge or two vs one. The battery in question is an AGM though not a lead acid, so I don't know if that matters.
    AGM stands for Absorbed Glass Mat. It has lead plates and uses an acid/water mix as electrolyte. AGM -is- a lead-acid battery. So is GEL.

    AGM has fiberglass mats between the lead plates, which absorb and hold the electrolyte. Also, the cell caps have a catlayst which recombines the hydrogen and oxygen separated during charging. They are sealed.

    Gel batteries are also lead-acid, but instead of glass mats, they use an agent which "gels" the electrolyte. These also are sealed.

    FLA stands for Flooded Lead Acid, and is a normal type battery with electrolyte that sloshes around. Some have removable caps and some are sealed and have catalyst cell caps.


    Also, I've heard that AGM batteries can be mounted on their sides - anyone know if that's true?
    Most can, a few can't - check the manufacturer's specs to be sure. Quite a few Gels can be mounted in any position, even upside down, but usually only the aviation AGMs are approved for upside down use.
    ...
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    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
    Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker

  8. #8
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    Quote Originally Posted by The Adam Blaster View Post
    From what I've read about AGM is due to how they are constructed, they don't vent, and they are air tight, and even if the casing is cracked, they won't leak.
    Yes, that's true with one exception - they WILL vent if they are overcharged sufficiently to build up enough pressure to blow the seal.

    Sealed batteries are called VRLA - Valve Regulated Lead Acid. AGMs and GELs are VRLA, and flooded batteries which are sealed are also VRLA.

    All VRLA (sealed) batteries have a pop-off valve (like an air compressor or water heater) that will vent excess pressure if overcharged far enough.
    ...
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    Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
    Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker

  9. #9
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    Is the valve a one-time use item?
    What I mean is, is the valve a pure safety feature in the low percentage event that too much pressure builds up internally, that trips the safety valve, and then the valve has to be replaced?
    Or is it reusable/permanent mechanism affixed to the battery case?
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  10. #10
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    I believe one large battery is a better setup than two batteries each of which has half the capacity. Two 12v batteries in parallel will usually not give you twice the capacity because of parasitic discharge.

    Some will say that two batteries gives you a backup in case one fails. That's the only potential advantage in my view. Still, my Horizon and my Bigfoot both have a 4D size AGM in them.
    Last edited by cnynrat; 03-30-2011 at 11:22 PM.

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