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care & feeding of deep cycle batteries

Discussion in 'Electronic Basics' started by [email protected], May 21, 2007.

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  1. Guest

    I just bought a 12V, 75 A-h deep cycle/RV battery from Wal-Mart ($45 +
    $9 core fee), with the intention of turning it into a portable
    "generator" (via 75W 12VDC -> 110VAC inverter) for overnight use, when
    we go on a long road trip / camping.

    I have a 3A load at 12VDC (two CPAP machines at 1.5A each) that I
    would like to maintain overnight (at least 8 hours), preferably
    lasting for at least two days before charging is necessary (at a
    motel, say).

    I also bought an el cheapo Black & Decker 2A "charge maintainer" /
    battery charger for about $17. I assumed that pretty much the only
    difference between this and a ~6A regular battery charger is the speed
    it takes to charge a battery. (At 2A, it should take a day and a half
    to fill up my 75 A-h battery.) Was my assumption correct? The
    charger box did say it could be used for charging deep-cycle
    batteries.

    Any comments on using a deep cycle battery in this manner?

    The best part about shopping at Wal-Mart is the very convenient return
    policy. I'd tried using a $15 motorcycle battery, hoping it would put
    out enough juice; alas, after only 7 hours (at 1.5A), my CPAP and
    inverter shut down from lack of juice. So, to the Wal-Mart return
    line I went. "Anything wrong with it?" the sales lady asked. "No, it
    was just too small." End of questioning; a refund was issued (along
    with the $9 core charge, yay!). So, if I goof, I can always return
    everything and walk away.

    Thanks,

    Michael
     
  2. Guest

    Hi Michael
    You should only ever use the top 50% of the capacity of any lead acid
    battery. Totally flattening it is very damaging, and you might find
    you only get a few full discharges. Most apparatus has a cutout when
    the voltage dips too low to protect the batteries.
    For the load you are wanting, you should triple the capacity of your
    battery. Remember to charge as soon as possible after discharge,
    otherwise irreversible sulfation will rob you of future capacity.
    You might just get away with buying another 75Ah and using one for
    each machine. If it fails half way through your third sleep, is it a
    life threatening matter? jack
     
  3. Guest

    I'd get a little bit better charger too, one that senses the battery
    voltage, which climbs during charging and then levels off at the
    point where things should stop. The good chargers sense this point
    and it helps prevent overcharging, which helps extend battery life.
    I agree that you need 3-4 times the capacity to avoid a deep discharge
    too.

    Steve Roberts
     
  4. Guest


    Ok, thanks for the information. I may well just spring for the 110 A-
    h battery; it was $15 more. Big n heavy, though.

    Not really a life threatening manner if a CPAP fails; dad and I just
    feel a bit more sleepy in the morning, that's all.

    Michael
     
  5. Guest

    I was going to suggest a bigger (faster) charger.
    You could reasonably go to 7.5A (~10% of the capacity).
    Wet cell lead acids do alright with a simple voltage charger and the
    max current limited by the size of the charger. They actually benefit
    from a bit of overharging. Stirs up the electrolyte. Be careful that
    you turn the charger off before you take the clips off the battery, or
    you might end up with bits of battery embedded in you with a sulfuric
    acid douch to sterilise the wounds :) jack
     
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