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UPS battery replacement

Discussion in 'Electrical Engineering' started by eric, Jan 16, 2004.

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  1. eric

    eric Guest

    Hi experts. I own an old UPS power back-up for my PC. I'd like to
    substitute its little 12V 4A battery with a heftier battery (such as a
    sealed car battery) in order to extend power availability during blackouts.
    In my area, these blackouts last for about an hour or two at a time, perhaps
    once a month.

    I don't need quick recharging. Trickle is fine, so I'm thinking that my
    little UPS charger should work.

    Do you think I could simply wire the new battery in to the UPS's battery
    terminal connectors? Or will my idea not work due to increased "draw" on
    the UPS charger?

    If it isn't obvious, my idea is to save a couple of thousand $ on the
    alternative, which is to buy a large UPS.

    Thanks for the advice.
  2. Guest

    When you change battery types, you need to ensure that
    your charger will charge the new battery properly. The
    UPS may contain a "smart" charger that would keep a
    gel cell in good shape, but would not keep a car battery
    properly charged. If the charge circuit is strictly a
    trickle charger, and you can adjust the charge voltage
    to whatever the new battery calls for, you should be
    fine. I think you could change battery types and get
    no indication of a problem, but nevertheless fail to
    charge the new battery properly with the result being
    a decrease in the life expectancy of the new battery.

    It's not a case of increased draw on the charger - it's
    a case of the UPS charge parameters not matching the
    correct charge parameters for the new battery.
  3. John Gilmer

    John Gilmer Guest

    Well, you can always "piggyback" a small trickle charger for the automobile

    Well, these cheap UPS system really weren't designed to operate
    indefinitely. I get if you put a much larger capacity batting on line you
    might find that you start to have failures induced by excess heat.

    At some point it starts to make sense to just get a box that is designed to
    be connected with BIG batteries.
  4. eric

    eric Guest

    Thanks for the advice!
  5. Some small UPS systems use a type of heat sink on their switching
    transistors (the primary source of heat) that is designed to keep the
    parts cool through sheer thermal mass, rather than radiate the heat into
    the air. Thus, they are not rated for continuous operation.

    Think holding a lighter up to a large block of steel and putting your
    hand on the top. The block will stay cool to the touch for a few
    minutes, but eventually will heat up enough that you will have to remove
    your finger. The sheer thermal mass of the block keeps the component
    (your finger) cool.

    These heatsinks are usually rated for the length of time that the
    battery built into the UPS would last. Substituting a larger battery can
    result in a run time long enough for the thermal-mass heatsink to warm
    up past safe operating temperature, possibly causing UPS failure or even
    a fire.

    Higher end UPS's (usually 1000VA and up) tend to have cooling fans and
    can operate continuously, but I recommend against doing this with a
    small 200 or 400VA consumer unit. There's usually no way to find out
    what type of cooling system the inverter uses or how it's rated.

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