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Using a bigger battery for a computer UPS?

  1. Sep 3, 2008 #1
    Hi guys,

    I m considering changing the battery to a bigger one so as to increase the backup duration.

    The battery that came with it is 12V 7.5 Ah. The battery I m planning to put is a 12 V 100 Ah. Now my question is, will the big battery increase the standby duration without overloading and damaging the hardware in the UPS.And will it charge the bigger battery?

    I have an APC Back-UPS ES 500 VA,300 Watts.And having the battery externally is what I had in mind.Thoughts please ?
  2. jcsd
  3. Sep 3, 2008 #2


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    Yes it's a common way of making a decent backup power. Take the electronics from a UPS and hook up a whole bunch of cheap car batteries in parallel.

    A couple of important points - the battery supplied with the UPS is probably a deep discharge gel battery, it can be discharged to within say 10-25% of empty without damage. A car battery can only be discharged to 50-60% repeatedly.
    The charging rate will be optomised for the sealed batteries but an unsealed car battery can cope with a faster rate. If you are using an unsealed battery make sure it is well ventilated - explosive hydrogen is given off when it charges,
  4. Sep 3, 2008 #3
    what exactly do you mean discharge to 10-25 % without damage and 60 % in case of a car battery.CAn you elaborate on that if you dont mind ?

    When I go for multiple batteries, will the input voltage double in both paralell and serial connections assuming I connect two batteries in this case ?Will this also increase the watts that the UPS can handle ?
  5. Sep 3, 2008 #4
    Car batteries are not good for medium or low current applications where the current is drawn over a long period of time. They can suffer damage if discharged too deeply. They are really for starting engines. Leisure batteries (for caravans - trailers) are more suitable.

    High current batteries are pretty dangerous. If you get a short circuit a heck of big current can flow and there may be a blinding flash like an arc welder. You MUST have proper fusing and/or a circuit breaker. A larger battery might demand a hefty charging current and blow a charger which has been designed for a smaller battery.
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