UPS battery replacement

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.
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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.
-Z
eric wrote:

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Thanks for the advice! Eric

blackouts.
perhaps
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Dumb question I'm sure but other than an industrial grade UPS that supports everything on an entire office floor, what small UPS today costs more than a few hundred dollars, and that's for a rather large one.
On another plane, you idea of substituting a large capacity battery for the small one in your UPS should function just fine, although it will take considerable more time to reacharge after use. Just be sure to use a battery of the same technology, that is a 12V leadd acid battery for a smaller 12V lead acid battery.
It you try to substitute say a lead acid battery for a NC or NIMH cell, then you likely will have large problems since the charging characteristics of these differing technology battery types are relatively unique.
If you select a lead acid battery, be sure to distinguish those designed to serice a low-discharge situation from that that of a deep-cycle variety. Were I doing this, I would chose a large capacity, deep cycle marine service battery, such as those sold by Sears in contrast to a car battery.
Just my thoughts on the subject.
Harry C.
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eric wrote:

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

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.
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I agree that some of the small UPS heatsink might not be large enough to work for more than 5 minutes (which is the maximum battery runtime for the little 12V 4AH battery, so little heatsink is OK as long as a little battery is used).
With larger battery (although, I would not recommend going larger than 12V, 15AH battery capacity - it will still give you 1 hour + backup for a typical PC), you might have to piggy back a heftier heatsink to the orgininal heatsink if transistor failure due to overheating is to be avoided.
Also, after 2 days of battery charging, make sure that battery voltage is within 13 to 14 volts. Under 13 volt means battery is under-charged (which is normally OK - you just get less minutes of discharge time) but over 14 volt means overcharge (you get lot of battery runtime, but lifetime of your battery will be shortened). But biggest danger with wet lead-acid battery (regular car battery) is that oxygen & hydrogen is produced by overcharging (you'll see bubbles rising from battery acid). So in a tightly sealed room, dangerous concentration of these gases means an explosion is possible when a spark or fire is created.
For this reason, I suggest you get "sealed lead-acid battery" which is no larger than 12V, 15AH. It does not produce dangerous gases & its smaller size means only limited amount of gas production is possible even if the safety seal on the battery is defective and outgassing does take place accidentally.
Safety should always be #1 consideration in electric/electronic appliance.
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