I got some semi-old UPS batteries in stock and hooked them up to my Eaton S3 700VA to test it out, and it seems to work pretty well: charge, discharge and feedback from the UPS to the UPS-software (using nut), all seems fine, got longer battery runtime in my test-setup :-)

Any advice on NOT to connect a bigger battery to your UPS?

It's understood:

  • In case of multiple batteries, must use parallel connection.
  • Don't overdo it, don't hook up a 200A battery to a 7A UPS
  • Voltage should of course be the same
  • Secure and well-sized wiring (my test-setup is NOT well sized, I know)
  • Don't draw more power from the UPS than it's designed for, only do this to get longer runtimer, NOT higher watt support.

My 7A stock-battery for this Eaton UPS need replacement, why not get a 12A battery instead (I'm okay with the battery can't fit in the cabinet).

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  • 3
    \$\begingroup\$ This is a case where, as long as the device is stupid, everything will work great; any potential problems come from it trying to be clever :) \$\endgroup\$
    – hobbs
    Jul 19, 2023 at 18:35
  • 1
    \$\begingroup\$ If your humongous battery is already charged when you connect it, the UPS won’t know the size of it. You may find some edge case where original battery runtime was only a few minutes at max load and by making it much larger, it will thermally protect the inverter eventually which you never saw in original configuration. For more UPS abuse and DIY energy, secondlifestorage forums might be a better place to discuss than here where only specific design questions are answered. \$\endgroup\$
    – winny
    Jul 19, 2023 at 19:31

2 Answers 2


So why not do it? Mostly, safety, and playing with safety is not more fun.

If something happens, like accidental short circuit, and your house burns down, and perhaps someone dies along with it, guess who is going to be responsible, and even if nobody dies, the insurance company might disagree with you how the wiring should have been done by someone with applicable training, experience and permits.

If you need a bigger UPS, buy a bigger UPS, don't play with safety and hack a smaller UPS.

The wiring looks dangerous. No fuses, so a short circuit anywhere will heat up those copper wires red hot, melt insulation and ignite it on fire. Sure it is possible with one battery too, but the original wires are short and have thick insulation and the wires are protected from accidents and movement inside the case so there is a reduced chance of it happening. The more there are batteries, the more there is available short circuit current.

The built in charger is also not rated for charging excessively high capacity batteries. It will take longer to charge them and there may be safety mechanisms to detect that the battery did not charge to given voltage in given time so the charging process might trigger a safety timeout and declare the battery bad and refuse to charge them. If there is any coulomb counter which tries to determine battery state of charge or capacity left before warning about degraded batteries, it will be thrown off by the extra capacity and it may not be possible to calibrate it over the specified ratings.

The inverter is rated to last for rated time with rated load and rated battery capacity. If you have same rated load but higher capacity batteries, the inverter can run longer without battery running out and shutting down. It is not rated for that and might run longer than intended and run hotter and wear out faster.


Aside from what Justme said, you're going to have the usual problem of paralleling batteries which are not matched. The weaker battery can actually drag down the stronger one, for instance.

In some cases, charge electronics depend on the battery being a certain type and characteristic (just try to upgrade a cheapie solar path light from NiCd to NiMH and you'll find out about that). However, lead acid is pretty low-impedance (hence its use in starting cars), so that may not wok with them. I mention that to say that as long as the charging circuit in the Eaton has the ability to limit itself thermally and not overload on a larger battery, I see no reason you couldn't put a 200AH battery on the thing. Well, I suppose if it's coulumb counting, you could overflow the counter :) and get weird telemetry as a result.

Couple of used golf cart batteries for instance - golf cart batteries are considered "dead" when they won't go 18 holes anymore, and that means they still have at least 1/3 of their capacity.


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