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Pitfalls on substituting larger Ah battery pack (with same voltage) on UPS?

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TheMG:

--- Quote from: sibeen on July 30, 2020, 01:16:32 am ---Many years ago I worked on a UPS system that was kept in a room that was constantly at around 50°C and the batteries wouldn't last a year.

--- End quote ---

I've seen something like that before. Someone had the bright idea to put a network rack (with UPS of course) in a boiler room!!! :palm:

duak:
Actually, I did the same thing.  I've got an older little APC unit (200 W maybe) to run a router and maybe a laptop.  The powerco scheduled a day long outage so I paralled a charged battery of about 3 times the AH to extend the time.  It worked so well I just left it.  After a few months, I noticed the UPS would randomly shut off without any power disturbance.  I also noticed the UPS was quite hot and so was the external battery.  Out of the unit, the internal battery was down to about 8 V and wouldn't accept a charge.  It was 3 years old and perhaps on the edge of old age (or maybe C19 got it).  Perhaps the heat killed it but good.  I haven't got a new battery or perform a post mortem to see if the UPS still works.  I can't remember what shape the external battery was in - it was over ten years old anyway.

Bottom line: check out the charger to see if it can safely supply the charging current and doesn't overheat.  Sometimes assumptions apply to every case, but sometimes they don't.

sibeen:
One other caveat I'd add about adding extra battery capacity is that on some of the APC models, especially the cheaper ones, the inverter was only sized to run the load for the short time that the original battery was rated for; so the thermal capacity of the inverter was designed to run from 7 to 10 minutes at full load. The thermal capacity was crap in other words and the heat sinks on the switching devices were the cheapest possible. I'd give it a test and monitor the temperature on the output transistors and perhaps bulk up the heat sinks if approaching a problem temperature.

Circlotron:

--- Quote from: sibeen on July 30, 2020, 01:41:33 am ---One other caveat I'd add about adding extra battery capacity is that on some of the APC models, especially the cheaper ones, the inverter was only sized to run the load for the short time that the original battery was rated for; so the thermal capacity of the inverter was designed to run from 7 to 10 minutes at full load. The thermal capacity was crap in other words and the heat sinks on the switching devices were the cheapest possible. I'd give it a test and monitor the temperature on the output transistors and perhaps bulk up the heat sinks if approaching a problem temperature.

--- End quote ---
Yep, that, also the thermal mass of the inverter transformer may only be enough to supply full load until the original battery quits. If that is the case, a big battery will kill it. Kinda like the tyres on a Bugatti Veyron will only last 15 minutes at full noise, but the fuel will only last 12 minutes.  :-DD

Another thing to consider is electrical safety. How good is the isolation between mains and the battery terminals? When the battery was completely inside the UPS nobody could touch it, so isolation could afford to be so-so. When outside it is a different set of requirements.

james_s:

--- Quote from: Circlotron on July 30, 2020, 02:43:05 am ---Yep, that, also the thermal mass of the inverter transformer may only be enough to supply full load until the original battery quits. If that is the case, a big battery will kill it. Kinda like the tyres on a Bugatti Veyron will only last 15 minutes at full noise, but the fuel will only last 12 minutes.  :-DD

Another thing to consider is electrical safety. How good is the isolation between mains and the battery terminals? When the battery was completely inside the UPS nobody could touch it, so isolation could afford to be so-so. When outside it is a different set of requirements.

--- End quote ---

As long as you run it well below the max rating it should be ok. I would recommend doing that anyway, the max rated capacity of consumer grade UPS's is absolutely brutal on the batteries. For example I have a little 550VA/300W unit that takes a 5Ah battery. I tested it with a 150W incandescent load and it pulled around 11A from the battery and that increased to 15A by the time the battery was nearly depleted. At the full 300W that's around 22-30A from a 5Ah battery, abusing a SLA battery with a continuous 4C-6C load will kill it after just a handful of cycles. I try to size a UPS for around a 30 minute run time unless there's a backup generator and the UPS only needs to carry the load long enough to get that running or just long enough to shut down the PC.

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