# Can UPS with small internal battery used with large external battery ? what are the issues?

I have APC BX1100C-IN 1100VA 230V Back UPS UPS , there is internal battery in it. Its gives 15 minutes backup in case of power failure.

I am planning to increase the backup time from 15 minutes to 6 hours. By using External Tubular 12 volts 180ah battery.

What are the possible issues in doing so ?

Can UPS be kept online for 6 hours ? will it heat ? or burn ?

Can this UPS be able to charge my 12volts 180ah battery ?

are there any other issues ? in doing so ?

• Who knows? But if you try it, let us know what happens. One thing that might happen is that the UPS may have a timer and may shut down even though there is plenty of charge left in the battery. – mkeith Sep 26 '17 at 5:45
• If you ask the manufacturer, it can't be done and voids your warranty. In real life practice, you will need more than one battery. Even the smallest APCs I've taken apart for similar goals have used a minimum of two batteries. The charge circuitry may time out and give fault indication due to massive Ah increase if you recharge it from flat. Try it and report back! Also, can you clean up your question? There is no space before question mark nor comma. And "It gives". – winny Sep 26 '17 at 5:55
• 6 hours instead of 15 minutes. Geez, that is not what the electronics in a UPS is designed for. During the 15 minutes the UPS will also heat up and that's OK since it will not be hot for that long. But 6 hours, it is not designed for that so it would not surprise me if the UPS will fail/smoke/catch fire/burn your house down. – Bimpelrekkie Sep 26 '17 at 6:05
• I didn't warn about fire and burning because the OP mentioned it in the question. For sure if you try this, OP, be aware that it may indeed catch on fire and fail permanently. So, take suitable precautions. Don't test it out in your house! – mkeith Sep 26 '17 at 6:45
• This is an perfect example of using the wrong equipment for the task. Buy a more efficient converter (eg: victron). – Jeroen3 Sep 26 '17 at 8:12

Ok, let's consider some ballpark numbers. To begin, we need to determine what is the real capability of the UPS. A quick Google search reveals that this model uses TWO 12-V batteries, each about 7 A*h. So to start, you can't use only one battery, you would need two of them.

Now, 24V times 7 Ah is 168 W*h total capacity. Manufacturer's website says that 15-min run time happens if the load is 300 W, which means that the UPS is only about 50% efficient, which is not that impressive.

If you want to increase the run time of this device from 1/4 to 6 hours, you need to increase capacity of your batteries 24 times. This means you need two 12-V 170 Ah batteries, so you got the necessary capacity of individual battery about right.

Now, the UPS manual says that it takes 6 hours to charge the original battery. This means it will take 6 days to charge your new battery. I see no problems with this, since the lead-acid batteries are charged with constant-voltage, and usually LA batteries do not suffer from "overcharging"(unlike Li-Ion).

Regarding the heat, if this UPS can sustain 15 minutes of 300-W load, it likely can sustain 6 hours as well. However, since the efficiency of this UPS is quite low (their claim is 72% at full load), the dissipated heat is quite substantial, so you might need to add some blower to your re-designed UPS.

• this is some good input., looks like i need to check same with 12 volt battery ups, as this one needs 2 12 volts battery. – AMB Sep 26 '17 at 7:52
• How do you know that those batteries are in series, not parallel? Is it the usual way in UPSes? – Chupacabras Sep 26 '17 at 8:06
• @Chupacabras that is very good observation. – AMB Sep 26 '17 at 13:47
• I@Chupacabras, I know they are in series because it is always so (more electrically friendly), and because I watched this video, youtube.com/watch?v=_wcF0v75WuU showing 27V charging voltage. – Ale..chenski Sep 26 '17 at 17:10
• @AMB, you said you have the UPS. Why don't you open it up and see/measure by yourself? – Ale..chenski Sep 26 '17 at 17:15