I have an APC BX1100C-IN 1100VA 230V Back UPS UPS with an internal battery. It 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 an external tubular 12 volts 180ah battery.
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.
Another point not raised already is that the inverter will have a duty cycle. If the battery only lasts 15 minutes with a 6 hour recharge, it would be reasonable to design an inverter with a "1 hour on 6 hour off" duty cycle, or roughly 20%.
Expecting your inverter circuit to run for 6 hours is asking a lot, and while it will probably work initially, the unit's overall life will be shortened.
Adding active cooling to the inverter will help somewhat extend the life, but will still be stressing the undersized circuit.
Considering that UPS are designed to do its intended job, which is to switch and provide emergency power for a limited time. Just enough for us to save all of our projects and shut down the PC. By that design requirement, they usually uses inverter with fewer switching MOSFETs and utilizes passive cooling from smaller heatsinks. For extended period of time, this would likely be a problem, especially for bigger loads. Overheating and of course, damage.
If you're a DIY kind of guy and know what you're doing, you can add some cooling fan and drill some additional vent holes on the case for better ventilation. But this still useless if you run large loads.
180 Ah battery? I could go crazy off-grid solar power to charge it! Your UPS won't be able to charge it, at least, not that fast. It might take hours or maybe days depending on the UPS battery and charger rating. I'm too lazy to do the math but I can conclude that you'll need a properly rated charger to probably meet your satisfaction.
For the sake of safety and peace of mind if you do large loads and uninterruptible power isn't your priority, you should get yourself a proper tool for the job, an actual inverter. It uses quite a few more MOSFETs for the rated power, bigger heatsink (the case itself usually a giant heatsink) and active cooling system (fan).
One more thing. Are you running inductive loads (fans, motors, etc.) out of your UPS? This could be a problem too. Most UPS on the market outputs square wave or modified sine wave which is more suitable for switching PSU (PC power supplies, printers, monitors). Square wave could cause inefficiencies (motor runs slower and hotter). Make sure your UPS is pure sine wave (test it by running a fan, if the fan hums like crazy and runs slower, then it's not pure sine wave).
