Unfortunately most of the "consumer" level UPS including the APC models are not really meant for continuous duty. What I found that will handle continuous application without overheating is the APC SUA1500 or DLA1500 (Dell Computer distributed version), they're low frequency inverters and they do have an internal fan inside to keep them cool along with the weight characteristics of a boat anchor even without batteries. I find them almost a dime a dozen at yard sales and thrift stores. Ebay is another option, but shipping cost is ridiculous now due to the weight. I modified them to have an external battery cable connection with Anderson SB50 connectors using gently preowned 100 Ah telecom AGM batteries. I too am ditching the lead for LiFePO4 batteries. With 100 Ah cells from fully charged to low voltage disconnect with my unscientific test on a 500 watt linear load was just a little over 4 hours. The onboard charger is limited to 5 amps and it pretty much took a good 24 hours to fully recharge it. Living in a subtropical zone and dealing with summer storms, having a UPS on electronics that don't particularly like hard power down events is definitely a must. As of last count, I have one in the office, entertainment center, mbr nightstand, and garage. They've served me well riding out 2+ hour power cuts giving me plenty of time to decide if I want to fire up the generator or not.

U chose the wrong model of ups for this project. There are some apc models that support external battery, they have way more powerful charger and have active cooling and can run at their rated power for hours.. i have br1000g and the ups comes with 2x9ah lead acid + it has a 3pin port in the back that allows me to hook an external battery pack of 2x2x9ah lead acid battery sold by apc , the connector is nothing specal it's just spade connectors with + & - directly connected via the pcb to its internal battery and 80amp fuse. & The 3rd pin just needs to be grounded to tell the ups external battery has been connected. I believe it has 3amp charger & 600w max out on battery backup. I have ran it on battery at 600w load with the internal lead acids and it runs for 4-5min . Apc claims it can run 18min with the external pack which i will test out in few days after i done building my Lifepo4 battery.

UPS "heatsinks" are just that -a small solid block of metal. The premise is that the battery as retailed is only worth about 3 minutes run time. In that 3 minutes the heatsinks will heat, but after 3 minutes the battery will flatten and the UPS inverter will go off. Getting the heat OUT for long term use is hard but possible. -Fans, vents and down-rating are your friends.

QUESTION does the ups go into any monthly equalizing charges ?? Secondly, once the target volatge is achieved does the UPS completely disconnect from charging the battery ??

What is the model of your UPS? Was your UPS equipped with lead acid battery originally? I am also planning to use external battery for my UPS but I wonder what is the charging voltage of the UPS. As far as I know, 24v lead acid batteries charge only up to around 25.5V but yours seem to charge up to 27V which is strange.

What is the UPC you talk about?

Which BMS are you using for your pack?

I tried modifying a cheap cyberpower UPS by replacing the internal lead acid battery with a LiFeP04 of the same size. Ran for ages but got worryingly hot (the board not the battery), sometimes going into thermal shutdown. Also if the resting voltage of the battery was more rhan 12.9 volts (so very low state of charge) the internal charger would refuse to charge it.