UPS failure

[Batterynoob]

I have the APC Back-UPS Pro, BR1500G with external battery pack BR24BPG

Yesterday my power went out in the building and the unit failed to turn on. Trying to figure out what went wrong

I bought the unit in 10/2022 and it worked fine at the time. I would unplug the device from the wall and it was powering my computer. Sometime a month later I decided to turn off the unit and not use it as the fan noise when it was on was very very annoying. I left the device plugged into the wall but powered it off.

1) Will the device continue charging if its plugged into the wall but the main unit power is off?

I turned it on again a couple months later to see the battery status and the battery on the LCD indicated it was full so I assumed I didn't need to keep this thing on all the time. I just need to use it as a backup battery for my computer and less so as a surge protector.

Fastforward a few months, the unit is stil plugged into the wall but powered off. My power goes out yesterday so I pull this unit off the wall and bring it to my computer. When I try to power it on, it won't turn on!! I press the power button and it just flashes for a quick less than a second but nothing turns on. I try to long press hold hte power button and same thing, nothing turns on. The external battery was attached to the BR1500G.

I didn't know how to troubleshoot it at the time so I eventually gave up and left the UPS plugged back into my wall and left. Few hours later the power came back in my apartment. about 3 hours after that I came back home to check the battery and it was able to power on and the battery was full!

I'm very confused by this.

1) If I leave the LCD off but keep the battery/external plugged into the wall, will it maintain charge when I need it? 

2) Why didn't it turn back on?

3) If the battery was dead and the reason for it not turning on, then why was it fully charged within 3 hours? I thought these devices need 8+ hours to fully charge?

Am I using it incorrectly?