We are having rapid short phase drops as they switch each line. The a/c overhead made banging noises and then a loud buzz while the lights dimmed many times. The aircompressor stopped, the conveyor stopped as they are all 3 phase. It's just a matter of time before something fails. A electronic ballast in the hall with 3 tubes appeared to fail one day.
This is an ATT business class modem. If the modem gets scrambled, no connectivity, which means the POS stops working after 4 hours and a hard reboot is needed to restore offline sales capability. People don't care much cash anymore, and the credit card readers will only allow 4 hours off-line before they refuse to work.
So what are you going to put this setup that you have made on? Doesn't seem practical for anything you have mentioned.
The battery appears to be a 17-20 Ah unit. If you pull 600 or even 300W from it, it isn't going to give you a very long run time if the power goes out.
Even with the charger running, you are heading downhill at anything over about 250W which would be less than most desktop computers and an LED monitor would pull let alone any add on's.
If things were as bad as you describe, I'd be getting hold of a proper used UPS, removing the battery pack and hooking it up to some used car batteries or whatever was better you could find and then running my equipment off that. Would probably last all day and you wouldn't have to worry about lost sales, damaged equipment or anything else. When the need had passed and the power was stable, you could sell it off and recoup most of your money. As a system you might get more for it.
Whatever the cost, which I imagine should not be more than $2-300 if you bought a used UPS and batteries, it would be cheap insurance against the potential losses to the business and you could probably write the cost off anyway. Once you sold it off afterwards, you may even be in front.