Hi!
New to the forum but been a long time fan!

I have an APC SmartUPS that I was lucky to obtain from a customer site where it had sat dormant for quite some time. To my surprise the unit was at the time in an ok condition and the batteries were still in reasonable order.
However UPS recently died

its an APC SUA3000RMI2U so 3000VA line interactive rack unit. It died in a peculiar fashion and instead of a dead battery it just emitted a continuous tone one day and refused to start or do anything useful. I figured it was the batteries but couldn't be sure as according to the manual, APC support, google and everything one I spoke to a dead battery should give a series of beeps and show the dead battery light.
Through tinkering and checking what I could using only a very basic multi-meter, all while being extremely mindful that I only have limited knowledge of the technology, I did eventually stop the continuous tone whenever it was plugged in, batteries attached or not, and it started to emit the right beeps and show a dead battery light. Sigh of relief here as the tone was very annoying and loud.
I also found this video to be very helpful although not about the same unit
https://youtu.be/Fj7e3WGUKO8Taking the battery tray out its got 8x 12v Pb batteries in a 2x4 arrangement giving a 48v output. I've checked all the cells and found only one of them is bad showing less than 6v. So I thought well could I run the UPS of only a single 1x4 pack? just to get some UPS at least until I can source replacement batteries.
Nope that doesn't work. It states bad batt still

.
This makes me wonder about the state of the other batteries are they too dead. Well checking each voltage i get 13.10~volts on each of the 4 I'm using. I ensured they were all charged up tested again with same result. I then left the project not having any money to do much else. The batteries have sat under my desk since then, so about a month not doing anything, I figured if the batteries were dead they would show a big voltage drop. Checked them just now and they are all showing the same 13.10V~ as before. The question now, are these batteries dead or do I have a dead UPS?
My question to the community is how can I test the UPS without buying a load of new batteries and wasting the money. The unit is quite nice and I'd like to re-use it but just don't want to spend money on it if its not going to work.
Does anyone know if there is a way to power the UPS from a powersupply to essentially fool it into thinking it has batteries connected? Thus testing the UPS is going to work with brand new batteries.
I'd appreciate any help
(I'd like a new Online UPS, but new batteries = $200, new UPS = $3000+, so simply cant justify a new UPS at the moment)
Thanks
Bill