All UPSes periodically need their batteries to be replaced, and it’s possible to do the job properly yourself, avoiding vendor delays and saving quite a bit of money in the process.
The batteries inside UPSes, whether HP, Dell, or APC, are standard valve-regulated non-spillable batteries. Do a little research on the batteries your UPS uses, and what your battery supplier stocks. There’s no need to use the exact same battery brand that your UPS originally used, but do ensure you only use high-quality batteries of the correct, equivalent type. A suitable replacement will always be available from Battery World, or some such battery supplier.
1U UPSes use smaller batteries than the larger UPSes, and they tend to have a much shorter lifespan, so I’ve lost track of the number of times I’ve replaced the batteries in 1U UPSes like the HP R1500.
I should note, the wiring looks more complicated than it really is, it’s just 6 batteries in series. It beats me why HP have flipped over every second battery, requiring the use of long cables from the positive on one to the negative on the next battery. If all the batteries in each row were laid out the same way up, it’d be a 2cm cable from one battery to the next, without any wires crossing over & looking a mess.