I recently got a very good deal on a refurb APC SMT2200RM2U battery backup. The unit seems to be in perfect working order, however the seller either gave me really cheap or old batteries.
The batteries go from a 100% to 70% charge in less than a minute. From there, it drops slower, but still very fast. At a 20% UPS load, I get about 20 minutes of runtime. I have done a runtime calibration.
What I am strongly considering is getting four 35Ah 12V SLA batteries, wiring them in series externally and connecting it to the APC unit. I'm specifically looking at these: https://www.amazon.com/35AH-Sealed-Battery-LEISURE-Wheelchair/dp/B00A82FCEE
Is there any reason to worry about this configuration? Is it going to overload the APC's charging circuit? Overheat it? Anything else that I would need to consider?
Four of these batteries would provide 3.5 times the capacity that come with the original battery packs when you buy this unit new.