The short answer is “yes, it is possible.”
I’m replacing the battery on my bike right now and while I’m waiting on the new one and the proper float charger to arrive, I wanted to try out this hack. As my momma always used to say, “probieren geht über Studieren.”
The longer answer is that using a UPS to charge a motorcycle battery is trivial to set up but painfully slow.
It hooked up fine with a set of SAE (aka battery tender) cables* fitted with spade connectors. The UPS ran from the battery and also charged it. The “uninteruptible” supply that I have is a ten-year-old APC 400 watt model. It was designed around a 12V 12AH absorbed glass mat (AGM) battery, which is precisely the specs of my bike’s battery. This apparently matters because the charging strategies are different for the various types (and sizes) of 12V lead-acid batteries.
I was seriously disappointed to find the UPS only charging my very dead battery at around 250mA, far short of the manufacturer recommended 1200mA, or even the 750mA that the smaller Battery Tender brand maintainer delivers. I assumed that the UPS was smart enough to switch off eventually so I just let it sit overnight.
When I came back the charge current had dropped to near zero and the voltage across the battery was 13.58V, which is what you see in the photo above. It’s very hard to read the Simpson analog ammeter is showing any positive current, but its at 5mA or so, on the 500mA scale. This float voltage of nearly 13.6V is at the high end of what appears to be acceptable for this chemistry.
Since many smart chargers reportedly switch off the charging current while measuring the “open” voltage of the battery to determine its state of charge, I was somewhat surprised that I couldn’t hear any pulsing or see any fluctuation on the instruments. Maybe this one is sneaky about it, or maybe it never switches off?
*Note: the colors of the alligator clamps in this cable set was reversed with regard to the color of the cable at the other end of the SAE plug, which is confusing because the black clip is actually on the positive terminal of the battery.