The message XFMail.051117232007.Ted.Harding@nessie.mcc.ac.uk from (Ted Harding) Ted.Harding@nessie.mcc.ac.uk contains these words:
With the motocycle battery, the UPS decides that the battery is too low at around 2 minutes, and simply turns itself off. The result is that both machines get caught before shutting down.
I suspect that this may be due to the m/c battery having a naturally slightly lower voltage (12.0v) than the UPS expects from a battery in good nick (with the old battery this would be around 12.5v or even a bit higher), so that "dropping" to 12.0v is taken as a sign of debilitation anyway, and the further drop as current is drawn to supply backup power finally does for it.
My UPS has accumulators in excess of twelve years old, and while it isn't an all-singing, all-dancing software-controlled one, it still functions effectively, and on the occasions when it has had to allow me to shut down due to a power outage, it hasn't tried any sneaky tricks like that.
How old is the motorcycle battery? Could a cell be down?