A500 RTC Battery replacements

[Castellen]

From memory, the A501 uses the Oki/Epson MSM6242 real time clock, which has a minimum data retention voltage of 2.0V.

You need to measure the supply voltage at the RTC itself as due to the series diode, it will be less than the battery voltage.
Datasheet for the MSM6242 here

We've flogged this subject to death in various other threads as well, more info here for starters.