The specifications for the MSM6242 real time clock's input voltage is from 2V - 5.5V.
If you disable the charging circuit, you can use ANY non-rechargable battery so long as it falls within this range.
Alternatively, you could simply use 3 Ni-Cad or Ni-MH batteries in series, which will give you a nominal voltage of 3.6V. The capacity of the battery does not matter, though obviously you're limited to the phyical constraints of the A501.
To disable the charging circuit on the A501, simply lift one leg of resistor R911 (470 ohm, so will be marked yellow-violet-brown), or lift either end of diode D932.
For schematics of the A500 and A501 look here:
http://amiga.serveftp.netSee the schematics section.
Anthony.
