It says 6 years at 20C degrees but this is with a continuous trickle charge.
So if the PCB is powered off 50% of that time the life expectancy would
be 12 years! Theoretically

I am starting to suspect that changing the battery is a red herring.
The problem is not that the battery is at the end of
life. The problem is that it loses it's charge! Instead of
using a Lithium battery like on CPS2, PGM used a trickle
charge NiMh battery. The only snag is that a NiMh battery
will lose it's charge over time.
Replacing the battery is not as simple as first thought, since
the new NiMh battery that you buy will not be charged fully! One
solution might be to do a swap while the PCB is powered but this
means we must know the circuit.
So my advise is to dig out your BL's and power them up once
in a while. A rough guess is that a 24h charge will last a year.
Here is some more charging info about the battery:
http://www.varta-microbattery.com/en/mb ... erf_en.pdf
rtw