I was wondering how long the FRAM type Nvram will last in typical pinball conditions. The FRAM is specified for a trillion, or 10^12 reads/writes which seems plenty enough.
Currently my only game at home is system 11b Bad Cats, but I think WPC doesn't differ very much from it in RAM access - I will check this when I visit my gameroom with WPC games.
Measured from the RAM /CS pin, System 11b CPU accesses its RAM about 100k per second. Considering FM1608 is specified for 10^12 reads/writes, this means in system 11b the FRAM will last 10^12 / 100000 = 10 million seconds. This means 2777 hours, or 116 24h days.
Of course, if you only keep the game poẃered on for 1 hour/day then the FRAM lifetime would be over 7 years.
Now with the CMOS RAM: Backup battery drain current on original System11b board was 3.6 uA with RAM chip in place, and 2.8 uA with RAM chip removed. Replacing the leaky Schottky diode D1(1N5817) with a 1N60 reduced leak current and consumption dropped to about 1 uA with RAM in place.
With this 1uA consumption, expected lifetime of 2000 mAh AA batteries is over 200 years And you dont have any lifetime limitation on access cycles. Just stay clear from known to leak batteries (e.g. Duracell) and replace at the "best before" date.