I found that with my Bally games I can get at least 3 months memory retention. I think the long memory retention is due to using a PCD5101 at U8.

I read that the PCD5101 were the last 5101s that were manufactured & they were the most economical with current draw, quite a lot less than any other 5101s, so this makes them the best 5101s to use in this application. (Ed from GPE had mentioned this somewhere).

I found on a Williams system 6 game that I needed to install 2 x 1.5 farad caps in parallel to get decent retention as the Williams games seem to consume more current than the Bally games. The blocking diode is bypassed to allow current to flow to the caps. A resistor is also installed to slow the capacitor's charging rate. I think I get 2 months retention or so, which is great. The Bally games only needed a single 1.5 farad.

Personally I much prefer a memory capacitor to a coin battery for backup. (my Firepower had been already converted to a coin battery backup as can be seen with the holder) If a blocking diode fails & a coin battery receives a charge it can explode (apparently).
So, essentially I guess you would be relying on a blocking diode alone not to fail.

Setup on my Firepower. Note a PCD5101 RAM is installed.
IMG_9855 (resized).JPGIMG_9856 (resized).JPG

Yes, good point, I agree!, but in this case the 2 months I was referring to was when I was away for that entire time & they wern't switched on at all, so this was a good time to test if the memory backup held up.

I also had a spare Bally mpu (out of the game) with a charged up memory cap, I tried that back in a game after about 3 months & the bookkeeping was fully retained.

Usually about 1 week max without having a game!!