SNES battery replacement but no lost data?

Yesterday I tried my hand at replacing save batteries in 5 of my SNES carts. Though I didn't test each one specifically beforehand, they are all candidates to have the data suddenly disappear due to age (has happened to me and it's not fun).

So the installation was fairly straightforward and I started each game and got to save point, powered off, removed the cart and put it back in to see if the information had held. 4 of them showed zero data as expected but were able to accept and retain new save information. However...

...one copy of Super Mario World retained all of its previous save data in three slots despite the new battery installation. How is this possible?

 

CMOS memories take very little power, and there are decoupling caps on the board to filter out supply noise. If the time interval between removing the old battery and installing the new one is short enough the caps will hold enough power to keep the RAM contents intact.

 

That explains why my dying Zelda battery would retain the save if I turn the console off, wait a little and turn it back on, while leaving the cart over night and trying again in the morning wouldn't work.

 

could try holding a constant 3v using a bench supply??

 

I pulled a battery from a Link to the Past cart and didn't want to heat the battery up too much in order to relocate the legs to the replacement battery. I eventually hopped down to Batteries Plus in town, and they used their spot welder to do the leg-swap at no charge (and took like 5 seconds). Finally put the new battery in later that afternoon after my errands were done...and a save game was still there. This was hours gone by for that cart without a battery.

 

It happened to me a few times, but never with bigger JRPGs.