system
Given the following post, respond as if you're commenting on Reddit:
human
How can batteries retain a charge for 40 years?

In a recent video from Techmoan, he opened a new old stock battery powered 8-track player. 5 of the 6 C batteries were still fully charged. He opened another one, but only 1 of the 6 batteries was fully charged.

Those 12 batteries looked identical and presumably were manufactured close together. The wikipedia article on self-discharge gives 10 years as the longest average time to discharge.

What factor caused half those batteries to discharge and why didn't all 12 either stay charged or fully discharge? How likely is it that batteries can retain a charge over 40 years?
gpt
A 10 year shelf life isn't the same as being fully discharged in  10 years. 

Losing 3% a year for an alkaline battery would put it at 74% ish charge. Not so good for selling as a new battery.

Over 40 years that battery should still have about 30% charge.

Note that storage conditions are going to matter too. Humidity temp and whatnot are going to affect the rate.

Also with slow discharge rates the voltage vs state of charge curve becomes pretty flat till it drops off really quick at near full discharge. You'd probably need to apply an actual load and see how long the battery lasts instead of just reading voltage to find out how charged they really were.

I suspect that when originally loaded a couple of the cells were just newer and not all were replaced at the same time.