Attempt 'rejuvenating' old gel-cell lead-acid batteries with a bench supply?

[Gyro]

Poking a cadaver might seem gross at first, yet valuable as a study 

From the last check, the 3rd and the 4th elements appeared to be in reasonable shape, one was capable of about 3 Amps short-circuit current, the other, 7A.  Both were way under the expected voltage, at only about 1.9V with no load.

...

Surely even 7A is nothing for short circuit current on a cell of that size of SLA, it ought to be about an order of magnitude higher. I remember accidentally making the jaws of a pair of snipe nose pliers glow red hot trying to discharge a remaining cell on a smaller faulty SLA than that (6.5AH iirc) before disposal.

[RoGeorge]

With NiCd cells I made a rejuvenator...
...
Worked well enough

I did something similar in the past, for 1.2V NiCd elements.

Some of them were short-circuit internally by metallic dendrites that grow by themselves (self-grown metallic dendrites were observed growing inside vintage integrated circuits, too, not only inside batteries).  A few seconds of a 12V lead-acid battery connected with thick wires directly at a 1.2V NiCd battery will burn any internal dendrites in the NiCd battery, and make it work again, but not for long.  In a few days or weeks, the short circuit appeared again.

[SeanB]

It worked well enough to get a month or so per zap, and I always had a spare battery pack on hand to swap out for it. Yes the dendrites always grow back, but for daily cycling it was worth it, as new cells would do the same in a few months anyway, as the Panasonic charge circuit was bound up in the microcontroller on the board, and simply did a sort of constant current charge while monitoring current to light a LED, and disabling the phone side when charging. No room to add anything inside, and a PITA to get to those boards inside, as they were well hidden by other boards soldered over them, along with all the soldered on paper and foil shields.

[Gyro]

A good article on NiCd cell failure modes and recovery that I remember from Wireless World Magazine...  https://www.americanradiohistory.com/UK/Wireless-World/80s/Wireless-World-1985-06.pdf

P.S. Magazine page 60.

[RoGeorge]

Just FYI, same SLA battery I was poking at in the previous page, now 1 month later.