This is where the original battery is located:

It’s held in place with a velcro sticker. This is what it looks like:

Here is the Datasheet for the TL-5242/W.

This is a NON-rechargeable 3.6V Lithium Battery. No wonder it’s dead after 35 years…

This reference is hard but not impossible to find today. However I could only find it in the US (Amazon, Mouser, RS), NOT in Europe. Worse, all these vendors won’t ship it to Europe. So, I opted for a “locally sourced” equivalent:

I found this other Tadiran 3.6 V battery (this time just the battery) for 7.37 € which is very likely to be equivalent to what’s in the sealed box above (which I did not try to open because if I puncture the battery in the process, it’s going to be a lot more trouble than the info was woth to me.)

I also found this encloure that fits the battery (8.63 € with a whole bunch of other sizes that I’m saving for other projects) and directly provides well isolated wires:

It’s a tiny bit larger than the original but the Mega STE has enough space to accomodate it.

Total: 16.20 € with next-day Prime delivery.

Then I decided to reuse the dupont connector from the original battery. I did so by removing the metallic connectors from the black casing:

I am re-using the black casing as it prevents from accidentally reversing the polarity… as long as I don’t mix up the red and green in the process! Therefore, I am proceding one wire at a time!

In the photo above, I already replaced the black wire and I am about tp replace the red wire.

Note that the section of the new cables is smaller than the originals but I’m not too worried about that since this battery’s sole purpose is to keep a small Real Time Clock chip powered. So the current (Amps) will be way below the tolerance of those wires anyway.

So I’m reusing the black casing but not the metal plugs. For those I am crimping new dupont plugs on the new wires. Here I am pre-crimping one such plug:

After that, I insert the red wire in this case from the other side of the crimping tool and I finish the crimp. Here is the result:

Last step, stick a bit of velcro on the casing:

And it’s ready to go back into the Mega STE.

PS: tested : works flawlessly ! :)

PS2: later found this approximate equivalent that might work, but it’s about the same price, takes longer to ship, is not as modular the next time you need to replace the battery… and it’s less fun… ;)