In ,
john hamilton typed:
I'm replacing a small button battery (CR 2032) with two AAA
batteries, I'm intending to leave the old discharged button
battery in its place and solder the leads from the AAA
batteries directly on to the button battery.

Bad idea: How do you know the button won't become a load or short ckt down
the road? Don't use it. You probably can't get a good solder connection to
it, anyway - wrong materials - not solderable. Solder to the pins inside
where the wires connect to them.

Is the heat that I would need to apply to the button
battery with the soldering iron to solder to it, likely to
make the battery prone to 'leakage' in the future?

A definite possibility since you don't mention heat-sinking and any level of
expertise of soldering skills. There's a lot more to soldering than just
melting the solder onto something. I'd give this about a 25% chance of
workng IF you have ever been taught about soldering, zero otherwise.

Or
perhaps cause it to go bang at the time of soldering?

Depending on the condition of the button, where you heat it, how you heat
it, how you heatsink it, yes, it's possible. Especially if ± sides of the
battery get connected by a heat sink or solder-drip, whatever.

Why wouldn't you just buy another coin battery? They're cheap and easy to
swap out.

HTH,

Twayne`


Thanks for advice.