Coincidentally, my thermostat just stopped, with dead and corroded
batteries. I emailed Duracell, quoting their guarantee: "Should any
device be damaged due to a battery defect, we will repair or replace it
at our option."

They started to weasel in their first we-have-received-your-letter reply:
"We’d also like to share that there are many things that can cause
batteries to leak /vent. Some of the more common causes include exposure
to extreme temperatures, moisture, where and how batteries are stored,
and/or becoming shorted out by other batteries or metals; trying to
recharge an alkaline battery. Research also shows that mixing old and
new batteries and different types of cells are also common causes of
battery outflow."
In other words "It's almost certainly your fault".

I'm supposed to call them and "have their package handy". Yeah, right -
I have the package that they came in! I have a bit of
obsessive-compulsive in me, but I don't save old battery packages.

Sheesh.

Bob