On 2006-08-01, mc wrote:

Is there a rule of thumb for judging a battery as "still OK" or
"dead" based on the calculated Thevenin resistance?

Indirectly, yes. At the current that the battery is designed to supply, the
voltage should be at least 90% of full voltage.

Actually, in this era of digital voltage meters, I measure Vth, not Rth.
Vth drops from well over 9.0 V for a new battery to 8.8 V for one that is
showing its age.

I was under the impression that the open-circuit voltage wasn't a
reliable measure of how much "juice" is left in a battery.