doesn't it have a LOW-BATT indicator on the LCD screen?

Yes. The Fluke 77 has a rectangular battery symbol in the upper left
corner of the screen. When it comes on, the battery is certainly dead.

You mean useless. If it were dead...


However, voltage measurements go insane somewhat before the
indicator comes on. I was checking my supply of AA alkalines in the
fridge and noticed that I was tossing far too many batteries as dead.

This is Most Amusing, because I was checking a pile of NiMH AA cells, and
most of them read a few tenths of a volt. Then, a few hours later, they were
all fine.

In case you're wondering... Of course I use an alkaline battery. A lithium
would be an even better choice. MCM has name-brand lithium 9V batteries for
about $5 right now.