I have one of the early Ryobi 18V Li-Ion battery packs (I think it's
2.4Ah -- ten separate cells: five paralleled pairs), which the charger
shows as full but the test button on the battery indicates low charge
state (red glow rather than orange or green). I measure 18V across the
terminals, and it drives a tool for several minutes but then will not do
anything at all just a few minutes later.

Any idea what could be going on here?

Perce