On Thu, 18 Jul 2019 09:49:23 -0400, "Percival P. Cassidy"
wrote:

On 7/18/19 1:40 AM, ehsjr wrote:

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.


Internal resistance buildup due to age,

But with a high internal battery resistance, would the tool it's
powering still run at full speed/power? For the short time that it
does work, it performs well even with a circular saw in thick plywood.

Yup. You said 5 paralleled pairs. Think what happens if say four of
those pairs have high internal resistance and one pair is good.
You'll be able to get high current for a short time from the good
pair, which will discharge way more rapidly under load than it
would if all the pairs were good. As it discharges, it can no longer
maintain the high current so the tool slows.

The tool does not slow with this battery. I make a couple of cuts, and
then a few minutes later when I want to make another cut, there is
nothing at all: it does not struggle to cut but simply does nothing; it
appears to be totally dead.

Perce

To the OP: "Slowing down" vs "working one second and totally off the
next" is a function of battery chemistry (and how the electronics
handle state of charge). I've used Craftsmen and Ryobi tools for a
long time. The older NiMH amd NiCd batteries would slow down but
still be usable for that one last cut. However, the Li-ion batteries
are designed to protect themselves from being overly discharged. If
the voltage gets too low, the battery is disabled until it is
recharged. As others have stated, yours sounds like it needs to be
replaced.

Pat