Twice in the last couple of weeks someone else using my van has gone to
start it and been unable. Engauging the starter only resulted in rapid
"clicks".

Attaching a battery booster both times easily started the car and all
was well.

Yesterday (the second time) I grabbed a volt-meter along with the
booster pack.  The battery was reading 10v at rest, and dropped nearly
to 9 under load.

I boosted it and the car started, drove it home and put it on a charger
-- where it drew 6-8 amps for a couple of hours.

This morning I checked it at rest in the cold before doing anything and
it read above 12v as it should.

With the engine running there is over 13v going across the terminals, so
the alternator etc. seem to be fine.

I was sure there was a bad cell in the battery (only 37 months old,
OEM), but then this morning seeing 12+V confuses me. Especially since my
wife had charged her cell phone overnight using the car battery, I had
expected it to read low again.

Thoughts?
And would it be worth exchanging for another MOPAR battery with whatever
proration credit or just go to my favorite battery store?