GeoLane at PTD dot NET wrote:

(...)

It's feeding 16V to the battery when it's running and the battery is
connected.

(...)

I dropped to
the next lowest range - 500 mA, but still didn't get a reading.

Taking those two symptoms at face value, I'd say that at
least one of the cells in your battery has opened up
and the battery is no longer 'fit for service'.

I wonder what voltage reading you would get with a
'tested good' car battery?

--Winston