[DeTomaso] Electrical Help Needed

[Will Demelo]

This spring I noticed that my car would loose cranking power after a couple of days. I was getting a draw of .5amps at rest.
The car always cranked a little slow on hot start, but never failed me until this spring. I changed nothing in the car electrically this passed winter.
I always attributed the slow hot start to having 2 solenoids in the car (original and 1 on the mini starter). The mini-starter was rebuilt 2 yrs ago.
I had the battery (1 yr new ) checked and it was fine. I had 2 people on the forum reply that they had a similar problem and it was the alternator/regulator. So I went out and bout a new alternator. (GM 1 wire).
I bypassed the original solenoid and fired up the car. I found I only had 10.5V at the fuel pump( new) so I checked the battery. It only had 10.5V while running. I shut it down and tried to start it. It would barely crank. I put the large battery charger/starter on it and fired it up. The voltage was 12.6V while the charger was on it. I removed the charger and it was only at 11.4V at the battery and slowly went down and I shut it down at 9.8V. I measured the current on the neg at 21amps. I’m assuming this is a draw and not the charge? Any way to test the alternator (built in regulator) while on the car?
Your help would be appreciated.
Will