TOPIC: Battery not Charging - how to test?

1966 Starflite 100.

I have my battery on a battery tender between runs. The fish finder always shows 12.6 volts on a fully charged battery at the start of the day. This spring I had the battery tested at the shop and it tested good.

This weekend we did about 35 miles with several stops. The voltage slowly but steadily dropped all day. On the way back we ran about 8 miles at WOT without stopping and the voltage still dropped down to 12.2. It still cranked and started, but not as well as it did in the morning.

Earlier this summer I put a meter across the battery and it did show an increase of about 1.5 volts with the motor running vs. not running, but the battery still lost voltage. I might be at the lake tomorrow and could try this again.

I assume you start at the voltage regulator, how do I test that and then what's next?

Check voltage at battery with motor off and note reading. Start motor and rev it up, if voltage increases to a value above what it was with engine off, then it is charging.
Not sure what amperage your 1966 engine has for charging, but it will most likely be small, like 4-6amps. So if it drains even while running, it sounds like your current draw is greater than your charging ability