Weird problem with batt. charging. Suddenly dead battery

Hello,
I keep having problems with the charging system of my battery in my stock 2004 superjet.

First of all, my battery is not charging. The voltage on the battery stays 12V, also when i apply throttle.
BUT: As long as the battery is charged, the superjet will start just fine. But at some point, after a day of fun on the water, nothing. Not a click from the starter, the battery is death. Voltage has gone to 6V. Its done. Replaced the battery, and same thing happened after a couple of runs.

What i did and measured:
Replaced the voltage regulator (2 times)
Measured AC voltage from the charging coil (ligting coil, 2 green wires) = 30V - 60V AC and goed up with RPM
Measured resistance of lighting coil (2 green wires), around 1.2Ohms if i remember correctly. (it was good according to the specs, but i dont remember them)
Removed battery while running engine, no difference. Keeps running good.

I have no clue where the problem could be at this point. It seem weird that the voltage doesnt go down slowly. It starts fine and hard for a while, but after a couple of hours and starting and stopping the battery died suddenly. No slow crank or something, just death. The problem obviously is not the voltage regulator, and the coil seem fine too.
 
Battery is new, twice. Ruined 2 batteries already.

Nope, no electric bilge pump etc.
The new voltage rectifier is OEM (second hand, but OEM), already tried two regulators.

But if the starter relay i bad, the battery should still be charged right?

I also measured the AC voltage of the coil with the regulator attached. It is very low, around 2V AC.
So AC voltage without regulator: Around 40V AC
AC voltage WITH regulator: 2V AC
Does this say something about the coil?
 
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