And this is the way I should have done it!

The error I made (powering the overvoltage relay and alternator field via the 5A breaker that is pulled to  ground when OVP unit triggers) is what caused total power loss when battery failed  open.

With an internally regulated alternator it needs to power itself after startup even when disconnected from battery. Because in case  of a OVP nuisance trip, the alternator will continue producing power. The 33,000uF (or bigger) cap on the B+ terminal should ensure sufficiently clean (low ripple and noise) to power itself and the engine electronics is case of no battery (disconnected or failed open).  (Size it for your engine bus current draw, desired alternator RPM range and maximum ripple you can tolerate. C = I / (RPM/60*3*2*Vpp) --- three-phase bridge-rectified alternator. I my case 10 amps, 3,000 RPM and 1V peak-peak ripple ~ 0.033 or 33,000uF.

You should probably have a way to monitor the voltage on the B+ terminal to ensure the OVP didn't trip on a real alternator runaway, before trying to reset the 5A breaker. (BTW, a 1A breaker should suffice -- it's only powering the overvoltage relay).

Now I'm undecided whether to add a backup battery or not.

Finn