I have an Victron MPPT 75/15 that charges the 300ah house bank well. There is then an Orion 12/12 30 amp which takes from the house bank and above 13.1V charges the engine/starter battery. This has also worked well until the last few days. With no changes to the settings twice now I have left the pontoon and within 5 or 10 minutes with the engine running have watched the engine battery voltage sag from its normal 14.4V towards 12V. I have hurried back and made it before the engine stopped. With the engine running the 150amp alternator feeds the batteries through a Victron Argodiode. I have not been able to repeat the problem alongside. I can not open the throttle however since it is not reasonable for boats nearby.
Today I switched the Orion off took the boat for a 40 minute run. No problem. I don’t see how the Orion can be causing a drop in voltage, and I do not believe there is a short circuit. Nothing appears to be overheating. The wiring is good since the voltage is normal both sides of the Orion. It’s as though the alternator is reducing its voltage to the point that the alternator shuts down when the Orion is on. The system worked well for the last few years. Thoughts?
If the engine is idling, then the alternator will not be able to produce much current.
Is the house bank taking more than usual current? This can be an indication of a developing fault in the house bank battery - particularly if more than a few years old.
OK, so I had the wrong end of stick…
Still, if you have a Clamp Meter, do check the currents flowing in the Orion connections…
Also: If the engine is charging both batteries through the Argo fet, why is the orion on - as you appear to have it wired to charge the starter battery from the house.
In this configuration, you have a current loop, as the output of the orion will also try to charge the house battery through the argofet, whether or not the engine is on.
How do you expect the Orion to discharge the engine battery on its output? It is solely unidirectional, meaning it can only ever charge from the house battery to the engine battery. If the engine is running, but the alternator suddenly fails to produce enough power to keep the system running, then i would start looking on the alternator side.
Its unlikely that you are using a smart alternator, so its output voltage will be a fixed value. Meaning as long as the engine is running, above the minimum necessary RPM, then its going to output whatever its fixed voltage is, regardless of other potential charging sources.
Since it doesnt output its fixed voltage anymore while idling, either the engine idle speed dropped, which can happen but is unlikely, or something else changed in the current path between alternator, argodiode and engine battery. Maybe a loose or overheated terminal, corrosion, something else. Check both positive and negative paths.