This is a nerdy one.
We have a huge 210 Amp Balmar alternator on the engine. It’s controlled by a separate Balmar MC-614 Charge Controller.
We also have a huge 400 Amp solar array mounted on the pushpit. It’s controlled by a Morningstar TriStar MPPT charge controller.
The purpose of the two charge controllers is to give our huge(ish) battery bank what it wants. What our battery bank wants is maximum amps until it’s at 14.4 volts, then 14.4 volts for a couple of hours until it’s mostly charged, then 13.2 volts for a long time to “top off” the charge.
Even though they don’t really communicate with each other, usually the solar charge controller and the alternator charge controller work well together, mostly because the batteries aren’t that picky.
However, we’ve had a few times where we’ll be motoring along on a sunny day and the tachometer signal will disappear, often while we’re anchoring or entering a harbor or are otherwise distracted.
Here is what the deal is: The batteries are full but the charge controllers are somehow off synchronization. The alternator controller is trying to hold a trickle charge of 13.2V, but the solar power controller has decided that the batteries really need 14.4V and pump in the juice. The alternator controller sees the higher voltage, thinks that the world is coming to an end, and faults out.
It would be fine if all it did was shut down the alternator until we re-started the engine, but the alternator controller also puts out a synthetic tachometer signal that the engine control panel uses to tell the meat how fast the engine is running.
So, now, if the tach shuts down while we’re running, we close up or cover the solar panels. Problem solved.