Battery Switch

[GBNI]

The manual on my 42MY with twin Cat 3116 states in several places never to run both engines with the battery switch at “All”. This should only done if running one engine and wanting to charge both sets of batteries.
Why? What would happen?

[bluebyu]

One thought. One engine has bad alternator, you will never know.
(don't ask me I how know this)

[TowLou]

Both alternators electrically will kinda argue with eachother. One will charge more then the other sencing eachother in the circut.

[jwag956]

We had a boating moment the other night - one of our 8D batteries was going bad - lots of outgassing, high temps, bad scene overall. Ok - disconnected battery, temps etc came down. But how to isolate that battery and run both engines and not burn out an alternator - well - for our GB42 1990 - we disabled the battery at the ER switch, put the battery selector into ALL and off we went. The alternator on the side of the disabled battery showed charging, nothing got hot and we made it back to home port.

So - another possible use case. (note as we talked to a few electricians, GBs of different eras are wired differently, and many have been re-wired - so look carefully).

[Bongi]

Great post… Which begs the question, how were the various model years wired at the factory?

I ask because I recently swapped out the solenoid on the genset starter and wasn’t able to fully isolate the batteries to the genset through the switches… so there was some interesting wiring from the factory. I don’t think this was rewired by a P.O.

Even with the Genset battery switch in the off position (at the panel with the 3 battery switches midships at the engine room bulkhead), there was still 12v coming down the wire…to be fair there are two positive 12v lines inbound to the genset starter on my 1986 42C #946. Possibly this is part of a ‘dead in the water’ arrangement to ensure you get a shot at using the genset to revive flat batteries. On my boat, I have two 8Ds (one for each Cat 210hp 3208) and a smaller Optima helical AGM for the genset.

Also, once one of the engines is running, can one assume the other engine at start up ‘sees’ the alternator current from the running engine?… the broker who sold us the boat implied GB has no crossover switch for situations like this because it is implicit in how ‘these’ boats are wired. How might the battery selector switch influence this?