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?

[SteveK]

Originally Posted by TowLou

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

OK, don't disagree they compete, but I do ask if there is more or less total charge in the process.
Here is a thought.
I have run on all and watched the amp meters applying charge. Normally one engine charges the start battery and takes little time to charge fully and sits idle, since it is still cranking out the amps they must now be going into the house battery bank in addition to the other dedicated house bank alternator.
Am I imagining the house bank getting charged more on two v. one?

[tiltrider1]

With out knowing what type of alternators and how they are wired I can’t answer the question. Maybe nothing and maybe the wiring is to small and will fry.

[TowLou]

Originally Posted by Soo-Valley

OK, don't disagree they compete, but I do ask if there is more or less total charge in the process.

Here is a thought.

I have run on all and watched the amp meters applying charge. Normally one engine charges the start battery and takes little time to charge fully and sits idle, since it is still cranking out the amps they must now be going into the house battery bank in addition to the other dedicated house bank alternator.

Am I imagining the house bank getting charged more on two v. one?

Yes a thought you can do it. Long term may not work out to well. It is seperated from the beginning. Making your alternators compete with eachother could fry them or even blow a diode when you move your switches around with the motor running then you will have a drain in your system that you will not find till its to late.

[Steve DAntonio]

There is no harm in running in ALL, however, if one alternator fails, you won't know it because the other is carrying both engines. You'll know it when the second one fails.

When paralleled, one alternator often does shut down, as it senses the higher charge voltage from the other, but if they are paralleled you'd only know one was shut down by measuring current output at the alternator. The same thing happens when you run a battery charger on a start circuit when underway, alternator output often goes to zero. If that charger is always on underway, via a genset, it can mask a failed alternator.