Sub battery cable question

[Aln2202]

Twin DF 300ap and getting ready to connect wiring. Looking at diagram it shows to connect the sub battery cable to a battery.
My question is what function does the cable provide and does it matter which battery it is connected to?
Can it be connected to the house battery or must it be connected to one of the start battery?

[Harper]

Probably best to connect to the start battery. If you have a battery switch that can combine both batteries if needed to start, then connect it to the output terminal on the battery switch that goes to the engines. that way, it's connected to the start battery on normal starts and to both on "emergency" starts.

[Aln2202]

Harper thanks for the quick reply. That brings me to my next question.
I have a BEP cluster inside my console where there is 2 start batteries and a house battery the sub cable is attached to the harness that is behind dash inside console.
Is it ok to attach the positive directly to BEP cluster?
Which terminal is best?
Will there be any issues with making the sub cable wires longer as long as it is the proper gauge and conectoions?

Thanks again

[Moonlighter]

You asked the function of that sub-battery cable - it provides power to the engine's ECU so it is critical that it gets power at the same time the engine is turned on.

So I agree with Mike - connect direct to the start battery, or it is also fine to connect to the switch on the BEP cluster for that battery. Thats where mine is connected, i also have a BEP dual battery charging cluster. (3 switches - start, house and emergency parrallel, plus the VSR.)

No dramas extending the wire as you proposed. Just make 100% sure the join is made properly and waterproofed.

[Aln2202]

Great guys thanks for the insight. Moonlighter do you recall which terminal on the cluster?
I assume it does not matter if it connected to the switched terminal on one of the start battery. Just want to be 100% certain.

[Moonlighter]

Cant recall. I recall looking when the boat was new and thinking that it looked right. Think there was another positive wire also attached to the same terminal (bilge pump?)

In my case, ive just got a single engine. Twin batteries. BEP "quad charging cluster". When I am ready to launch the boat, I simply turn both start and house battery switches on, and thats how they stay until I put the boat away in the garage after finishing for the day. The only time I would even consider turning the start battery off when i am on the water is if i was overnighting and even then the VSR would protect it from discharging so i probably wouldnt bother.

This system has served me well, flawlessly, for 5 years now.

So in that context, it wouldnt matter in my case where i connected the white wire as long as it got power whenever the engine was running. But safest is the start battery switch because it gets priority charging when the engine is running, and cant get run down while the engines are off. If the start battery failed, then the white wire would get power when i switched the emergency parrallel switch on.