I'd like comments on the following wiring scheme. Twin engine stern drive boat.

Both engines connected to starting battery. One alternator connected to starting battery bank (twin 27), one alternator connected to house battery bank (twin 8D). Batteries are switched through the new blue sea dual circuit battery switch which separates house and starting battery loads from each other but allows combining batteries when necessary. I intend to mount a 135 amp alternator to power my house bank and would rather wire it directly to the adjacent battery to reduce voltage drop instead of through the starter to the battery switch and back to the battery.

The original Formula wiring is the set up with one battery switch controlling two batteries. The alternator is connected to the starter on each engine and in turn the starters are wired to the same lug on the battery switch. The result is only one alternator is active. Further, only the battery activated by the switch gets charged. My proposed set up would allow both batteries to be charged when running without risk of running the starting battery dead at rest or imposing too high a load on the dominant alternator.

Comments and criticisms both appreciated.

Mike

You will need to incorporate an isolator to do that safely and effectively. There are several builders of these, but not too many that will handle 135 amp. input. (you can go online and find something suitable at a price you are willing to pay) That piece will come with detailed wiring instructions that will answer all your questions. I just went through this with my motorhome and have some info. on the manufacturers that I can forward if you wish. --- Jer

Mike,

I have a spare Professional Mariner Model2-70-3 battery isolator out of my 382 Formula if you need one. Thought it was the source of a problem, replaced it and didn't fix the problem.

Chris

Why would I need an isolator? There are wo separate banks of two batteries wired in parallel. No electrical connection between the banks other than common grounds. Both engines would draw power from a common alternator and battery, the second alternator is electrically isolated from the engines (other than the common ground ) and directly feeds the house battery and loads.

with a isolator you can charge both batterys enen though the switch might just be on batt 1 , thats really the only reason it is a good idea to have it