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I presently have two banks of Batteries wired through a traditional battery switch, 1 - BOTH - 2 - OFF. The batteries are charged from the engine alternator or a battery charger hooked to shore power. The charger can charge 3 battery banks at 5 amps each.
The engine is a diesel that takes a good draw to get it turned over to start.
My issue is the electronics. When drifting, I often lose the electronics due to voltage drop when I start the engine. I presently have the electronics running through a segregated circuit pannel hard wired to battery bank 1, not running through the battery switch. I can lose the electronics even if I have the battery switch on battery 2 position, I am not sure why. Maybe because I am using a common ground between the two banks going to the engine.
In any event I am thinking to solve the problem with a third battery bank solely devoted to the electronics, and not hooked in any way to the engine. I would only charge this through the shorepower battery charger. Simple enough so far...and fine as long as I am back to the shorepower each night.
But for sure if I have a 3rd bank of batteries, I need some way to switch the electronics to one of the other banks if I am off shore power for an extened period of time. I could do this with an A-B switch, but it probably makes more sense to be able to somehow be able to charge this 3rd bank from the engine.
I could incorporate a second battery switch, but i am thinking that there are all these new fangled things today like battery isolaters etc. which I do not understand but which probaly makes sense here.
Can somebody suggest a better way to incorporate a 3rd battery bank into my system
 

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I had a similar problem with my Groverbuilt. Diesels have a higher compression ratio than comparable sized gas engines. Therefore, they take more energy to crank during starting and tend to drop battery voltage. My solution was to install a battery isolator.

I have two batteries, both always active. One battery, a group 24 starting battery rated for 1000 amps does nothing but crank the engine. The other battery, a group 31 deep cycle runs electronics, lights, winch and everything else. I can charge both batteries from a single altenator. The isolator prevents current from leaking backwards from a fully charged battery, to a partially discharged battery. When I fish and leave the electronics on, they are draining the deep cycle and the starting battery remains untouched. I can drain my deep cycle and would not loose any charge on the starting battery. When I start the engine, there is no voltage drop to the electronics since the starting battery is on a totally different system.

In order to install this system, you will need two seperate battery switches. I have mine rigged up so I can combine both batteries or totally isoloate one in an emergency. The system puts less stress on the batteries and has worked great for me for the last 6 years.
 

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Very common problem with a somple solution. You need to install a battery that onyl powers the electronics. That battery can be wired into the charging system (both on board and shore) with an isolator or combiner, that will prevent that battery from being drained by the other battery. Most installations will also incorporate a battery switch (just a simple on/off switch, not the A/B both type) so that the battery can be brough on line in an emergency to start the engine. Leaky Rivot posted a wiring diagram of this type of set up some time ago.
 

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Hey Rob,
This topic has come up too many times to count.:rolleyes: I'm sure you've done a search here on batteries.
There was some good info in past posts and some guys put up some basic drawings for adding a third battery.

I'm putting in a seperate switch for my electronics this year as well.

I'm using 3 simple on-off switches. 1 for the start only, 1 for the electronics and a third to combine the both if needed.
That way, I know my electronics are completely seperate from everything else.


West Marine has some pretty good info in their catalog about wiring switches..........


 

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I had the same problem and tried to follow West Marine's advice last year. It looks very straightforward in a diagram and I bought all of the gear, but the heavy gauge cable recommended by the mfr doesn't make neat 90 degree bends and there simply wasn't enough room to mount such a complicated a design



 
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