Steve-
Yup, KISS all the way!
While I like complicated stuff, and electrical things are a fun hobby, the battery power on a boat can be simplified a lot.
Why carry two types of batteries?
A question or two-
Do you really need isolators, combiners, multiple bank chargers, and a Ph.D. in Electrical Engineering to keep 12 volts on tap? And at what expense?
When I added the electric trolling motor a year ago, I decided to add some amp hours with extra batteries and Keep It Simple, Stupid.
Here's what I did:
Went to four group 27 deep cycle batteries connected with two off-1-all 2 switches combined into a single parallel 12 v battery system. I can turn on any one, two, three or all four batteries on into the system at any one time, but, for 99% of the time, I just run it as one big battery.
The value of this is that with 460 amp-hours of capacity, I can't run out of electricity, even running the 38 amp trolling motor for hours on end. I never have to worry about which battery to use or charge.
Then too, I need just one battery charger (since they can be all charged in parallel) of 40 amp capacity. I have a single battery three stage charger that can charge at 40, 20, 10, or 5 amp rate and automatically cuts the rate down based on the resistance voltage of the batteries. It can also put out 100 amps for emergency starting, and has anti-sulfation cycles and a couple of other features. Cost $100 at Wall Mart, vs. $300 for tye typical marine two/or three bank charger. (Just keep it in the cabin away from gas fumes since it's not sealed spark proof marine approved.)
The connecting of a lot of batteries in parallel is usually frowned upon, mostly because a weak one or shorted one will allow the other batteries to discharge through it, even boiling it's electrolyte. The solution is to keep only high quality fresh batteries and to check their condition frequently and replace them at any sign of deterioration. Also, as always, keep each battery in a battery case with the strap on tightly.
Also, I use regular RV batteries (Interstate or the like) rather than expensive gel cell or gas mat batteries because they're cheaper, have MORE capacity, and because I'm not usually boating upside down.
The only problem with this approach is that it doesn't lend it self to dual motor installations because when two motors are connected to the same battery their voltage regulators can't agreee on what to do.
I'll have to try to figure out if this approach can be done for dual installations, though, because
dual motors
+ dual starting batteries
+ house batteries
+ isolators and / or combiners (black boxes that do unknown tasks)
+ general over complicated and confusion added systems
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seem to defy understanding and trouble shooting!
Here's one area where the KISS Principle works for me!
Joe.