I'll give you the other view.
Do not isloate your batteries. With the simple electrical system on a mac and the low starting requirments of our motors there is no need.
- Keeping them separate means you will get a reduced life out of your house battery.
Keeping them separate really wastes the capacity of the starting battery.
Keeping them separate also means you need a set of switches and a battery combiner to keep things charged.
In any case you DO NOT want a Off-A-Both-B switch. These are outdated and can leave the opportunity for user errors. If you do separate the batteries you want 3 simple on-off switches. Refer to this
article.
If you get two batteries (I assume you are refering to group 27 typically 85-105ah or group 31 95-125ah batteries which have far mor amp hours than their group number suggests) I would make sure they are matched in size and wire them together in parallel. This will result in making the sum of their amp hours available to you.
The general guidline is to never use more than 50% of a batteries ah capacity. For a given days load, say 30ah used, you would use 33 % of a single house g27 rated at 90ah. If your 2 g27's were wired in parallel you would have used only 16.6% of your capacity. This shallower dip into your batteries capacity will result in longer battery life. Battery life is direct result of how deeply and often you dip into it's reserves. While a battery may be able to withstand 200 discharges to 50%, it will be able to many times more discharges to 25% of capacity.
Other reasons to go the single bank route,
- Simpler charging. You only need a single bank shore charger. Your connection to the motor does not need a combiner.
Simpler wiring. You only need a single master on off switch.
Simpler use. You will not by accident leave your switches in the wrong position.
More power is available yet you are being gentler to your batteries.
I feel that this is a very appropriate system for the simple mac electrical system. Some claim my electrical system is anything but simple with all the stuff I have. Still, there is no need for a system designed to cross oceans in our boats.
I suggest to this simple system you add a high quality amp hour meter (volt meters are useless when the batteries are in active use). Save all your extra switch and combiner money and spend it on a meter that will give you an accurate picture of your electrical use in real time. This will allow you to make the proper consumption decisions and never get into a dead battery situation. Having a single bank also lets you buy a cheaper meter.
I have used and advocated this system for the Mac for many years. Many others here have adopted it as well an are pleased with it's simplicity and functionality.
I do carry a small jump start battery just in case but the only time I have used it was for the truck.
I have no concerns about the redundancy I have available. I've never even come close to a dead battery bank even on multi-day overnights anchored out at parks with no additional power and 5 people on board including 3 kids who swear they never knew a switch had an off position. And this is with the two small group 24 batteries I had which were mis-matched of different ages and dual purpose instead of deep cycle. But say I did kill the single bank. I have the jump start battery tucked away and charged before each trip. Call it dead too. I can hand start my Tohatsu 50 (I've seen it done, it's not theory). I have my 6hp dinghy motor on a transom bracket that is only hand start and it will move the boat at 6 knots (tested and proven). Plus horror of horrors, this is a sail boat. Somehow people got where they needed to go without motors for centuries. I have no worries about being stranded at all.
I just changed my battery system last summer. I went with two GC2-6v golf cart batteries wired in series. This provides me 215ah at 12 volts. As you can imagine golf cart batteries are designed for a high number of very deep cycles and a high vibration environment. The make a perfect high capacity, long life bank and at $90 each they are quite a bargin. Note that their footprint in comperable to a G27 but they are taller.
These are wired to a single positive stud for connection to the motor and shore charger and then on to master on/off switch for the loads. the negative goes to the link 10 meter shunt.
