Troop; you can get a switch from $25 to $50.

Firecadet; You can also get a Blue Sea combiner for about the same price as well. It basically does the same thing. Troop likes Yandina; I like Blue Sea.

Nothing wrong with either brand, and I am not going to recommend one over the other. But like anything else, there are options out there - and go with the one you like best.

I use a switch, a pair of dual purpose gp 27 batteries and just alternate use of batt 1 one day, batt 2 the next day. This makes the system simple, no wiring changes and I always have one batt in reserve. If in the unlikely event both get discharged, then I would switch to Both to get the boat started. I never use the both position normally, though.

Al:

The cables are usually more expensive than the switch.
Plus I don't want to drill/mount in my cockpit area.

I do have trickle-charger connectors attached to the poles on my starting battery. IF I ever go a month without starting the boat, I give it an overnight charge on the Battery Tender.

You are right in that cables are expensive; but they are not that expensive. Sure would be a lot more handy to simply flip a switch than to disconnect the battery - especially if you are broken down in a pitching-rolling boat.

You went to the expense of putting Optima batteries on board, why not finish the installation with a nice switch?

The best place I have found for cables is www.genuinedealz.com and for I tink a buck an end, they will terminate them for you. Of course, you got to buy the connector, but you at least don't need a $100 termination tool.

I like my installation and the fact that it is almost fool-proof; I can't forget and leave the switch in the wrong position...

True, true.

I had heard that when you put "both" batteries on as you run the boat around, that it is harder on the alternator as it's designed for charging one battery. If charging two batt's then you should up the alternator.. ???????

It is true that it may take longer for the alternator to charge both batteries simultaneously, but it is not necessarily hard on the alternator.

If the alternator's current output capacity is marginal to the required load, then it would be time for a bigger alternator.

Before I read the other responses, this is the exact thing the tech told me on an 07 Cobalt 252. He also stated that the engine's alternator would be not be sufficient to charge both batteries at once if I were to have the switch on 1+2.

There are several people that post on this web site that have added their own dual batteries. You should ask them if they have had any alternator issues.

I've had a dual batt setup since 2004 when I installed the switch and wiring. I always alternate the use of the batteries so I am never charging both at the same time. It's hard to generalize as to whether or not the alt could charge both not knowing the state of charge of each battery. I'd do that only in a situation that required both to get it going. So far the 20 year old alternator is still working well!

My boat came standard with a group 27 120AH multi purpose battery and a 75A alternator. I added a second group 31 180AH multi purpose battery, and Blue Sea System's Add a Battery ACR and switch. At the same time I upgraded the boat with a stereo amplifier, additional speakers and sub, and added a back up bilge pump. I modified the wiring such that the group 27 is used exclusively for starting and back up bilge pump, while the group 31 handles house systems and primary bilge pump. The beauty of the ACR is that the house and starting systems are kept isolated except when a charging source is present. So, the group 27 does not get run down much, while the group 31 sees quite a bit of use both with engine running or not. Essentially, it's like having a single group 31 battery, and a group 27 in reserve. While I do have a 20A dual charger and shore power, I have not used it so far this season. The ACR is doing a very good job of keeping both batteries charged, and so far I've had no issues with the alternator. Having said that, I have not come close to discharging the group 31 battery by 50%. If I did, and did not have an extended run afterwards, I would want to use the dual charger to top both batteries. Heck, I'd probably want to run the charger even if I did have an extended run. Note that there isn't much discussion of the switch above. I really don't use the switch much, so far I have switched to off a few times when doing some electrical work - I've never switched to 'combine batteries', and hopefully never will.

Here is a thought, for all of us with inboard engines---a switch being igntion protected, is much much safer than using any kind of jumper cables, because with cables there is always a possibility of a spark when you make the last connection--and sparks in the bilge or engine compartment of an inboard are a BAD thing. That was one of the main reasons I put in the dual battery system. I did boat the first 2 seasons with one battery and the battery pack but got to thinking, I do not want to take a chance with that. And the switch and wiring was probably the same price as a good battery pack.