Isolator kill switch
 

Lots of boaters have dual batteries for different reasons... stereos, speakers, radar, trolling moters, bilge pumps, etc etc etc. So they use one battery just for the engine, and the other battery for when the engine isn't running. That way their engine battery is always fresh and not run down.
And if, for some reason, they need the extra juice from the second battery, it's available with just the turn of a switch to the "both" setting.

Additionally, when set to the "off" position, it prevents any unwanted drain on the batteries by isolating them from the electrical system. (Useful if your boat spends long hours on a trailer between trips to the water).

Thanks,,, more questions to come and more money to spend .. LOL

first off, you should have starting batteries, not deep cycle, its probably the number one mistake I run into working on boats, deep cycle batteries don't have the cold cranking amps that's needed to start an engine. The reason you use an isolator switch so that both batteries get charged, but you won't run one battery down, you'll allways have one in reserve

they will say on teh battery what type they are, it will either be starting, dual purpose, or deep cycle. There should be a rating on it for CCA(cold cranking amps), MCA (marine rated cranking amps), and Amp Hours reserve.

Hey spare can you pm me info on some economical batteries I need for my boat? Or post on here if the op don't mind?

the best battery that personally have had good luck with is the Deka 24M7, its a 1000 MCA maintenance free starting battery, there are some people that don't like them, but I've had great luck with them. I wouldn't waste your money of a AGM or gel battery for a boat, they just aren't worth it.

Yup... A friend recommended Deka to me a while back... never had a problem with it yet.

And for the record Roffey.. a deep cycle battery is usually used for an electric trolling motor.. something that drains the battery waaaayyyyy down during the day. Assuming that your engine's charging system is working properly, a good starting battery is all that you'll ever need.

My set up is one starting battery for starting only and one deep cycle with all of my electronics on it, set up on a 3 way switch. I can use the deep cycle to start if needed. Seems to work out well for me.