Battery replacement

debbie1112

Recruit
Joined
Oct 18, 2007
Messages
5
I need to replace the batteries on my boat. I own a 21 ft. Sea Ray w/ Merc 5.0 I/O engine. I'm not sure of the difference between deep cycle, starting, & dual purpose batteries. Which do you suggest I purchase. Thank you!
 

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
Re: Battery replacement

If you are using it for starting the boat and running a few electronics and stereo, then I would get a good high amp. marine starting battery. Tell us a bit about the way you use your boat and more advice will follow....
 

debbie1112

Recruit
Joined
Oct 18, 2007
Messages
5
Re: Battery replacement

Actually,I had 2 starting batteries & they both died at the same time. I had them checked. I have a battery switch but always have both batteries turned on. #1 I thought it would give me more power to crank the engine & #2 I thought if one battery died the other would take over to recharge it. ? I have a stereo system , lowrance, & then the usual lights, bilge,etc.. I did read a few of the similar threads & got some helpful information.
 

JoLin

Vice Admiral
Joined
Aug 18, 2007
Messages
5,146
Re: Battery replacement

The reason you have a switch between 2 batteries is so you can isolate one from the other. Basically, reserve one battery strictly for starting the motor, use the other when motor is off and you're running your accessories. That way you know you have a strong battery available to start 'er up. Set the switch to 'both' when the motor is running so you charge both.

One battery can't/won't charge the other. Only thing that will happen is that they'll reach 'stasis', or a similar level of (dis)charge. If one battery's charge is low and the other high, when you switch to 'both', they'll meet in the middle. If one is really, really low, you could create a situation in which neither now has enough amps to start your motor.

Conventional wisdom is that you run 2 different styles of battery- a high amp, fast-load "starting" battery, and a deep cycle type for your engine-off electrical needs. Both, however, should be of the same geneal construction - lead/acid, gel, etc... so you can use the same type of charger for both.

As far as the hybrid, combination "deep cycle/starting" battery, it's more an oversized starting battery than a true deep-cycle one. Not knocking it, but it isn't a deep-cycle battery.
 

Boatist

Rear Admiral
Joined
Apr 22, 2002
Messages
4,552
Re: Battery replacement

I agree if you leave the switch in both the bad battery will always discharge the good battery to the same bad battery. Only time I would put it in both is when you know the engine will start up and you want to charge both. Still as soon as I shut engine off would switch back to the normal starting battery.

On my boat charge both batteries thru a battery isolator and never had a battery that did not last 6 years.

Some also use a battery combiner which will allow the starting battery to charge to a certain level then switch in the second battery for charging.
 
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