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  Starting vs. Deepcycle/Starting Combo batteries

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Author Topic:   Starting vs. Deepcycle/Starting Combo batteries
sternorama posted 10-15-2007 04:45 PM ET (US)   Profile for sternorama   Send Email to sternorama  
Boat battery selection... for a small runabout with dual batteries and a 1/2/all/off switch, on a boat with a bait tank...(on a trailer, in So. CA) would you suggest:
1 starting, 1 deep cycle
1 combo, 1 starting,
2 combos
or something different.
What about lead/acid or AGM? Any favorite brands or Wal-Mart?


jimh posted 10-15-2007 06:05 PM ET (US)     Profile for jimh  Send Email to jimh     
AGM batteries are lead-acid batteries. I think the difference you want to make is flooded cell batteries versus valve-regulated absorbent glass mat batteries.

The need to have redundant starting batteries is proportional to the difficulty in starting the motor by hand. If you can't easily pull-start the motor, I recommend two batteries that can both start the motor by themselves. What horsepower is your motor? What is the starting current required?

sternorama posted 10-15-2007 06:13 PM ET (US)     Profile for sternorama  Send Email to sternorama     
Thank you for the correction on the nomenclature! Yamaha F115 motor, I'll have to research the starting current required...
davej14 posted 10-20-2007 03:49 AM ET (US)     Profile for davej14  Send Email to davej14     
Cabelas AGM batteries seem to be a good value. I would recommend two combo's with each having the MCA required for starting your motor. Be sure your charger is compatible with AGM sealed batteries.
jimh posted 10-20-2007 06:11 PM ET (US)     Profile for jimh  Send Email to jimh     
I believe that a deep-cycle battery becomes a "combo" battery when it has enough capacity to provide the amperes needed for starting a motor. In this regard, precisely what constitutes a "combo" battery depends mainly on the current required to operate the electric starting motor on your outboard.

It is typical that for a given physical size, a cranking battery can deliver almost twice as much current as a deep-cycle battery. If your outboard motor has modest current requirements for starting, you may find a deep cycle battery in a reasonable size which will provide the current you need. Thus it becomes, for you, a "combo" battery.

davej14 posted 10-22-2007 09:55 AM ET (US)     Profile for davej14  Send Email to davej14     
Jimh is correct but I want to clarify that a starting battery cannot ever be considered a deep cycle battery. The reason that a starting battery can produce more instantaneous current than a deep cycle battery is that the lead plate construction is quite different. A starting battery has thinner plates so that in a comparable case size their effective surface area is increased. These more fragile plates do not tolerate deep discharge cycles well. After a small number of deep discharges a starting battery will have significantly less capacity.

If a deep cycle (combo battery) has an MCA rating => the MCA required by your motor and you have a significant electronic load then you are better off with this type of battery, especially in a single battery system. Make sure you use MCA ratings and not CCA ratings which are basically meaningless when you select your battery.

andygere posted 10-22-2007 01:49 PM ET (US)     Profile for andygere  Send Email to andygere     
I went through a similar analysis about a year ago, and arrived at a pair of Optima Blue Top combination batteries. They were one of the few that had enough MCA to satisfy my motor's starting requirements, and I wanted my "house" battery to be able to provide starting service if the start battery went bad. I opted for the AGMs because they hold a charge for a long time, which helps when the boat sits idle for weeks at a time. They are a bit more expensive, but I have not been dissappointed. Also, they had the same MCA rating as the lead acid wet cell Group 31s that they replaced, in a Group 24 size. This cleaned up my splashwell considerably. I bought them at Costco, which had the lowest prices I could find. Various reviewers claimed a service life of 5 years or more, which would make them an excellent value.
sternorama posted 10-23-2007 03:26 AM ET (US)     Profile for sternorama  Send Email to sternorama     
Thanks for the help. On the Optima site I noticed that there is some cross-over between the yellow top and blue top models now. Andy-did you buy the D34M?

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