Dual Purpose v. Starting Battery

Electrical and electronic topics for small boats
ferdinando
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Dual Purpose v. Starting Battery

Postby ferdinando » Sat Aug 31, 2019 3:32 pm

My boat had two nine-year-old Optima-brand dual purpose batteries. I wanted some peace of mind and bought two new Optima-brand batteries, but by mistake I purchased starting batteries. The boat house loads are just a chart plotter and an entertainment amplifier.

Q1: In comparison to the Optima dual-purpose batteries, is there any concern about using the new Optima-brand starting batteries?

Q2: Will the new starting batteries be okay for use with my boat?

--Fred

jimh
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Re: Dual Purpose v. Starting Battery

Postby jimh » Sun Sep 01, 2019 7:34 am

A1: there is not really a big concern, as long as you do not intend to continually discharge the starting batteries to a deeply discharged state and then recharge them.

A2: yes, as long as you do not intend to continually discharge the starting batteries to a deep-discharge state and then recharge them the batteries will work well for you.

jimh
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Re: Dual Purpose v. Starting Battery

Postby jimh » Sun Sep 01, 2019 8:12 am

A "starting" battery is generally used for starting automobile engines. Today's automobiles have much greater house loads than your boat has with just a chart plotter and an audio amplifier. A starting battery is designed to be able to deliver enormous current for a short duration, then be immediately recharged. A better term for these batteries is SLI or Staring/Lighting/Ignition. The battery's three purposes: to start the engine, to operate vehicle lighting, and to provide ignition current to run the engine. The general expectation for an SLI battery is the loads on the battery will be imposed almost exclusively while the engine associated with the battery is running and providing charging current to the battery

A "dual purpose" battery is designed for starting and some other purpose. The other purpose is sometimes unspecified. Generally the other purpose is to run electrical loads while the engine associated with the battery is not running. The duration of discharging the battery while its associated engine is not running and charging the battery is not clearly specified. A reasonable assumption is a "dual purpose" battery will tolerate some moderate discharging while in use without the engine running.

A battery designated as a "deep cycle" battery is designed to tolerate many cycles of being deeply discharged and then recharged. The ability of a "deep cycle" battery to tolerate many cycles of being deeply discharged and then recharged is greater than the ability of a "starting" battery and a "dual purpose" battery.

The physical difference in battery construction for these three different designations is usually the thickness of the lead plates. A starting battery will usually have thin lead plates. This will allow a very high surface area of lead to be contained in a particular battery case size, leading to very high current delivery in short bursts. However a thin-plate battery will not be as tolerant of deep-discharge/recharge cycles, and using a starting battery in that way will tend to reduce its service life. A deep-cycle battery will usually have thick lead plates. This limits the surface area of lead that can be contained in a particular battery case size, reducing the ultimate peak current delivery capacity. However, a thick-plate battery is more tolerant of deep-discharge/recharge cycles, but eventually too many of those cycles will begin to reduce the battery service life. A dual-purpose battery is assumed to have a plate thickness somewhere between the thin-plate construction of a starting battery and the thick-plate construction of a deep-cycle battery. The effect of deep-discharge/recharge cycles will be also somewhere between thick and thin plate construction.

As a general approach to battery life, limiting depth of discharge and maintaining the battery at full-charge with a properly regulated charging voltage will tend to improve battery service life, no matter what type battery is in use.

ferdinando
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Joined: Thu Oct 22, 2015 3:51 pm

Re: Dual Purpose v. Starting Battery

Postby ferdinando » Sun Sep 01, 2019 8:32 pm

I don’t think I should have a problem. The most I would use the battery with the engine off is the stereo maybe for an hour or less. I also have the batteries on a continuous battery maintainer, maybe that’s why my last pair of Optima-brand batteries lasted over nine years.

jimh
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Re: Dual Purpose v. Starting Battery

Postby jimh » Mon Sep 02, 2019 7:28 am

FRED--I am still using a boat battery that I bought in 2010. It was the primary engine starting battery, but now it has been moved to be the HOUSE battery, where--as on your boat--the loads are minimal. However, as you comment, I need some peace of mind and bought a new battery for the engine starting service.