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rob's garage


I was at my auto parts store the other day looking for a battery for my boat. I told the guy behind the counter that I just needed a battery, that it didn't matter what kind it was.

The guy was insistent that I purchase an actual marine-style battery. I took his advice and bought one.

In retrospect, can you tell me what the difference is, if any, between a regular car battery and a marine battery?


Ooh boy, Dave. Battery discussions have been going on as long as guys have been able to arm wrestle. No matter what anyone says - expert or otherwise - there will always be a difference of opinion on the subject.

With that in mind, here come the goods.

Batteries come in many shapes and sizes. This is a given - just check out the selection at a battery supplier. From here it gets complicated further by the different applications, capacities and ratings but for the purpose of this explanation Dave, I'm going to keep this to a comparison of marine and automotive batteries.

The major difference is the ability of each type to discharge. You see, batteries only store energy, they do not create it. They simply accept energy (amperage pushed by a voltage), from whatever device has been connected to it to supply this energy (the charging system or an external battery charger).

A car battery is primarily designed to provide a large amount of current to get an engine started, and that's pretty much it. Once started, the battery just sits there being fed by the charging system to supply it with the energy that was lost during the starting (cranking) process. Dave, this is why they are called cranking batteries or engine start batteries. They can pump out large amounts of energy or current or amperes very quickly - but only for a short time. They supply little if any energy once the engine is running.

On the other hand, a Marine battery is designed for a couple of purposes - some call it a hybrid - a cross between a cranking and a "house" battery.

House batteries are used extensively in recreational vehicles of all types and are not expected to supply large amounts of cranking energy. However, they are capable of supplying energy (or discharging) over very long periods of time.

Not only are marine batteries expected to crank over an engine, they are also required to supply energy for lighting, pumps, sound systems and anything else that might be essential for comfort or safety while out on the water. As might be expected, these batteries can supply a combination of large amounts of cranking energy followed by a continuous supply of energy to keep accessories running.

The marine battery is capable of doing this through different construction techniques. As car and marine batteries are both categorized as "flood-acid batteries", that is, the internal components are immersed in a solution of sulphuric acid and water called electrolyte, the easiest and cheapest parts to modify are the sets of energy storing plates that are immersed in the electrolyte.

Marine battery plates are thicker than conventional batteries. This is one of the contributing factors to the increased cost of a marine battery. But hey, if you really want to spend more, Dave, get the top-of-the-line batteries and shock-proof the plates by imbedding them in epoxy.

Anyone that has spent time in rough water will appreciate shock resistance in a boat.

Other differences show up on the labels that are glued to batteries.

Cranking batteries are rated for the following:

  • Amp hours (Ah) or 20-hour rate is a measure of current that drains the battery capacity in 20 hours to a voltage point of 10.5 volts at 80F.
  • Cold Cranking Amps (CCA), or Cold Cranking Power (CCP) is the maximum discharge current in amps that a new, fully charged 12 volt battery at 0F can deliver for 30 seconds and maintain a voltage of 7.2 volts.
  • Reserve Capacity (RC), or Peak Capacity (PC) is a measure of time to discharge a fully charged battery at 25 amperes, at 80F down to an end voltage of 10.5V. (Almost similar to Ah rating.)

Marine Batteries are rated for:

  • Marine Cranking Amps (MCA), which is the same test procedure as Cold Craning Amps except that the test temperature is 32F. (This test is done warmer as a car is expected to operate in more severe operating temperatures). Batteries lose efficiencies the colder they get. Another way to look at it is who in their right mind is going to out in a boat in subzero temps anyway?
  • Previously mentioned ratings other than CCA. This part can be a little confusing as there are often different combinations of the ratings depending on the manufacturer.

But, this is one case where Go Big or Go Home really works. The larger the numbers for battery ratings, the more reliable it will be as long as the right battery is pressed into the right service.

I'm glad you took the guy on the other side of the counters' advice on this one, Dave.

Oh, a parting thought. Contrary to popular belief, batteries will NOT discharge through a concrete floor. Putting a battery on a piece of wood simply insulates the bottom of the battery from the cold.

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