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Author Topic:   Battery Size
jimh posted 03-28-2006 09:21 AM ET (US)   Profile for jimh   Send Email to jimh  
A short summary of battery size characteristics

There are three dimensions to battery size:

--physical size
--storage capacity
--cranking current

The physical size of a battery is usually stated in terms of a somewhat obscure collection of group sizes. The most common ones are

GROUP - - L x W x H (in inches)

24 = 10-1/4 x 6-13/16 x 8-7/8

27 = 12-1/16 x 6-13/16 x 8-7/8 (i.e., about 2-inches longer)

31 = 13 x 6-13/16 x 9-7/16 (even longer and now taller)

The typical OEM Boston Whaler battery box will accept a Group-24 battery.

The storage capacity of a battery is stated in Ampere-hours (A-h). This is a measure of the total electrical capacity the battery can store. If a current of one ampere is withdrawn from a battery for one hour, that is 1 A-h. The rated Ampere-hour figure is the amount of current that can be continuously supplied by the battery in a 20-hour period. This is better explained by an example.

If a battery has a rating of 40 A-h, this implies that it will supply a current of 2-amperes for 20-hours. This capacity is available only when the battery begins with a full charge and is in new condition. As a battery ages it will gradually lose some of its storage capacity, and it may be more realistic to de-rate the Ampere-hour figure somewhat to judge the true capacity.

The typical Group-24 battery will have an A-h rating from 40 to 80, depending on its construction and quality.

The A-h rating will also indicate how long the battery must be charged to become fully charged. To fully charge an 80 A-h battery from completely discharged, you would need to use an 8-Ampere charging current for 10-hours, or a 16-Ampere charging current for 5-hours, or a 20-Ampere charging current for four hours, and so on. (The charging current usually tapers off as the battery terminal voltage rises so these examples are not quite obtainable in practice.)

Ratings of cranking current refer to how much current can be delivered for 30-seconds until the battery voltage drops to 7.2-volts. This is measured and rated at two temperatures. The rating at 0-degrees-F is called the Cold Cranking Amperes (CCA). This is not particularly applicable to marine installations. The rating at 32-degrees-F is called the Marine Cranking Amperes (MCA).

Because of the sophisticated electrical systems of modern outboard motors, it is important that the battery voltage during cranking remain high. If the battery voltage falls, the electronic system needed to start the motor may not be able to function, and the motor will not start, even though the battery can still turn over the motor. For this reason, it is now common that motors require starting batteries of over 1,000-MCA rating.

Chuck Tribolet posted 03-28-2006 09:38 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Not only does the charging current taper off, it's not 100%
efficient either.


glen e posted 03-28-2006 10:34 PM ET (US)     Profile for glen e  Send Email to glen e     
Excelent post JIm.... as you quote:

"For this reason, it is now common that motors require starting batteries of over 1,000-MCA rating"

That's why I own 4 Cabellas AGM Group 24's @ 1195 MCA's...very strong batts with no maintenance feature

bamatenn posted 03-31-2006 04:04 PM ET (US)     Profile for bamatenn  Send Email to bamatenn     
What about the weight of different battery groups? Do all brands of Group 24 batteries with the same A-h and MCA generally weigh the same?
jimh posted 04-01-2006 04:25 PM ET (US)     Profile for jimh  Send Email to jimh     
Batteries of the physical same size , A-h rating, and cranking current ratings do not necessarily weigh the same.

Battery weigh is determined by the construction. This varies considerably with manufacturer and type of battery.

seahorse posted 04-02-2006 09:49 AM ET (US)     Profile for seahorse  Send Email to seahorse     
Good informative post, Jim, however the amp-hour rating is hardly used anymore. Back in the "good ol' days", OB's required a 70 AH battery. Today, most specs are in CCA or MCA.

A good rule of thumb is to purchase the largest battery that you can afford that is above the minimum specs.

A quality heavy duty group 24 lead-acid battery will weigh 45-50#. If you weigh one of those "big box" cheapie marine batteries, they may be as light as 38#, and are definately not strong enough for any but the smallest electric-start oarboards.

seahorse posted 04-02-2006 09:52 AM ET (US)     Profile for seahorse  Send Email to seahorse     
Damn! the caffeine must not have kicked in yet. The last word on my previous post should read outboard, not oarboard.
Moe posted 04-02-2006 02:33 PM ET (US)     Profile for Moe  Send Email to Moe     
While it isn't as important on starting batteries as Cold Cranking Amps (CCA) or Marine Cranking Amps (MCA), and Reserve Capacity, and often not even published for starting batteries, Amp-Hour rating is THE rating for deep-cycle batteries, where CCA/MCA isn't as important.

A battery's amp-hour capacity depends on the discharge rate and it decreases as the discharge rate increases due to something called the Peukert Effect. Most companies publish amp-hour rating at the 20 hour rate, which is the rate that will take the battery from a defined full charge to defined full discharge (typically 10.5 volts). For example, on a 100 amp-hour battery, that's a 100AH/20H = 5A discharge.

The Reserve Capacity rating, usually found on both starting and deep-cycle batteries, shows the battery capacity at a higher discharge rate, 25 amps. That could be, for example, powering a 250W 120VAC inverter at full output. A 100 amp-hour Group 27 deep-cycle battery typically has a Reserve Capacity of about 180 minutes or 3 hours. This translates to 3H x 25A = 75 amp-hours at this higher discharge rate.


jimh posted 04-02-2006 05:03 PM ET (US)     Profile for jimh  Send Email to jimh     
Let us assume that a vessel has navigation lighting that draws 2-Ampere, and it has a battery with a rating of 1,000-MCA, a capacity of 50 A-h, and is fully charged. These are typical values.

If you leave the navigation lights on, the battery will be down to half-charge in a day. In two days it will be completely discharged.

(This is a bit simplistic as the current drain will be proportional to the battery voltage; as the voltage decreases so will the current.)

Even a very small current drain will discharge a battery in a relatively short time.

cruiser6003 posted 01-04-2009 01:52 AM ET (US)     Profile for cruiser6003  Send Email to cruiser6003     
I must respectfully disagree with Seahorse's assessment that amp-hour rating is not used much anymore.

For deep cycle batteries it is probably the most important measure. Both CCA and MCA are near useless to evaluate a battery that will be put through it's paces as a trolling motor or any type of house load.

Joel Wilkins
Columbia 45, #98
m/s Miss Magoo
S. Pasadena, FL

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