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ContinuousWave: Small Boat Electrical
Standard Battery Size for 20 Outrage
|Author||Topic: Standard Battery Size for 20 Outrage|
posted 01-12-2014 11:18 PM ET (US)
As I have explained in another thread, I am currently switching over to a dual battery configuration. As part of this I will be purchasing either 2 new or one additional battery. Without checking first, I ordered 2 Group 27 Battery boxes because as far as I knew that was the common size in these size boats. I guess I was wrong because much to my surprise when I looked at the existing battery, it is an Interstate SRM-24 Battery. I have not checked the sizing yet to see if there is a difference. I guess what I would like to know is- should I buy another Group 24 battery or replace with (2) new Group 27 batteries? I know the two should not be mixed in a dual battery environment. I am using both these for cranking and house power. Thanks for any help!
posted 01-13-2014 11:28 AM ET (US)
The designation of a battery as belonging to a particular group, for example, a Group-24 battery, is a way to describe the physical dimensions of the battery, the location of the battery terminals, and the type of battery terminals. These designations do not describe the electrical properties of the battery. For example, they do not describe the storage capacity of the battery or its ability to provide a certain current output.
The best method for choosing a battery for use on a boat is to find the recommended minimum cranking current Amperes needed by the outboard engine for starting. For example, a particular engine may specify it needs a battery rated for 800-Amperes of cranking current. If you are boating in moderate temperatures you can use the marine cranking ampere (MCA) rating for a battery. If you will be boating in freezing conditions, use the cold cranking ampere rating, or CCA rating.
A Group 27 battery has different dimensions from a Group 24. I think if you compare them you will find that a Group 24 battery could fit into a Group 27 sized battery box, although perhaps a bit loosely. The Group 27 is about 1.75-inch longer than a Group 24.
The Interstate Battery model SRM-24 is a deep-cycle battery with a rating of 550-CCA and 690-MCA.
I don't have much concern about using two batteries of different Group size together. The concern is more about electrical characteristics. If both batteries are of the same type and age, they can likely be charged in parallel or operated briefly in parallel without too much concern.
posted 01-14-2014 02:25 PM ET (US)
On most small boats with outboard engines, the principal purpose of the battery is to permit the electric starting of the outboard engine. In this regard, the battery must be sized for the outboard engine, and not for the boat. The engine starting battery also usually runs the power trim and tilt system, and, if equipped, a power steering system, and trim tabs.
Considering an electrical system with an isolated battery, separate from the engine starting battery, that will just power house loads, we can question whether the size of the boat is a reasonable way to select a battery. The larger a boat, the more house loads it is likely to have. However, individual boats can vary greatly in their house loads.
If a small boat is used for fishing, it may have many added house loads, such as pumps for circulating water into bait wells or fish boxes, electric propulsion motors for trolling or maneuvering at low speeds, power winches for controlling down rigger lines, and so on. The total electrical power needed can be quite variable from boat to boat, so it is not really possible to select with any certainty a battery with a certain electrical capacity based on the length of the boat.
The best approach to selection of a house battery is to tally the non-engine electrical loads, and estimate the amount of current consumed by time, usually calculated in Ampere-hours. One must then consider if this consumption will occur when the main engine is operating or when it is shut off. A power consumption plan can be devised, leading to a selection of a house battery with storage capacity that will provide a reasonable number of hours of operation of the house loads when the engine is not running, and leading to a reasonable re-charging of the house battery when the engine is running.
The capacity of a battery to store electricity and delivery it later is described by the Ampere-hour capacity of the battery, or, sometimes, by a so-called Reserve capacity.
An Ampere-hour is a unit of 1-Ampere of current supplied for 1-hour. The capacity of a battery to delivery stored electrical energy is also a function of how rapidly the current is to be delivered. Usually an Ampere-hour rating is based on a current that can be delivered for 20-hours, and for a battery terminal voltage at the end of the discharge of 10.5-Volts. For example, if a baterry is rated as a 40-Ampere-hour battery, this suggests that it can provide 2-Amperes for 20-hours. This does not mean it can also deliver 10-Amperes for 4-hours. For more explanation see
The rating for Reserve Capacity is usually understood to be the number of minutes the battery can deliver 25-Amperes and still maintain a useful voltage, usually considered to be 10.5-Volts.
For more terms and definitions, see
posted 01-14-2014 09:12 PM ET (US)
Thanks for all the information Jim. The engine specs state I need 630 MCA/ 490 CCA. Does it matter if I go much over this rating as long as I meet or exceed the specs?
The existing #24 Battery is 690/550 and it looks like the Group 27 is 750/600. At this point I am leaning towards sticking with the SRM-24 and adding an additional one of the same. The roughly 1.5" less in length will give me a little more room under the console and I can use every spare inch. The 24 has a bit less run time than the 27 but since I will have 2 batteries to share starting and house, I see that as less of an issue. I would love to go AGM but not sure I can make that investment right now.
posted 01-15-2014 11:04 AM ET (US)
If you use a battery that can delivery more Amperes than the minimum, the only harm that occurs is to your wallet. Actually, to have some margin in the battery rating is a good idea. As a battery ages it will tend to lose some of its capacity. By having extra capacity when new, the battery will have a longer useful life in your application.
posted 01-18-2014 01:35 AM ET (US)
Thanks Jim. I may reconsider and just go with the 27s for the extra runtime.
posted 01-18-2014 09:30 AM ET (US)
The Group size tells nothing about electrical properties.
posted 01-19-2014 12:31 PM ET (US)
I understand that is the physical size but the Group 27s do appear to have a bit more run time than the Group 24s at least when it comes to the Interstate battery chart I have been looking at. As always, thanks for your valuable and detailed advice!
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