A guide to marine batteries
Tom Vondrasek | 6th Dec 2023 | 6 minutes to read
Since boats or watercraft have been using internal combustion engines of a large size, they have required batteries to operate. This includes outboard motors unless they are around 20 horsepower (hp) or less, with smaller motors like this using an old-style manual pull-starter and therefore requiring no battery.
These types of motors are used to propel small boats, tinnies, or dinghies and, once running, these engines have their own electrical power source.
Outside of these smaller motors, marine batteries are a necessary part of boat ownership, and understanding how they operate can help you to make smart decisions if you ever have to replace your boat’s battery.
Below are some important topics to understand when looking for a marine battery:
Types of Boat Motors
Boat motors come in two main types:
Inboard Motor
These sit inside the boat and come in all shapes and sizes. They can be petrol or diesel and can be anything from a specially designed boat motor to a car engine adapted to run in a boat. Engine size varies according to boat size and the boat’s purpose — a boat designed for water skiing will have a different motor to a cabin cruiser that is used for fishing and pleasure cruising.
Outboard Motor
These sit on the outside of the boat at the back and are specially designed for boats. Some larger boats have more than one outboard motor. These can be petrol (both 2 and 4 stroke), diesel, and even full electric for some of the smaller outboards. By far the most common, however, is the petrol engine.
The vast majority of these boats require a start battery to kick the engine into life. They need to be dependable and reliable as when you are a long way from shore and the weather starts getting rough, you want the security of knowing the battery will do its job.
Marine Batteries Explained
Like a car start battery, a marine battery is a plastic box with 2 terminals (+ & -) that contains an acid (electrolyte) and lead plates. This box generates electricity and is a standard lead acid battery (wet cell).
If your boat uses a petrol or diesel engine you will still require a battery to run it.
Once it starts, the alternator or generator kicks in and provides the electrical power. Boat motors, especially outboards, have their own version of an alternator/generator that also recharges the battery, so you are ready for your next start.
Built Tough
Boat start batteries need to be built to be especially resilient, due to the harsh conditions they operate in — more like a 4x4 off road battery than a standard car battery. Offroad vehicles suffer from violent and harsh forward and back, side to side movements when travelling on tracks, and water has the same effect — except you can go up and own as well!
Wave pounding is also constant, as the waves keep coming. For this reason, marine batteries are built stronger as they need to take this punishment without cracking or breaking.
Electronics
Marine batteries have one primary role and that is to start the engine. In today’s modern world they also run fish finders and marine radios, as well as charge your mobile phone or other electronic devices. For this reason, some are a combination of a start battery and a deep cycle battery.
When selecting a marine battery, it’s important to think about what you’ll be using your boat for.
If you have a modern engine, whether it is inboard or outboard it will have some form of electronics and engine management. This is not as critical as a vehicle as it doesn’t need to run a lot of ancillary functions like central locking. If it does, bigger boats may have a separate deep cycle battery and the start battery simply starts the engine.
Marine Battery Power
Being multifunctional marine batteries have a number of useful ratings.
CCA - Cold Cranking Amps
This is the rating for starting the engine. The larger the engine the more CCA is needed.
- Cold – The engine is cold or has not been started. This takes more power than a warm engine.
- Cranking – Rotating or turning over the engine
- Amps – The Ampere is an electrical measurement for current flow. The greater or higher the number of Amps, the more electricity flows down the cable.
For more information on CCA, see our article: Get To Know Your Car Battery.
Ah – Amperes Per Hour
Marine batteries are a power source or power supply designed to run portable 12 Volt DC (Direct Current) electrical equipment. The rating on the front of a marine battery is in Ah or Amperes per Hour (Amps per Hour); this is the maximum number of amps or current the battery can flow in an hour.
So, if it has 100Ah rating it can flow up to 100 Amps of electricity in 1 hour. If there was a 100 Amp flow, then unfortunately, it would be dead flat in 1 hour. If there was a 10 Amp flow, it would be dead in 10 hours.
For more information on Ah, see our article: A Guide to Deep Cycle Batteries.
RC – Reserve Capacity
This rating is for how long in minutes a 12 Volt battery can supply 25 Amps of power before it drops below 10.5 Volts. This is approximately how long your battery can run your engine if the charging system fails and your engine’s electricity supply is from the battery only. The measurement is taken when the battery is fully charged.
Selecting the Correct Marine Battery for Your Boat
Selecting the correct marine battery is not so difficult when your old one needs replacing. If you have an old one and it was doing OK before its life cycle ended, replace it with the same or something similar. If you want to run more electronics, you can simply get a battery with an increased Ah rating.
However, don’t go overboard here as the charging system may not cope.
It is also important to check what the charging system can do before upgrading the battery.
A few things to note are:
Warranty – Marine batteries can have different warranty periods, so the longer it is, the higher the price of the battery.
CCA/Ah/RC - Increasing some or all of these increases the battery price and size.
Replacing a Marine Battery
When it comes to replacing a marine battery always check your owner’s manual. Most are extremely simple and straight forward and require basic hand tools to take out and replace the battery. When buying, make sure the terminals are in the same spot as your old battery so the cabling will fit.
If you have a cabin cruiser that runs a lot of ancillary items, then you definitely need to check before fully disconnecting the old battery. If you’re still unsure, then check with a specialised boat mechanic to get an expert opinion.
