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Marine Battery Basics

Understanding batteries and their use has become a very important part of boating. Batteries are necessary to start your boat and to run all the electrical devices on board that help make boating easier and more fun. In this Navigator we start with the basics of batteries and then explore the different applications of batteries used on boats.

What is a Battery?

Deep Cycle Gel Battery from VictronFirst off, let's cover some basic information about how batteries work. Sometimes it's helpful to think about batteries like the money in your bank account. If you want to be able to pay your bills, you need to put money into the account. If you keep taking money out without adding any money to your account, there will soon be no money left when you need it the most. Batteries are similar in that there is a limited amount of energy (measured in amps), so we need to add amps back in or risk not having any left when we need them.

At their most basic, batteries store electrical energy chemically – so that the power can be available when you need it. Marine batteries are typically comprised of several cells with each containing three parts – a positive electrode, and a negative electrode immersed in a liquid (or gel) called an electrolyte (usually an acid). When electricity is applied, a chemical reaction causes ions to flow through one way and electrons to flow in the opposite direction. This movement causes electric current to flow through the cell and through the circuit to which it is connected.

Battery Applications

There are three types of batteries for boats:

Optima SC34A Start BatteryStarter Batteries – These batteries are used for starting your engine – though on many boats they are wired in such a way that they can be combined with your house bank in an emergency. Starter batteries (also called cranking batteries) have thinner plates with greater surface area to give a fast current surge (between 75-400 amperes, for 5-15 seconds) for cranking an engine. They are capable of being quickly recharged by your engine's alternator, but are not meant to maintain a high power output for long periods as deeper discharges will decrease the lifespan of these batteries. This is why they are typically not used for anything other than starting your engine. Starter batteries list their Marine Cranking Amps (MCA) or Cold Cranking Amps (CCA) to indicate their starting power – so make sure you always have more cranking amps than your engine manual calls for.

AGM Batteries from LifelineDeep Cycle Batteries – These batteries are used to run everything else on your boat that requires power – be it your refrigerator, your stereo, lights or your instruments while cruising – and are usually referred to as your “house bank”. To go back to our banking analogy – these batteries are where the majority of your energy (money) is stored for your future needs – a little like a savings account. Deep cycle batteries are constructed with thicker, heavier plates that can be discharged more deeply over longer periods of time. They can be discharged down to 50% without damage, but you should try not to take them down that low very often. It's best to size your battery bank to contain at least 3-4 times the amount of energy (amp hours) you think you may require between charge cycles so that you are usually only using about 25% of the potential energy stored.

Dyno Dual Purpose M24M Battery Dual Purpose Batteries – Based on the name, it's probably obvious that these batteries fall somewhere in between the other two types of batteries. They are best used in small boat applications where you will only have one battery on board (instead of two separate banks of batteries) – usually because space in an issue and you need the best of both worlds in one battery.

To learn more about additional battery topics, click on the links below:

Choosing the Right Marine Battery

How to Size a Marine Battery Bank

Charging a Marine Battery

Marine Inverter Basics

We hope this Navigator has helped you understand batteries and the job they perform on your boat. If you have further questions, please don't hesitate to contact our battery experts at (800) 426-6930.