To most people, a battery is a battery. While marine and auto batteries usually look similar, the internal components of these batteries — as well as their design and intended purpose — is often very different.
Marine batteries come in three types: starting (or cranking) batteries, dual purpose batteries, and deep cycle batteries. In this guide, we’re going to explain the differences between these battery types, with a specific focus on deep cycle batteries.
Starting vs. Dual Purpose vs. Deep Cycle
Before we get into the mechanics of deep cycle batteries, it’s important to explain how they differ from starting and dual purpose batteries.
Starting batteries, which are also known as cranking batteries, are designed to discharge a large amount of energy for a short period. This makes them ideal for starting engines, such as an automotive engine or an inboard or outboard marine engine.
Instead of discharging a large amount of energy for a short period, deep cycle batteries are designed to discharge a small amount of energy over a long period, often until most of the battery’s capacity has been discharged.
One way to think of this difference is by visualizing a sprinter and a marathon runner. A starting battery performs like the sprinter, delivering lots of power before running out of breath. The deep cycle battery is the marathon runner, offering less speed but much greater endurance.
Dual purpose batteries are capable of starting engines and deep cycling, although they usually can’t match the performance of a dedicated deep cycle battery.
How deep cycle batteries work
The biggest difference between a deep cycle battery and a typical starter battery is the amount of energy it discharges and the way in which the energy is discharged.
Deep cycle batteries are designed specifically to discharge a large percentage of their capacity, in a process called “deep discharge.” Starter batteries, on the other hand, are only designed to discharge a small amount of energy at any time.
When a starter battery is deeply discharged, it can suffer damage that affects the battery’s total lifespan and ability to charge to capacity.
Most deep cycle batteries are built to discharge up to 75% of their capacity without suffering any damage. The “safe” amount of energy to discharge varies from manufacturer to manufacturer — some batteries are designed for a 45% discharge, while others can discharge 75% or more of their total energy capacity without any negative effects on long-term performance.
Uses of deep cycle batteries
Because deep cycle batteries are designed to discharge a small amount of energy over a longer period than starter batteries, they’re most commonly used for appliances and motors that require a steady, consistent supply of energy.
For example, a trolling motor — which uses electricity to power a propellor — works best using a deep cycle battery. For larger, more powerful trolling motors, several deep cycle batteries can be connected in series and used as a power source.
Deep cycle batteries are also used to power small vehicles, such as electrical wheelchairs, golf carts, and forklifts. Many of the instruments and navigational devices inside a boat draw energy from a deep cycle battery when the inboard or outboard motor is inactive.
Finally, deep cycle batteries — especially larger batteries — are often used as storage batteries for solar and other renewable energy systems.
Deep cycle batteries and amp hours
Most deep cycle batteries are rated in Amp hours (Ah) — a unit that measures the amount of energy the battery can supply over a long period. For example, a battery rated at 1Ah is capable of supplying a one amp current over a period of one hour.
Small deep cycle marine batteries usually have ratings of 25-50Ah, while larger batteries are usually rated at 100Ah.
While amp hours might seem like a measure of time, the actual amount of time a deep cycle battery can act as a power source depends on the current required by your trolling motor, or other electrical devices.
The greater the current, the faster the battery’s capacity will be depleted. This makes it vital to understand the amount of energy required by your trolling motor or electrical equipment before you select a deep cycle battery.
Do you need a deep cycle battery?
If you need to power a trolling motor, store energy generated by your renewable energy system or operate 12v appliances, a deep cycle battery is probably the right choice for you. Our guide to the best batteries for a trolling motor lists several reliable deep cycle and dual purpose batteries.