What is a Deep Cycle Battery?

 Before we dive into what is a deep cycle battery, it is important to understand some battery basics, such as what a cycle is, what depth of discharge means, and what deep battery discharge really means.

A complete cycle is considered to be the complete discharge and recharge of a battery. Discharge is measured by the amount of capacity removed from the battery - depth of discharge (DoD) is used to indicate how much battery capacity was used during a single discharge. Fully discharged is 100% DoD.

DoD is inversely proportional to the state of charge (SoC), which is the amount of charge remaining in the battery. 100% DoD = 0% SoC.

With this understanding of DoD and battery cycling, you may want to know what deep discharge really is. Deep discharge is classified as 80% to 100% of the battery's discharge capacity.

What is a Deep Cycle Battery?

A deep cycle battery pack is a battery that is designed to handle 80-100% DoD deep discharge on a continuous basis. Does this mean that high rate batteries cannot be used for deep discharge? Yes and no. One or two deep cycles will not damage the battery, but this is the key to battery design, because if cycled continuously, a high rate battery will be severely damaged over time in a profound way.

Types of Deep Cycle Batteries

In SLA (sealed lead-acid) batteries, electricity is generated in the electrodes. In high rate batteries, there are many thin plates that provide a larger surface area to produce energy quickly. In a deep cycle battery, the plates are thicker than those inside a high rate battery because the chemical reactions that generate energy enter the plates and therefore need to be very thick to handle the reactions. If the plates are thin, as they are in high magnification and starter batteries, the plates will degrade quickly and will not withstand multiple deep discharges over time.

The two subcategories of deep cycle SLA batteries are deep cycle gel and AGM (Absorbent Glass Mat) deep cycle. Gel cells use a substance that turns the electrolyte acid inside the cell into a thick gel so that it does not overflow. Compared to AGM, gels have better heat distribution and venting, and therefore better performance throughout their life cycle. In AGM cells, a glass mat is placed between the negative and positive plates to hold the electrolyte in place. The mat keeps the cell from overflowing and optimizes the surface area of the plates, resulting in better electrolyte distribution than in conventional cells.

In lithium deep cycle batteries​, energy cells are used. Energy cells are designed to provide continuous current for long periods of time, making them ideal for cycling and deep-cycle applications. In addition to the internal cell composition of the lithium battery, the circuitry is also important because the protection may limit how much current can be drawn from the battery. Different lithium batteries will have different levels of protection.

Applications

Before choosing a battery, you need to know exactly what equipment you need to power, when you need to power it, how often you need to power it, and how much power you need.

For example, medical carts used in hospitals will be away from the electrical outlet and will require battery power throughout the unplugging process. In this case, the nurse or doctor tells the battery when to use it by turning on the medical cart. It will be used for several hours each day. The battery needs to provide steady power for a long time, which means it is used in deep discharge, so a deep cycle battery is needed.

Another example of a deep cycle battery is the battery in a cell phone. Depending on the age, these batteries are designed to last a full day on a single charge and can be used every day. You can tell when the battery needs to be used by turning the phone on or unplugging it from the charger. If you're like most users, you'll wait until the phone is completely dead before plugging it back in - 100% DOD / 0% SOC. You want this battery to provide consistent power throughout the day, so you must use a deep cycle battery.

There are many other applications that utilize deep cycle battery technology, including marine, recreational (including RV batteries), mobility scooters, all types of electric vehicles, and solar applications.