What Is a Deep Cycle Battery


Staff member
A deep cycle battery is a rechargeable battery designed to have the majority of their capacity used before being recharged. A deep cycle refers to one complete charging and discharging of the battery. Battery designs generally assume a number of discharge-charge cycles a rechargeable battery can experience before its capacity falls below a pre-determined percentage of its initial rated capacity. Deep cycle batteries are designed for discharge cycles to a depth of discharge (DOD) as high as 80% on a regular basis. Deep cycle batteries are also characterized by the ability to provide continuous power over longer periods of time, instead of delivering quick bursts of energy (such as starting engines) as with SLI (Starting, Lighting and Ignition) batteries.

The ability to deliver a steady, continuous output with long cycle life enables the widespread use of deep cycle batteries in consumer electronics, recreational and industrial applications. Available in many sizes and types, deep cycle batteries may be found in mobile phones, laptop computers, digital cameras, flashlights, headlamps, headphones, two-way radios, power tools, medical devices, telecommunications systems, wireless security systems, solar power systems, emergency lighting systems, uninteruptable power supplies (UPS), golf buggies, electric wheelchairs, caravans, electric scooters, hybrid-electric vehicles, plug-in hybrid-electric vehicles and light-electric vehicles.

There are many deep cycle battery chemistries available in the market today. Some of the chemistries used in deep-cycle applications are: lead-acid, nickel cadmium (NiCd), nickel-metal-hydride (NiMH), and lithium-ion (Li-ion). All Li-ion, NiCd and NiMH batteries are deep cycle batteries meaning the batteries can be deeply discharged with a controlled current and can be repeatedly cycled. NiMH batteries deliver similar battery characteristics of NiCd. Even still, NiMH batteries have been losing market share because they are not as cheap as lead-acid nor as energy dense as lithium-ion chemistries. Lithium-ion batteries are by far the best-performing deep-cycle battery technology as they offer the highest energy densities, the highest charging efficiency, the longest cycle life, and the lowest weight of all battery chemistries.

Despite the availability of other technologies, the term “deep cycle” is most often applied to lead-acid batteries. Lead acid chemistry is the oldest rechargeable battery technology which has remained virtually unchanged since it was invented. Lead acid batteries have a low specific energy of 30–50 Wh/kg and energy density of 50-90 Wh/l, whereas lithium ion batteries can have a specific energy as high as 300 Wh/kg and an energy density as high as 600 Wh/l. For larger power applications where weight is of little concern, however, lead-acid batteries are the usual choice because they have a price-to-power ratio superior to all other types.

A lead acid battery is constructed with a lead dioxide (PbO2) positive plate, lead (Pb) negative plate, and an electrolyte solution containing sulphuric acid. Lead acid batteries can be divided by construction type into two categories: flooded (or wet) lead-acid batteries, and sealed lead-acid (SLA) batteries. A flooded lead-acid battery, sometimes referred to as a vented lead-acid battery, has its electrodes immersed in the electrolyte solution. The battery is characterized by a vented design which allows the oxygen created at the positive electrode and the hydrogen created at the negative electrode during charging to be released from the cell. A sealed lead-acid battery is sealed to the atmosphere and uses a recombination reaction to prevent the escape of hydrogen and oxygen gases normally lost in a flooded lead-acid battery. SLA batteries are also called valve-regulated lead-acid (VRLA) batteries because a pressure-relief valve is used in these system. VRLA batteries are further classified by their technique for immobilizing the electrolyte, of which there are two types: gel batteries and absorbed glass mat (AGM) batteries.

Sealed and flooded deep-cycle lead-acid batteries are able to withstand discharges to 50% and 80% DOD, respectively. A deep cycle battery has thicker lead plates which allow this type of battery to survive more discharge cycles, as compared to car batteries. While flooded lead acid batteries have lower upfront costs and can generally be discharged (and charged) at high rates of current without harm, they require regular electrolyte maintenance (addition of water) to compensate for lost hydrogen and oxygen. On the flip side sealed VRLA batteries are maintenance-free as gases generated during charging are recombined into water in a so-called “oxygen cycle”. However, controlled charging (including effective charge termination) is required to reduce the negative effects of overcharging.