What is Depth of Discharge and why is it so important?

A battery’s depth of discharge (DoD) indicates the percentage of the battery that has been discharged relative to the overall capacity of the battery. Depth of Discharge is defined as the capacity that is discharged from a fully charged battery, divided by battery nominal capacity. Depth of discharge is normally expressed as a percentage. For, example, if a 100 A h battery is discharged for 20 minutes at a current of 50 A, the depth of discharge is 50 * 20 / 60 / 100 = 16.7 %.

The depth of discharge is the complement of state of charge: as one increases, the other decreases. While the state of charge is usually expressed using percentage points (0 % = empty; 100 % = full), depth of discharge is usually expressed using units of Ah (e.g, 0 is full and 50 A h is empty) or percentage points (100 % is empty and 0 % is full). The capacity of a battery may be higher than its nominal rating. Thus, it is possible for the depth of discharge value to exceed the nominal value (e.g., 55 A h for a 50 A h battery, or 110 %).

In most battery technologies, such as lead-acid and AGM batteries, there is a correlation between the depth of discharge and the cycle life of the battery. 

The more frequently a battery is charged and discharged, the shorter its lifespan will be. It’s generally not recommended to discharge a battery entirely, as that dramatically shortens the useful life of the battery. Many battery manufacturers specify a maximum recommended DoD for optimal performance.

Cycle Life

Cyclic life is the number of charge/discharge cycles are battery can sustain in its useful life and depends on how much of the battery’s capacity you normally use. If you regularly discharge the batteries at a lower percentage amount, it will have more useful cycles than if you frequently drain the battery to its maximum DoD. Depending on the depth of discharge and operating temperature, the typical lead-acid battery provides 200 to 300 discharge/charge cycles. The primary reason for its relatively short cycle life is grid corrosion of the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Cycling does not prevent or reverse the trend.

Another factor that affects your battery’s lifespan

Another factor affecting the lifetime of your battery is how well you maintain it, and more particularly the temperature it’s kept in. Batteries in a hot environment (over 30 deg C) may overheat, which shortens the lifetime of the battery. Very cold temperatures also have a negative impact on the battery, because it has to work harder and at a higher voltage to charge. To maximize your battery’s useful life, try to keep it in a relatively mild environment – not too hot and not too cold.

Different manufacturers and technologies can impact DoD performance

A good quality wet, deep cycle battery, that is properly charged and maintained will deliver best value for cycle life/DoD ,however many users require a low maintenance sealed option.

Out of our product offerings, the Deka Dominator would have the longest service life at High DoD’s. The Gel technology is well proven hence their use in mobility and heavy service applications.

The Lifeline AGM’s also have exceptional high DoD performance, as do the Deka Intimidators.

A US or Euro manufactured ,sealed ,battery will generally outlast and outperform the Asian product offerings.This is due to the quality of manufacturing process , the raw materials used and the flexibility of use, for example, high current charging, hot enviroments.

Ensure you ask the right questions when your customer is enquiring about Deep Cycle applications.

The REMCO is a great battery in a camper trailer or caravan ,moderate DoD needs and low charging currents (<25 Amps), but would be unsuitable in a power boat engine room,with heat, high DoD’s and large charging currents.In these case, the Lifeline or Deka products will perform far better.

 Happy Selling.