What is the depth of discharge and why is it so important?

Depth of discharge

As batteries can provide additional benefits for solar system buyers, batteries are becoming a popular accessory for solar systems. When the grid fails, batteries can provide backup power advantages, improve the utility of off-grid systems, and improve solar energy economy when your net metering policy or time of use (TOU) rate is not ideal.

More and more countries and utility companies are providing incentives for installing battery storage. With an increasing amount of options, there are more and more questions about how to compare batteries: which is the best, or which is best for you? One of the most important factors to consider when purchasing a spare battery for your home is the depth of discharge (DoD).

What does the battery’s depth of discharge (DoD) represent?

DOD, also known as “depth of discharge”, is used to describe the depth to which the battery is discharged or should be discharged. If we say that the battery is 100% fully charged, it means that the DOD of the battery is 0%.

The depth of discharge (DoD) of a battery indicates the percentage of a discharged battery relative to the total capacity of the battery. The depth of discharge is defined as the discharge capacity of a fully charged battery divided by the battery’s nominal capacity. The depth of discharge is usually 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 a supplement to the state of charge: one increase, the other decreases. Although the state of charge is usually expressed as a percentage (0% = empty; 100% = full), the depth of discharge is usually expressed in Ah units (for example, 0 is full, 50 Ah is empty) or percentage (100% is empty, 0% is full). The capacity of the battery may be higher than its nominal rating. Therefore, the depth of discharge value may exceed the nominal value (for example, 55 A h for a 50 A h battery, or 110 %).

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

The higher the frequency of battery charging and discharging, the shorter its service life. It is generally not recommended to completely discharge the battery, as this will greatly shorten the battery life. Many battery manufacturers specify the maximum recommended DoD for best performance.

depth of discharge 1

Cycle life

Cycle life is the number of charge/discharge cycles that a battery can maintain during its service life, depending on the battery capacity you normally use. If you regularly discharge at a lower percentage of power, it will have more useful cycles than when you often drain the battery to the maximum DoD. Depending on the depth of discharge and operating temperature, a typical lead-acid battery can provide 200 to 300 discharge/charge cycles. The main reasons for its relatively short cycle life are the corrosion of the positive grid, the exhaustion of active materials and the expansion of the positive plate. These changes are most common at higher operating temperatures. Cycling does not stop or reverse the trend.

If a lead-acid battery is discharged/used to 100% of its storage capacity every time it is used, its electrolyte will rapidly degrade, and the battery may not be able to retain energy during charging as before if it only discharges up to 50%.

This is why the term “cycle life or cycle” (with the qualifier “at X% DoD”) is used on the battery information board.

The relation between DOD and cycle life 2

Another factor affecting battery life

Today, most solar cells are guaranteed to last for 3 to 10 years. This partly depends on how often the battery is discharged and the number of cycles it goes through, but there are other factors that can have a significant impact.

Another factor that affects battery life is how well you maintain the battery, especially its storage temperature. The battery may overheat in a hot environment (above 30 degrees Celsius), thereby shortening the battery’s service life. Extremely cold temperatures can also have a negative impact on the battery, because it must work harder and charge at a higher voltage. In order to maximize the life of the battery, try to place it in a relatively mild environment-not too hot or too cold.

Which battery is the right option for you?

When comparing battery options, the depth of discharge is important, but it is not the only factor in the evaluation.

Outside of the Department of Defense and cost, one of the most important things to consider is whether the battery is suitable for your specific situation. Small batteries will be cheaper, but if you just want to reduce demand costs or avoid expensive use time, the effect is best. If you want to get off the grid, you need batteries that can store as much energy as possible (or, multiple smaller batteries stacked together). Homeowners with frequent (and prolonged) power outages can also consider buying a large battery to give you peace of mind.

best option battery manufacturer 3


When your customers ask about deep loop applications, make sure you ask the right questions.

All battery manufacturers recommend keeping the depth of discharge (DOD) below the maximum limit of 100%. Ideally, 80% DOD or less is recommended. This means that when using the battery, you should make sure that you only use less than 80% of the power before recharging it. In fact, since it is better to avoid deep-cycle discharge, the more frequent the charge, the better. It’s better to “use after full (full) charge” rather than “empty charge”.

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