Depth of discharge (DoD) and effect on battery life

Each rechargeable battery has a nominal capacity that represents the maximum amount of energy stored, i.e. 100%. However, this capacity cannot be fully used. At both the upper (100%) and lower (0%) capacity limits, critical points occur that can cause destruction of the battery cells. This phenomenon includes dendrite formation (short circuit between anode and cathode) or accelerated battery degradation.

What is the depth of discharge (DoD) of a battery?
Depth of Discharge (DoD) refers to the percentage of a battery's capacity that is discharged relative to its total rated capacity. For example, a DoD of 80% means that the battery has been discharged to 80% of its total capacity and the remaining 20% is the reserve.

To maximize battery life, it is crucial to monitor not only the maximum charge and discharge currents (C), but also the DoD value. A higher DoD means higher battery capacity utilization, but it also accelerates battery degradation.

Impact of DoD on battery life
In general, the lower the DoD, the longer the battery life. This is due to the reduced exposure of battery cells to extreme conditions that increase the risk of degradation.

Battery life at different DoD values:

• DoD 50%: The longest lifetime, suitable for applications with a long-term horizon where stability is a priority.
• DoD 80%: A compromise between capacity utilization and lifetime, ideal for most standard applications.
• DoD 95%: Maximum capacity utilization but significantly reduced lifetime. Used for applications where high capacity is key, but with an awareness of faster degradation.

Examples of DoD in practice

• Domestic battery systems: A DoD of around 80% is recommended to balance capacity and lifetime.
• Industrial systems: A DoD of 90% is suitable for applications with higher available energy requirements.
• Commercial systems: Some customers choose DoD up to 95%, but this requires better quality systems and monitoring.

Optimal DoD value
The most common compromise between lifetime and available capacity is a DoD between 80% and 90%. For example, for a 1 MWh battery, this means a realistic usable capacity of 800-900 kWh. This value provides a large enough buffer to protect the battery cells while ensuring a reasonable usable capacity.

For systems designed for SVR (Short Voltage Response) applications, some customers require DoD settings of up to 95%. However, this approach is risky because it leaves a lower buffer of only 2.5%. This can lead to faster degradation and deterioration of the SoH (State of Health) value.

The depth of discharge (DoD) setting is a key factor in maximizing battery life. Too high a DoD value can shorten battery life, while too low a value reduces usable capacity. The key is to find the optimal value that strikes a balance between capacity and long-term sustainability. it is important to consider the specifics of the application and to prefer the quality of the battery storage.


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