Every rechargeable battery (accumulator) has a nominal capacity, representing the maximum amount of storable energy, which is 100%. However, this capacity cannot be fully utilized. At both ends of the capacity spectrum – the upper (100%) and the lower (0%) – critical points occur that can lead to damage to battery cells. This phenomenon includes dendrite formation (short-circuit between the anode and cathode) or accelerated degradation of the battery.
What is Battery Depth of Discharge (DoD)?
Depth of Discharge (DoD) refers to the percentage of a battery’s capacity that has been discharged relative to its total nominal capacity. For instance, a DoD of 80% means that the battery has been discharged to 80% of its total capacity, leaving 20% as a reserve.
To maximize battery lifespan, it is crucial to monitor not only maximum charging and discharging currents (C) but also the DoD value. Higher DoD results in higher utilization of the battery’s capacity but also accelerates its degradation.
Impact of DoD on Battery Lifespan
In general, the lower the DoD, the longer the battery lifespan. This is because battery cells are less exposed to extreme conditions, which reduces the risk of degradation.
Battery Lifespan at Different DoD Levels:
- DoD 50%: Longest lifespan, suitable for long-term applications where stability is a priority.
- DoD 80%: A balanced compromise between capacity utilization and lifespan, ideal for most standard applications.
- DoD 95%: Maximum capacity utilization but significantly reduced lifespan. Used in applications where high capacity is critical but with awareness of faster degradation.
Practical Examples of DoD Usage:
- Home Battery Systems: A DoD of around 80% is recommended for a balanced trade-off between capacity and lifespan.
- Industrial Systems: A DoD of 90% is suitable for applications with higher energy demands.
- Commercial Systems: Some customers opt for a DoD of up to 95%, but this requires advanced systems and continuous monitoring.
Optimal DoD Value
The most common compromise between lifespan and usable capacity is a DoD in the range of 80% to 90%. For instance, in a battery with a capacity of 1 MWh, this equates to a usable capacity of 800–900 kWh. This value provides a sufficient buffer to protect battery cells while ensuring reasonable capacity utilization.
For systems designed for SVR (Short Voltage Response) applications, some customers require a DoD of up to 95%. However, this approach is risky as it leaves only a 2.5% lower buffer. This can lead to faster degradation and deterioration of the State of Health (SoH) value.
Graphic representation:
1. The relationship between the DoD value and the number of cycles
The following graph shows how different DoD values affect the total number of cycles a battery can handle. Higher DoD values result in lower cycle counts, illustrating the need to find a compromise.
2. Risk of degradation at high DoD
The table compares SoH values after 1000 cycles at DoD 80%, 90%, and 95%.
| DoD | Number of cycles | SoH after 1000 cycles |
|---|---|---|
| 80 % | 5000+ | 90 % |
| 90 % | 3000–4000 | 80 % |
| 95 % | 2000–3000 | 70 % |
3. Recommended DoD by application
- Households: 80–85%
- Industrial applications: 80–90%
- SVR application: 90–95%
Conclusion
Setting the depth of discharge (DoD) is a key factor in maximizing battery lifespan. Excessive DoD can shorten the battery’s lifespan, while too low a value reduces usable capacity. The key is to find an optimal value that ensures a balance between capacity and long-term sustainability. When configuring parameters, it is essential to consider the specifics of the application and prioritize high-quality battery systems.