The depth of discharge (DoD) significantly impacts the cycle life of a lithium-ion battery by affecting how many charge-discharge cycles the battery can undergo before its capacity degrades to unusable levels.
How DoD Affects Cycle Life
- Higher DoD leads to fewer cycles: The deeper a battery is discharged during each cycle, the fewer total cycles it can sustain before capacity fades. For example, at 100% DoD (fully discharging the battery), cycle life may be around 300 cycles for some lithium chemistries, whereas at 20% DoD (partial discharge), cycle life can extend to thousands of cycles (e.g., ~2,000 cycles at 20% DoD and even up to 15,000 cycles at 10% DoD for certain lithium-ion types).
- Battery life is roughly proportional to (number of cycles) × (depth of discharge): This means battery degradation depends not just on the number of cycles but also on how deep each discharge is. While deeper discharges reduce total cycle count, fewer cycles are needed to use the same total capacity because each cycle consumes more energy.
- Trade-off between DoD and total throughput: Doubling DoD roughly halves the number of cycles, but the total amount of energy cycled (and thus usable battery lifetime in terms of energy throughput) may not differ as drastically. For example, a battery discharged to 80% DoD might last about 700-900 cycles before capacity drops to 50%, while at 30% DoD it might last around 1,950 cycles to the same capacity loss. However, the total energy delivered over the battery’s life is similar because deeper discharges deliver more energy per cycle.
- Deeper discharges increase stress and aging: High DoD subjects the battery to more stress, accelerating capacity loss due to chemical and mechanical degradation inside the cells.
Optimal Battery Use for Longevity
- Partial discharges (low to moderate DoD) prolong battery life by reducing mechanical and chemical stress.
- Cycling in the mid-state-of-charge range (avoiding very high or very low charge levels) is ideal for maximizing cycle life.
- Other factors such as elevated temperature and high charge/discharge rates also affect cycle life but DoD remains a primary factor in degradation.
Summary Table of Approximate Cycle Life by DoD for Lithium-ion types (NMC, LiPo)
| Depth of Discharge | Approximate Cycle Life (NMC) | Approximate Cycle Life (LiPO4) |
|---|---|---|
| 100% | ~300 cycles | ~600 cycles |
| 80% | ~400 cycles | ~900 cycles |
| 60% | ~600 cycles | ~1500 cycles |
| 40% | ~1000 cycles | ~3000 cycles |
| 20% | ~2000 cycles | ~9000 cycles |
| 10% | ~6000 cycles | ~15000 cycles |
These figures illustrate the strong inverse relationship between DoD and cycle life in lithium-ion batteries.
In conclusion, increasing the depth of discharge decreases the total number of charge-discharge cycles a lithium-ion battery can sustain, thus reducing its overall cycle life. However, due to higher energy throughput per cycle at deeper DoD, the total usable energy over the battery’s lifespan may be comparable within reasonable DoD ranges. Managing DoD by favoring partial discharges is a key strategy to prolong lithium-ion battery life.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/how-does-the-depth-of-discharge-dod-impact-the-cycle-life-of-a-lithium-ion-battery/
