What is Depth of Discharge (DOD) in Lithium battery

Depth of Discharge (DoD) for a lithium battery refers to how much of its total capacity has been used before recharging. It is expressed as a percentage of the battery’s total energy capacity. For example, if a 100 Ah lithium battery is discharged to 20 Ah, its DoD is 80% (80 Ah used, 20 Ah remaining). The DOD measures the battery discharge percentage. Lithium batteries of all kinds of chemistry come with a Battery Management system, which can monitor and control the depth of discharge for the lithium-ion battery. The Depth of Discharge control is significant for the battery life. Nowadays, phones have settings for the battery where the maximum capacity of charge and depth of discharge of the lithium battery can be set in the phone settings.

In the same way, the Depth of Discharge becomes essential as the battery becomes older, and if we reduce the DOD of the battery as per the use, then the life of the battery can be further enhanced. So, all kinds of lithium batteries come with a life cycle based on the depth of discharge. The higher the depth of discharge, the lower the cycle one will get.

1. • For example, a lithium battery might offer:
     • 3,000-5,000 cycles at 80% DoD.
     • 2,000-3,000 cycles at 100% DoD.
2. The BMS Role: Nowadays, the smart BMS fully controls the depth of discharge in the Lithium-ion battery and is set up by the remote control, Bluetooth, or computer application.

1. The Load Factor: The load factor plays a significant role in a lithium battery’s Depth of Discharge (DOD), directly impacting its performance, lifespan, and efficiency. Here’s how the load factor influences DOD:
   

   1. Definition of Load Factor

   • The load factor refers to the ratio of the average load to the maximum load over a specific period. In the context of batteries, it represents how consistently and heavily the battery is used relative to its capacity.

   2. Impact on Depth of Discharge (DOD)

   • Higher Load Factor: If the load factor is high (i.e., the battery consistently operates near its maximum capacity), the DOD will likely be more profound. This means the battery discharges more of its stored energy before recharging.
   • Lower Load Factor: A lower load factor implies that the battery is used less intensively, resulting in a shallower DOD. The battery discharges only a tiny portion of its capacity before recharging.

   3. Relationship Between DOD and Battery Lifespan

   • Lithium batteries have a finite number of charge-discharge cycles, and deeper DOD (higher load factor) accelerates degradation. For example:
     • A battery cycled at 80% DOD will have a shorter lifespan than one at 50% DOD.
   • A lower load factor (shallower DOD) reduces stress on the battery, prolonging its lifespan.

   4. Thermal and Efficiency Considerations

   • A high load factor (deep DOD) can lead to increased heat generation, negatively affecting battery performance and longevity.
   • A lower load factor (shallow DOD) keeps the battery operating in a more efficient and stable temperature range.

   5. Practical Implications

   • Managing the load factor to optimise DOD is critical in applications like electric vehicles or renewable energy storage. For instance:
     • Limiting DOD to 50-80% can significantly extend battery life.
     • Using battery management systems (BMS) to control the load factor and prevent excessive DOD is essential.

   6. Trade-offs

   • While a lower load factor (shallow DOD) improves battery lifespan, it may require a larger capacity to meet energy demands, increasing costs.
   • A higher load factor (deep DOD) maximizes energy utilization but at the expense of reduced battery life.

2. 

   Depthofdischargecurveof51.2VlithiumLifePO4battery

3. • Voltage Profile During Discharge:
     • As a lithium battery discharges, its voltage gradually decreases. This nonlinear relationship depends on the battery chemistry (e.g., Li-ion, LiFePO4).
     • For example, in a typical Li-ion battery:
       • At 0% DOD (fully charged), the voltage is at its maximum (e.g., 4.2V for a single cell).
       • As DOD increases, the voltage drops, reaching a minimum threshold (e.g., 2.5V–3.0V) at 100% DOD (fully discharged).
     • The voltage drop is more pronounced at higher DOD levels due to increased internal resistance and reduced electrochemical activity.
   • Voltage Plateau:
     • Some lithium chemistries, like LiFePO4, exhibit a flat voltage plateau during most of the discharge cycle. This means the voltage remains relatively stable until near the end of discharge, where it drops sharply.

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     DOD

   • Impact of DOD on Voltage:
     • Deeper DOD (higher discharge levels) results in lower battery voltage, which can affect the performance of devices powered by the battery.
     • Operating at very low voltages (near 100% DOD) can also damage the battery, as it may lead to over-discharge.

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   2. Current Discharge and DOD

   • Discharge Rate (C-rate):
     • The discharge current is often expressed as a C-rate, the ratio of the discharge current to the battery’s capacity. For example, a 1C rate for a 10Ah battery means discharging at 10A.
     • Higher discharge currents (higher C-rates) lead to faster DOD increases as the battery depletes its energy more quickly.
   • Voltage Sag Under High Current:
     • The battery voltage drops significantly at high discharge currents due to internal resistance. This effect is more pronounced at higher DOD levels.
     • For example, a battery at 50% DOD may experience a more considerable voltage drop under high current compared to when it is at 20% DOD.
   • Capacity Reduction at High Currents:
     • High discharge currents can reduce the adequate capacity of the battery (lower usable DOD) due to increased losses from internal resistance and heat generation.
     • This means that at very high C-rates, the battery may reach its voltage cutoff threshold sooner, even if it has not fully discharged its theoretical capacity.

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   3. Interplay Between Voltage, Current, and DOD

   • Dynamic Relationship:
     • The voltage and current discharge characteristics are interdependent and vary with DOD. For example:
       • At low DOD (e.g., 0–20%), the battery voltage is high, and the voltage drop under load is minimal.
       • At high DOD (e.g., 80–100%), the voltage is lower, and the voltage drop under load is more significant.
     • This relationship is critical for designing battery management systems (BMS) that protect the battery from over-discharge and optimize performance.
   • Power Delivery:
     • The battery’s power output (P = V × I) decreases as DOD increases because both voltage and current capability are reduced.
     • This is particularly important in high-power applications like electric vehicles, where maintaining consistent power delivery is essential.

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   4. Practical Implications

   • Battery Management Systems (BMS):
     • A BMS monitors voltage and current to estimate DOD and prevent over-discharge, which can damage the battery.
     • It also ensures the battery operates within safe voltage and current limits at all DOD levels.
   • Capacity Estimation:
     • Voltage and current data are used to estimate the battery’s remaining capacity (DOD). However, this estimation becomes less accurate at high discharge rates or very low/high DOD levels.
   • Application-Specific Considerations:
     • Maintaining a moderate DOD (e.g., 20–80%) in applications like renewable energy storage helps preserve battery life and ensures stable voltage and current delivery.
     • Managing discharge currents and DOD is critical in high-power applications to avoid excessive voltage sag and capacity reduction.

   Battery Voltage and Current measurements:

   Depth of Discharge (DoD) Chart for 12.8V LiFePO4 Battery

   DoD (%)	Cycle Life (Approx.)	Voltage Range (12V System)	Example Usage
   20%	6,000–10,000 cycles	~13.2V–13.4V (resting)	Frequent shallow discharges (ideal)
   50%	4,000–6,000 cycles	~13.0V–13.2V (resting)	Balanced use (good lifespan)
   80%	3,000–5,000 cycles	~12.8V–13.0V (resting)	A common recommendation for daily use
   90%	2,000–3,000 cycles	~12.6V–12.8V (resting)	Occasional deep discharges
   100%	1,500–2,000 cycles	~10.0V–12.0V (resting)	Avoid regularly (shortens lifespan)

1. Typical DoD Range:
   • Lithium-ion batteries (e.g., LiFePO4) are often designed to handle a DoD of 80-90% without significant degradation.
   • Some high-quality lithium batteries can even tolerate a DoD of 100%, though this may reduce their lifespan over time.
2. Impact on Battery Lifespan:
   • The deeper the DoD, the more stress is placed on the battery, which can reduce its overall lifespan.
   • Shallow discharges (e.g., 20-50% DoD) can significantly extend the battery’s cycle life.
3. Cycle Life and DoD:
   • Lithium batteries have a higher cycle life than other chemistries (e.g., lead acid).
4. Optimal DoD for Longevity:
   • Operating a lithium battery within a 20-80% DoD range is generally recommended to maximise its lifespan.
5. Comparison with Other Batteries:
   • Lead-acid batteries typically have a much lower recommended DoD (around 50%) to avoid damage.
   • Lithium batteries are more resilient to deeper discharges, making them more efficient and versatile.
     • This article was written by Mr Kunwwer Sachdev, founder of Su-kam, known as the inverter man of India.