Understanding Battery Performance in Off-Grid Systems: Days of Autonomy, Depth of Discharge, State of Charge & State of Health
- JVC Energy Team
- Mar 8
- 3 min read
When designing an off-grid solar power system, battery performance is one of the most critical factors. The efficiency, longevity, and reliability of your energy storage depend on understanding four key concepts: Days of Autonomy, Depth of Discharge (DoD), State of Charge (SoC), and State of Health (SoH).
In this blog, we break down each term and explain how they impact your battery system.
Days of Autonomy: Planning for Energy Security
Days of autonomy refer to the number of days your battery system can supply power without solar generation. This is crucial for ensuring energy reliability during extended cloudy periods or unexpected increases in energy demand.
Key Factors Affecting Days of Autonomy:
Load Requirements: The more energy your system consumes daily, the fewer days of autonomy you’ll have.
Battery Capacity: Larger battery banks provide more backup power.
Solar Resource Availability: Locations with frequent cloud cover require higher autonomy to compensate for lower solar production.
Backup Systems: Incorporating a generator can reduce the need for large battery banks by supplying power when needed.
Standard Recommendation: To be compliant with AS/NZS 4509, an off grid system without a generator backup requires a minimum of 3 days of autonomy.
Depth of Discharge (DoD): Maximizing Battery Lifespan
Depth of Discharge (DoD) represents how much energy has been used from the battery relative to its total capacity. It’s expressed as a percentage.
For example, if a 10kWh battery discharges to 3kWh, it has a 70% Depth of Discharge (DoD).
Why Does DoD Matter?
Battery Lifespan: The deeper a battery is discharged, the fewer cycles it can complete. A battery with a 90% DoD will wear out faster than one limited to 50% DoD.
Manufacturer Guidelines: Many lithium batteries recommend a DoD of 80-90%, while lead-acid batteries should ideally stay below 50% to extend life.
Energy Availability: A lower DoD setting means more stored energy remains as a backup, reducing the risk of complete depletion.
Best Practice:For Lithium-ion (LiFePO4) batteries, maintaining a DoD between 70-90% balances performance and longevity. For lead-acid, staying under 50% is recommended.
State of Charge (SoC): Real-Time Battery Monitoring
State of Charge (SoC) is the opposite of Depth of Discharge—it represents the percentage of charge remaining in the battery.
For example, if a 10kWh battery has a SoC of 40%, it means 4kWh of energy is still available.
Why SoC is Critical:
Prevents Over-Discharging: Allowing a battery to reach 0% SoC can damage cells and shorten lifespan.
Optimizes Charging Strategies: Charging batteries before they reach low SoC levels helps maintain long-term performance.
Enhances System Control: Smart inverters and battery management systems (BMS) use SoC data to optimize energy distribution.
Best Practice:Lithium-ion batteries are most efficient when operated between 20-90% SoC to prevent overcharging or deep discharge.
State of Health (SoH): Evaluating Long-Term Battery Performance
State of Health (SoH) measures the overall condition of a battery compared to its original capacity. It is typically displayed as a percentage.
For example, a battery with an 80% SoH means it can only hold 80% of its original capacity due to natural degradation.
Factors That Reduce SoH:
Excessive DoD: Deep discharges accelerate wear.
High Operating Temperatures: Heat speeds up battery degradation.
Frequent Charge/Discharge Cycles: More cycles reduce capacity over time.
Overcharging or Undercharging: Both extremes negatively affect lifespan.
Best Practice:Regular battery monitoring and proper maintenance can help prolong SoH above 80% for many years.
Bringing It All Together: Optimizing Battery Performance for Off-Grid Systems
To design a reliable and long-lasting off-grid system, it's essential to balance all four metrics:
✅ Plan for sufficient Days of Autonomy to avoid energy shortages.✅ Set an optimal Depth of Discharge (DoD) to extend battery life.✅ Monitor State of Charge (SoC) to prevent system failures.✅ Track State of Health (SoH) to anticipate battery replacement needs.
JVC Energy specializes in high-efficiency off-grid solar and battery systems, ensuring every system is optimized for reliability and longevity.