Battery DOD

What is battery DOD

Depth of Discharge (DOD) is the percentage of a battery’s nominal capacity that has been discharged from full. It is the inverse of State of Charge: SOC + DOD = 100 percent at any moment. The term is used in two senses: instantaneous DOD (right now) and cycle DOD (the deepest point in a charge-discharge cycle).

For solar storage design, the relevant metric is cycle DOD. Sizing decisions, warranty terms and cycle life predictions all use cycle DOD. A 10 kWh nominal battery sized for 80 percent DOD provides 8 kWh of usable energy per cycle.

DOD is the single most important design choice in battery sizing after capacity itself. Choosing the right DOD for the chemistry balances upfront capex against cycle life and lifecycle cost.

DOD explained simply

If your battery holds 10 kWh when full and you discharge it down to 2 kWh, you used 80 percent. That is 80 percent DOD. Cycle life depends on how deep you go each cycle.

For lead acid batteries, going deeper than 50 percent per cycle damages the battery quickly. For LFP, you can comfortably do 80 to 90 percent every day. Different chemistries, different DOD habits.

Why DOD matters

Battery sizing. Larger nominal capacity is needed at lower DOD to deliver the same usable energy.

Cycle life. Deeper DOD means fewer cycles before end of life. The relationship is non-linear and chemistry-specific.

Lifecycle cost. Lower DOD extends life but costs more upfront. Sweet spot depends on energy throughput needs.

Warranty terms. Manufacturers specify cycle warranties at a given DOD. Operating beyond invalidates terms.

User experience. Apparent runtime depends on usable capacity, which depends on DOD configuration.

How DOD affects cycle life

Cycle life is the number of full charge-discharge cycles before capacity drops to a specified threshold, typically 80 percent of original. At lower DOD, more cycles are possible.

For LFP, an approximate relationship:

Cycles ≈ 6000 × (1 / DOD)^0.5

For lead acid the dependence is much steeper.

Technical deep dive

DOD sizing formula

Tools built for battery and energy storage sizing use this same usable-kWh-over-DOD approach when speccing hybrid solar storage for installers.

DOD vs Calendar ageing trade-off

Beyond cycle life, calendar ageing happens whether the battery is cycled or not. Calendar ageing is faster at high SOC and high temperature, a dynamic explored further in how long solar batteries actually last in Indian heat.

For lithium-ion batteries, holding the battery at 100 percent SOC accelerates calendar ageing. Operating at 80 to 90 percent average SOC balances cycle life and calendar life.

Partial DOD cycling

Real-world solar storage rarely sees full DOD. Typical residential cycles:

• Daytime charge from 30 to 95 percent (65 percent DOD)
• Evening discharge from 95 to 30 percent (65 percent DOD)

Cumulative cycle equivalents are computed via rainflow counting.

DOD and round-trip efficiency

Round-trip efficiency varies slightly with DOD. Higher DOD includes lower-SOC operation, where coulombic and voltage efficiency dip slightly.

Real-world applications

Residential storage sizing for hybrid solar.

Telecom backup with deep discharge during long outages.

EV range planning (DOD = trip energy / battery capacity).

Industrial peak shaving with shallow cycling.

Off-grid sizing accounting for cloudy day autonomy.

Advantages of disciplined DOD

Extends battery life.

Reduces lifecycle cost per kWh delivered.

Preserves warranty validity.

Provides headroom for unexpected high-load events.

Limitations

Over-conservative DOD wastes battery capacity.

Customer education needed to avoid intuitive ‘use all of it’ approach.

EMS must enforce DOD limits through SOC cutoffs.

DOD discipline is harder to maintain in inverter modes that prioritise self-consumption over battery health.

Common problems

Customer using full DOD to maximise short-term backup, killing the battery in 18 months.

Inverter EMS configured for higher DOD than the battery warranty allows, voiding cycle warranty.

Mismatched DOD assumption between EPC and customer expectations.

Calendar ageing ignored in sizing for low-cycle applications.

DOD different on different days due to load variability, complicating cycle counting.

Best practices

Configure inverter minimum SOC to match manufacturer’s recommended DOD.

Size battery for design DOD, not nominal capacity.

Document DOD in the EPC specification.

Train the user on what DOD means for their warranty.

Use EMS modes that respect DOD limits.

Audit actual DOD distribution against design assumption annually.

Comparison: typical DOD recommendations

Indian market context

Indian residential hybrid systems typically design for 80 percent DOD on LFP and 50 percent DOD on tubular lead acid.

Telecom backup systems are designed for occasional 100 percent DOD events during long outages, accepting cycle life impact.

Inverter compatibility lists from major brands document the assumed DOD for warranty compliance.

PM Surya Ghar does not subsidise batteries, so customer choice of DOD is governed only by manufacturer warranty and EPC recommendation.

Standards and certifications

Common mistakes

Sizing the battery to nominal capacity instead of usable capacity at design DOD.

Allowing customer-set EMS strategies that exceed recommended DOD.

Comparing batteries by nominal capacity alone, ignoring DOD warranty terms.

Forgetting calendar ageing when DOD is shallow.

Mixing partial DOD cycling with rainflow counting incorrectly.

Not auditing actual DOD distribution post-installation.

Key takeaways

DOD is the depth of each charge-discharge cycle, expressed as a percentage of nominal capacity. LFP tolerates 80 to 90 percent DOD daily. Lead acid requires 50 percent or shallower DOD for reasonable cycle life. Sizing for the correct DOD, configuring the inverter accordingly, and respecting warranty terms are the disciplines that decide actual battery life.

Further reading

For how Battery DOD plays out in real projects, these guides go deeper:

• LiFePO4 vs NMC Battery for Solar in India
• Off-Grid Solar Battery Bank Sizing for India
• How Long Do Solar Batteries Last in India?
• How to Size a Battery for a Hybrid Solar Inverter

Sources

• IEC 61960:2017, Performance requirements for secondary lithium cells.
• IEC 62619:2017, Safety requirements for secondary lithium cells.
• IS 16893 (Part 2), BIS (secondary lithium-ion cells for electric road vehicle propulsion; not a stationary-storage standard).
• Manufacturer warranty documents from Pylon Tech, BYD, Goodwe, Sungrow, Solis, Tata Green.
• Plett G. L., Battery Management Systems, Volume I and II, Artech House.
• Reddy T. B., Linden’s Handbook of Batteries, McGraw-Hill.
• NREL Technical Report TP-7A40-83933, 2024 Battery Storage Cost Projections.
• BloombergNEF Battery Storage Outlook 2024.
• IEA Energy Storage Outlook 2024.