Cycle Life

What is cycle life

Cycle life is the number of full charge-discharge cycles a battery can deliver before its usable capacity drops below a defined threshold, typically 80 percent of original rated capacity. Some standards use 70 percent as the threshold. Cycle life is the primary durability metric for batteries used in cycling applications like solar storage and EVs.

Cycle life depends on chemistry, depth of discharge per cycle, temperature, charge and discharge rate, and storage conditions between cycles. The relationship is non-linear and chemistry-specific. The same battery may deliver 4,000 cycles at 80 percent DOD and 6,000 cycles at 50 percent DOD, which is why cycle life is the deciding factor in the LiFePO4 vs NMC comparison for Indian solar storage.

For solar storage, cycle life translates to lifecycle cost. A battery rated for 6,000 cycles at 80 percent DOD over 15 years performs roughly one cycle per day for the full life of the system, matching the typical solar storage duty cycle.

Cycle life explained simply

Imagine a rechargeable battery as a pair of shoes. Every time you charge and discharge, you walk in them once. The shoes wear out after enough wear. Battery cycle life is how many cycles before the wear shows. Some batteries are like sneakers (LFP, thousands of cycles). Some are like dress shoes (lead acid, hundreds of cycles).

Why cycle life matters

Battery longevity. The primary reason customers compare batteries, as explored in how long solar batteries actually last in India’s heat.

Lifecycle cost. ₹/kWh-cycle is the right way to compare battery economics.

Solar storage payback. A 10-year cycle life supports the financial model for solar plus storage.

Warranty terms. Manufacturers warrant cycle counts at specific conditions.

Replacement planning. Predicting when to budget for replacement.

How cycle life is measured

Standardised tests apply repeated charge-discharge cycles at defined conditions:

1. Charge at standard rate to full SOC.
2. Discharge at standard rate to specified DOD.
3. Pause for thermal equilibrium.
4. Repeat.
5. Periodically measure capacity at a reference rate.
6. Cycle count when capacity reaches threshold.

Industry-standard conditions per IEC 61960 and IEC 62620:

• Temperature: 25 ± 5 degrees Celsius
• C-rate: typically 0.5C or 1C
• DOD: 80 to 100 percent

Manufacturer warranties may use different conditions, so cycle counts in datasheets and warranties can differ.

Technical deep dive

Chemistry-specific cycle life

DOD vs cycle life

The relationship is non-linear. Approximate empirical fits for LFP:

This gives:

• 100% DOD: 4,250 cycles
• 80% DOD: 4,750 cycles
• 50% DOD: 6,000 cycles
• 30% DOD: 7,800 cycles
• 10% DOD: 13,500 cycles

Calendar life

Calendar life is the chronological life regardless of cycling. LFP calendar life is 10 to 15 years. NMC is 8 to 12 years. Both age while sitting unused.

Combined ageing

Total ageing = cycle ageing + calendar ageing.

Whichever exhausts first dictates end of life. For a battery cycled once daily, cycle and calendar life are usually balanced. For batteries used rarely, calendar life dominates.

Capacity fade curve

Capacity does not drop linearly, and the decline is tracked as falling State of Health. Typical LFP curve:

• 0 to 70 percent of cycle life: roughly linear 0.5 to 1 percent loss per year.
• 70 to 100 percent of cycle life: accelerating to 2 to 3 percent loss per year.

Real-world applications

Solar storage payback modelling.

EV residual value estimation, including second-life battery repurposing of retired packs for stationary storage.

Telecom backup replacement planning.

Hybrid inverter battery sizing.

Grid storage operating contracts.

Advantages of high cycle life chemistries

Lower lifecycle cost.

Longer warranty terms.

Lower replacement frequency.

Better fit for daily cycling solar storage.

Reduced material throughput and environmental impact.

Limitations of cycle life metric

Single number hides DOD, temperature and rate dependence.

Real-world cycle distribution differs from standardised tests.

Calendar ageing not captured.

Variability across manufacturers within the same chemistry.

End-of-life threshold (70 vs 80 percent) affects the headline number.

Common problems

Cycle life shorter than expected. Usually traceable to higher DOD, higher temperature, or higher C-rate than assumed.

Calendar ageing dominating cycle life in shallow-cycling applications.

Cycle counter miscounting partial cycles inside the BMS. Rainflow counting is the correct method.

Different cycle life cited at different DOD assumptions causing customer confusion.

Best practices

Compare batteries on ₹/kWh-cycle at intended DOD, not on nominal capacity alone.

Operate at recommended DOD and temperature.

Use rainflow counting for partial cycle accumulation.

Plan replacement budget around expected cycle and calendar life, starting from a proper hybrid inverter battery sizing exercise that accounts for DOD and C-rate limits.

Get cycle warranty terms in writing.

Comparison: cycle life metrics across applications

Indian market context

Indian residential LFP storage warranties typically guarantee 4,500 to 6,000 cycles at 80 percent DOD with 10-year coverage.

Telecom operators planning 15 to 20 year tower lifespans select LFP for cycle life security.

Utility-scale storage RFPs increasingly specify minimum cycle life as 6,000 cycles for solar firming applications.

Cycle-life testing methods are set out in IEC 61960 and the cell maker’s datasheet; IS 16893 covers EV propulsion cells and is not the stationary-storage standard.

Standards and certifications

Common mistakes

Comparing cycle life across chemistries without normalising for DOD.

Ignoring calendar ageing in shallow-cycling sizing.

Trusting brochure cycle count without reading the DOD and temperature footnotes.

Allowing operation outside warranted conditions.

Not counting partial cycles correctly.

Skipping rainflow counting in long-term cycle life accounting.

Key takeaways

Cycle life is the number of charge-discharge cycles a battery delivers before capacity drops to a defined threshold. LFP at 4,000 to 8,000 cycles, NMC at 2,000 to 4,000, lead acid at 500 to 1,500. Cycle life depends on DOD, temperature and C-rate. Comparison across batteries should normalise these conditions. Lifecycle cost is the right metric, not headline cycle count.

Further reading

For how Cycle Life plays out in real projects, these guides go deeper:

• LiFePO4 vs NMC Battery for Solar in India
• How Long Do Solar Batteries Last in India?
• How to Size a Battery for a Hybrid Solar Inverter
• Lithium vs Lead-Acid Solar Battery for Indian Homes

Sources

• IEC 61960:2017, Performance requirements for secondary lithium cells.
• IEC 62620, Performance of lithium cells for industrial applications.
• IS 16893 (Part 2), BIS (secondary lithium-ion cells for electric road vehicle propulsion; not a stationary-storage standard).
• IEC 60896, Stationary lead acid batteries.
• Manufacturer warranty documents from leading LFP and NMC suppliers.
• Plett G. L., Battery Management Systems, Volume I and II, Artech House.
• NREL Technical Report TP-7A40-83933, 2024 Battery Storage Cost Projections.
• BloombergNEF Battery Storage Outlook 2024.
• IEA Energy Storage Outlook 2024.