Battery cycle life and depth of discharge: a practical guide

1. Cycle-life ratings belong to a test protocol

Depth of discharge (DoD) describes how much of a battery's rated capacity is removed during a discharge. It is one input to aging, not a stand-alone lifetime predictor. A Sandia study tested commercial LFP, NCA, and NMC cells at different discharge rates, depths of discharge, and temperatures. It found substantial variation in time and cycle counts to 80% capacity, including variation among cells of the same chemistry. The result describes those cells and protocols; it does not establish a universal cycle-life curve for every product. See Preger et al. (2020).

A cycle-life number is useful only alongside the conditions under which it was measured. When comparing products, record the DoD or state-of-charge window, temperature, charge and discharge rate, voltage limits, cycle profile, and the maker's end-of-life criterion. NREL's operation and maintenance guide emphasizes documenting operating conditions when evaluating energy-storage systems.

2. What a larger battery can and cannot tell you

If the energy taken from a bank stays the same, increasing its usable capacity reduces the share used on a typical day. That is capacity arithmetic, not a promise of a particular cycle count, calendar life, or replacement date. Temperature, charge settings, system controls, and time in service still matter.

For an initial sizing estimate, divide the energy that must come from the battery by the usable fraction selected for the scenario. Then account for conversion losses, reserve, and the limits in the battery and inverter documentation. Treat the result as a planning input; it is not a warranty or a battery-life prediction.

3. Chemistry does not guarantee a service life

Cycle-life figures describe a particular cell or pack under a stated test. Chemistry alone does not establish usable capacity, cycle life, calendar life, cold-weather performance, or replacement timing. Read the exact product documentation and compare the conditions behind each rating.

Lithium iron phosphate (LFP)

A cycle count is meaningful only with its test conditions and end-of-life definition. Preger et al. tested commercial lithium-ion cells under different cycling conditions and found substantial variation in the time and cycle count to 80% capacity. That study describes its tested cells; it is not a universal curve for every LFP product. See the study record.

For a product comparison, check the maker's stated charge and discharge limits, temperature, rate, depth of discharge, and capacity threshold. Do not convert a lab cycle count directly into a calendar service-life promise.

Sodium-ion

Do not infer a pack's usable range, voltage limits, storage behavior, cold-weather limits, or inverter compatibility from the chemistry name. Follow the exact battery manufacturer's operating limits, battery-management-system cutoffs, and the inverter maker's compatibility guidance. Treat any cycle-life figure as specific to the product and test conditions that produced it.

Lead-acid and AGM

Cycle life also depends on the particular model and its use. For example, Discover's AGM performance sheet says cyclic life depends on capacity taken, discharge rate, temperature, and charging method. Its cycle-life chart gives the test current, final voltage, charge current, and temperature. Those results describe the listed test, not every AGM battery. See the manufacturer's performance sheet.

Across chemistries, use the exact model's warranty and maintenance instructions, and plan around the loads, environment, and service conditions you expect. A cycle count alone cannot establish how many years a home battery will last.

6. Build a cost comparison from stated inputs

Lifetime-cost comparisons depend on the exact products, their usable energy, installed price, warranty, operating conditions, and replacement assumptions. A cycle count alone does not establish a replacement date or total cost.

Mark unavailable prices or service-life inputs as assumptions. Do not present a scenario as a guaranteed saving or a universal battery replacement schedule.

7. Use product-specific setup information

Before choosing or configuring a battery, gather the load, battery, inverter, and installation information for the exact system:

1. Characterize the load: Estimate the energy and peak power the battery must supply over the intended backup period. Use measured or documented loads and include inverter standby and conversion losses only when supported by the equipment information.
2. Size from usable capacity: Account for the battery’s stated usable energy, discharge limits, temperature range, power rating, and any reserve you need. Do not use a fixed depth-of-discharge multiplier as a universal sizing rule.
3. Check battery and inverter compatibility: Confirm that the exact products support each other. Configure charging, voltage cutoffs, current limits, battery-management-system communications, and firmware only as their manufacturers specify. Do not copy setpoints from another model or chemistry.
4. Plan protection and installation: Follow the product instructions for overcurrent protection, disconnects, conductor sizing, location, clearances, and temperature limits. Check applicable local electrical requirements and use a qualified installer when required.

8. Frequently Asked Questions

9. References

• Preger, Y., et al. (2020). “Degradation of Commercial Lithium-Ion Cells as a Function of Chemistry and Cycling Conditions.” Journal of The Electrochemical Society, 167(12), 120532. DOI: 10.1149/1945-7111/abae37.
• PV O&M Best Practices Working Group. (2018). Best Practices for Operation and Maintenance of Photovoltaic and Energy Storage Systems (3rd ed., NREL/TP-7A40-73822). National Renewable Energy Laboratory.
• Discover Battery. (n.d.). AGM Battery Performance data sheet.
• CATL. (2026, June 22). “CATL Debuts World's First Field-Validated Sodium-Ion BESS, Bringing Sodium Storage to Commercial Reality.”