Battery Cycle Life

What Is Battery Cycle Life?

Answer: Battery cycle life is the number of specified charge–discharge cycles a battery can complete before reaching a stated performance threshold under defined test conditions. The rating must include its capacity-retention endpoint, depth of discharge, current rates and temperature. It is not a guaranteed number of service years: calendar aging, operating conditions and warranty terms also matter.

A cycle-life figure without its conditions is incomplete. Compare products at equivalent boundaries and endpoints rather than assuming every LFP, NMC or lead-acid battery shares one chemistry-wide lifetime.

Test Cycles and Equivalent Full Cycles

A test cycle may involve a specified partial discharge followed by recharge. That counted test event is not necessarily an equivalent full cycle, or EFC.

For a declared fixed discharge-charge reference of 100 Ah, two discharges of 50 Ah total 100 Ah, or one discharge-based EFC. They are still two partial discharge events. The example tracks discharge throughput only; adding both charging and discharge throughput without adjusting the denominator would double-count.

EFC normalizes throughput. It does not prove that two shallow cycles cause exactly the same degradation as one deep cycle. A lifetime model may consider the individual cycle depths and operating conditions. The SAM battery-life documentation describes depth-dependent cycle data and cycle counting rather than treating total throughput as the only aging input.

Reading an Actual Rating

As one model-specific example, the Victron Lithium NG 51.2 V, 100 Ah technical table lists cycle life with capacity at least 80% of nominal:

These are that model’s manufacturer-reported figures. They are not field measurements by SurgePV, chemistry-wide ranges or a warranty promise. Confirm the supporting conditions before applying them to a project.

More cycles at a smaller depth of discharge does not automatically mean proportionally more delivered energy. If a simplified fixed reference were 10 kWh, then 2,500 cycles × 80% × 10 = 20,000 kWh, while 5,000 × 50% × 10 = 25,000 kWh. These are illustrative constant-capacity throughput calculations, not forecasts for the cited battery, whose nominal energy is different. Real capacity declines and discharge losses require a time-dependent estimate.

Cycle Aging and Calendar Aging

Cycle aging is associated with battery use; calendar aging occurs with elapsed time even without cycling. Temperature and SOC history can matter to both. Neither a chemistry label nor a single annual percentage specifies their combined field behavior.

SAM offers different lifetime model options and explicitly distinguishes these mechanisms. Choose the documented model appropriate to the evidence; do not assume every model combines them identically or that a linear annual fade is a measured fact.

For an assumed 6,000 counted cycles and one matching cycle per day, the arithmetic is 6,000 ÷ 365 = 16.4 years. That is a schedule conversion, not proof the battery lasts 16.4 years. The rating conditions, aging, operating limits or other equipment may make that projection unsuitable.

A rating of “1,200 cycles” likewise needs its endpoint and test cycle definition. It does not mean the battery suddenly fails on the next use or provides 1,200 days of identical output.

Correct Cost-per-Cycle Arithmetic

For a hypothetical $5,000 battery cost, zero residual value and 6,000 assumed cycles:

Capital cost per counted cycle = $5,000 ÷ 6,000 = $0.833.

If each assumed cycle delivered a constant 10 kWh at the chosen output boundary, total delivery would be 60,000 kWh, making battery-capital cost $0.0833/kWh.

This simplified comparison excludes degradation, charging energy, inverter/installation cost, maintenance, replacement, financing and discounting. A realistic cost per delivered kWh uses accumulated output at the same boundary over time. More counted shallow cycles alone need not make a product cheaper per kWh.

Warranty and Project Handoff

Read the exact country- and model-specific warranty alongside the datasheet. Record the warranty duration, capacity endpoint, throughput or cycle limits if present, allowed uses, registration requirements and measurement procedure. Laboratory cycle life, a contractual capacity warranty and a modeled replacement year are different inputs.

Designers should retain the load/dispatch profile, SOC window, C-rate, temperature assumptions, lifetime method and replacement sensitivity. Compare alternatives with consistent energy boundaries rather than unsupported chemistry-wide costs or guaranteed payback.

Installers should follow placement and commissioning instructions and retain the supported configuration, settings and baseline monitoring records. A dashboard health estimate is not automatically the warranty’s capacity-test result.

Sales teams should distinguish documented ratings from project assumptions. Show how available energy and economics change under plausible scenarios, without presenting fabricated field projects as evidence. When evaluating SurgePV, ask which storage degradation inputs and financial assumptions the workflow supports.

Common Questions

How many cycles does a solar battery last?

Use the exact product’s rating and conditions. There is no single cycle count that applies to every solar battery or installation.

Does depth of discharge affect cycle life?

It can, and manufacturer curves may show different cycle counts at different depths. Compare the actual model data without assuming a universal percentage improvement.

How do I calculate battery cost per cycle?

Divide the stated net cost by the assumed counted cycles, then check whether the comparison uses the same cycle definition. For cost per delivered kWh, use cumulative output at a consistent boundary.