SaurEnergy Explains: What are Battery Cycles Metric, Why it Matters?

Battery Cycles is a commonly used phrase in talks about battery energy storage systems (BESS). The number of battery cycles is, in fact, a metrics that define the age or the usage limit of a battery before its performance drops.

In this explainer, we will explore what a Battery Cycle is and how it affects the economy and utility of a battery or battery storage system.

What are Battery Cycles?

Each time a battery is charged and discharged, it is said to have completed one cycle. The number of cycles a battery undergoes throughout its lifetime, or before its performance drops significantly, determines how long your investment lasts.

Over time, as batteries age, their ability to hold a charge decreases. They eventually need to be replaced. For example, a battery with a 1,000-cycle life can theoretically be charged and discharged 1,000 times before its capacity drops to approximately 80% of its original value. This is important for industries that rely on continuous power, such as renewable energy storage, electric vehicles, and medical devices.

Lithium-ion batteries, for example, often boast 3,000 to 6,000 cycles. Interestingly, the number of cycles is tied directly to something called depth of discharge (DoD). It is a commonly understood fact that the deeper you drain a battery’s stored energy, the fewer cycles it will have in its lifespan.

Understanding How DoD Affects Longevity

Depth of Discharge (DoD) has an inverse relationship with battery cycle life: deeper discharges result in fewer total charge-discharge cycles, while shallower discharges increase lifespan. Deep discharges increase stress on internal components (such as chemical plates in lead-acid or electrolyte in Lithium), causing faster capacity loss.

Regularly using a high percentage of a battery's capacity causes faster degradation, meaning a battery discharged to 100% daily will fail much sooner than one used at 30%–50% DoD. For example, a battery that lasts 1,500 cycles at 50% DoD may only last 500 cycles at 80% DoD.

Lead-acid batteries, despite their lower upfront cost, may require more frequent replacements when discharged deeply, increasing long-term costs. Lithium-ion batteries, although initially more expensive, often have a longer lifespan due to their ability to tolerate deeper discharges without as much degradation, making them more cost-effective in the long run for certain high-demand applications. 

Impact on C&I Businesses?

The number of cycles defines how cost-efficient your battery is for your business. Battery cycles directly affect the levelised cost of storage. This metric considers how much usable energy a battery can deliver over its lifetime relative to its total cost.

Batteries with more cycles mean reduced replacement costs. There is no need for a new system every few years. It also affects the reliability of your BESS, keeping your operations powered during peak demand. A battery with fewer cycles may look cheaper initially, but it can become more expensive when replacement costs are included. Frequent replacements also introduce downtime, additional labour costs, and supply chain risks.

Battery cycle life also affects warranty terms. Many suppliers offer performance guarantees based on a defined number of cycles. Exceeding these limits can void warranties and increase financial risk.