What is a battery cycle?

Rechargeable batteries have charge/discharge cycles and life cycles. They are related but not the same. Batteries are complex electrochemical systems, and there are several factors that impact battery cycles including battery chemistry, how the battery is used, and a wide range of environmental factors. This FAQ presents a few of the complexities and subtleties related to quantifying battery cycles.

The concept of rechargeable battery cycles begins with the discharge cycle that’s defined and the process of draining a fully charged battery to a fully discharged state. That scientific definition is not useful in practical applications since it’s almost never a good idea to fully discharge a battery. And it does not consider several important factors like how rapidly the battery is drained. Treating a battery ‘gently’ and discharging at a lower rate almost always increases battery life.

The scientific definition of cycle life measures how many complete charges and discharges a rechargeable battery can experience before it will no longer hold a charge. One problem with that definition is the term “hold a charge.” Does that mean that a battery at the end of its cycle life will be capable of holding zero charge? No. In practical applications, a battery is no longer considered usable when its maximum charge becomes some fraction of its initial capacity, like 80% or 50%.

Then there’s the challenge in real-world applications that batteries are rarely fully discharged, or even discharged to the same percentage of full capacity during sequential usages. For example, if a battery is discharged to 50% of its capacity and then fully recharged, that’s defined as half of a charge/discharge cycle.

Depth of discharge
The depth of discharge (DoD) is the percentage of the battery’s capacity that has been discharged. It’s related to the state of charge (SoC) that measures the percentage of total capacity remaining. A battery with a DoD of 25% would have a SoC of 75%. The more often a battery is exposed to a high DoD, also called deep discharging, the lower its cycle life (Figure 1).

Figure 1. High DoD significantly reduces Li-ion battery cycle life (Image: Panbo Marine Technology).

Temperature affects both the performance and life of a battery. For example, Li-ion batteries operate best at a nominal temperature of 25 °C but can be used up to 60 °C. Above that they present a safety hazard. At temperatures approaching 0 °C, their lifetime and performance suffer. That can present thermal management challenges when integrating Li-ion batteries into electric vehicles (EVs) where the nominal automotive operating temperature range is -40 to 105 °C.

Chemistry and voltage
Battery chemistry is a major factor in determining cycle life. Sealed lead acid (SLA) batteries usually have a life of 500 complete charge/discharge cycles. With Li-ion batteries, the lifetime depends on the specific chemistry being used. For example, lithium iron phosphate (LFP) batteries can have a useful lifetime of over 2,000 cycles while lithium manganese oxide (LMO) cells are often rated for 700 or fewer cycles.

Charging conditions, especially the charging voltage, are also important. Many Li-ion chemistries can’t be operated at over 4.20 V per cell. Higher voltages can provide a short-term boost to capacity but shorten useful lifetime and can compromise safety (Figure 2). The cycle life of most Li-ion batteries is specified with a nominal charge voltage of 4.2 V. Fast charging technology is beyond the scope of this discussion and involves the use of higher voltages plus real-time voltage and thermal management to prevent battery damage.

Figure 2. At voltages above 4.2 V, Li-ion cycle life suffers (Image: Battery University).

Summary
Understanding the numerous factors that affect battery cycles is important when using batteries in systems from mobile phone handsets and wearables to EVs and large energy storage systems. Quantifying battery cycle performance is a complex activity that must consider details of how the battery is used and its operating environment.

References
Battery Life Cycle vs. Cycle Life, UPS Battery Center
Deep Cycle Batteries, Crown Battery
How to Prolong Lithium-based Batteries, Battery University
Lithium battery math, better than you may think, Panbo Marine Technology
How do Battery Charging Cycles Work?, BatteriesPlus