Charge-discharge cycles and calendar aging limit battery life, but operating hours measure neither. For electric vehicle (EV) engineers, that distinction matters because service-life estimates influence how batteries are sized and managed throughout the vehicle’s life.
Actual service life depends on energy throughput and operating conditions, including how the battery is used and stored. This is especially true for off-highway electric equipment, where duty cycles can vary significantly among machines and applications.
Battery service life in off-highway electric equipment depends on actual energy throughput and operating conditions, not operating hours alone.
The average human life expectancy in the US is about 687,000 hours. That number is accurate but not particularly useful for estimating an individual lifetime because it says nothing about how that life is lived.
At equipment trade shows, the useful life of lithium-ion batteries is often quoted in hours of operation. Some manufacturers advertise “20,000 hours or ten years, whichever comes first.” Others promise “12,000 hours.”
Meanwhile, technical datasheets from lithium-ion battery suppliers say something different. Ratings may be stated as 4,000 cycles at 80% depth of discharge, at a specified temperature and charge rate. Cycles are the basis used to measure battery energy throughput.
That difference matters because an operating-hours figure isn’t a measured property of the cell. It’s a calculated estimate based on assumptions about how the equipment will be used, and those assumptions aren’t always clearly stated.
This article explains how to translate a battery cycle rating into an expected service life for off-highway electric equipment.
The same problem across off-highway electrification
Forklifts get the most attention because their fleets are the largest and their duty cycles the best documented. But electrification has spread the same specification problem across off-highway equipment where duty varies far more widely:
- Electric excavators, whose energy draw depends on material density, dig depth, and ambient temperature
- Construction equipment, such as scissor and boom lifts carrying different loads, or compact track loaders switching between attachments that change power demand substantially
- Electric wheel loaders, where bucket fill factor and travel distance per load set the energy per hour
- Telehandlers, where lift height and load can also differ dramatically
A battery for every machine on that list is quoted in hours by someone, and not one has a universal operation cycle. An excavator battery in soft soil and the same battery used by a machine in blasted rock will not reach end of life in the same year, whatever the hours meter reads.
Two aging mechanisms
Lithium-ion cells lose capacity two ways at once, and a complete specification accounts for both.
- Cycle aging comes from energy throughput of the battery pack, through mechanisms such as lithium plating and particle cracking. It scales with each cycle.
- Calendar aging proceeds whether or not the equipment runs. Typically, it is driven by the time a battery is exposed to high ambient temperature and the time at a high state of charge. A pack left fully charged in a hot yard degrades while doing no work.
For electric forklifts, the same number of operating hours can represent very different battery aging depending on duty cycle and storage conditions.
Which mechanism brings a battery to the end of life first depends on operation intensity. In heavily used off-highway equipment, cycle aging typically matters more, and calendar effects are secondary. In lightly used or seasonal machines, calendar aging can arrive first, which is why manufacturers often quote both operating hours and calendar years.
Operating hours capture neither mechanism directly. They are a proxy for throughput that assumes a fixed draw rate, and they say nothing about time, temperature, or state of charge at rest.
What changes throughput?
Three factors determine how much energy a pack delivers per operating hour. They apply across every machine class.
- Load and work intensity. Heavier loads, higher lifts, denser dig conditions, and attachments that add hydraulic demand all raise amp-hours drawn per hour. Higher draw means more cycles per year at identical hours on the meter.
- Operating temperature. For outdoor equipment this is the most under-counted factor. In cold operation, energy goes into heating the pack, or usable capacity shrinks if there is no heater. Either way means more frequent cycling. Heat works the other way, accelerating calendar aging. A telehandler working four winter months in the upper Midwest and the identical unit in Arizona are running different duty cycles and different aging profiles, and both are quoted the same hours figure.
- Installed capacity. A larger pack requires fewer cycles to complete the same work. Doubling a battery’s amp-hour capacity roughly halves annual cycle count.
The cycling math
An equivalent full cycle (EFC) represents cumulative discharge equal to 100% of the battery pack’s nominal capacity. For a 600-Ah battery, a cumulative 600 Ah of discharge counts as one EFC, whether it occurs in one discharge or several partial discharges. In this calculation, the 4,000-cycle rating is treated as 4,000 EFCs. The stated 80% depth of discharge is a test condition that affects the quoted cycle-life rating, not a multiplier applied to the EFC calculation.
Discharging a lithium battery to near zero may not lead to damage, though repeated deep discharges do accelerate degradation.
Take a typical case. A distribution operation runs a 36-V counterbalance truck 2,657 hours a year. The power study says the truck consumes 100 Ah per hour, and its lithium battery is 600 Ah and rated for 4,000 EFCs under the specified test conditions.
Move the machine into cold-weather work with a high-demand attachment drawing 120 Ah per hour, and the hours number is reduced to 20,000 hours under the same 4,000-EFC rating. The cycle number never changed. The hours never meant anything.
Life (years) = Cycle rating ÷ (Annual hours × Ah per hour ÷ Battery Ah capacity)
Calendar life runs in parallel and sets a separate ceiling, particularly for equipment that is seasonal, lightly used, or stored at high state of charge in hot climates. Service life is whichever limit arrives first.
The conditions that define a cycle rating
A cycle count without its test conditions is not a specification. The key parameters determine what the number means, and each moves it materially.
- Depth of discharge. The state-of-charge window the cell was cycled through. Shallower windows yield dramatically higher cycle counts, which is why a rating at 80% DOD and one at 100% DOD are not comparable figures.
- Temperature. Most ratings are measured near 25° C. Aging accelerates with higher temperatures, and low-temperature discharging and charging decrease capacity and lead to more frequent cycling.
- Charge rate. Higher C-rates raise internal heating and plating rates. A count measured at 0.5C rate will change at a higher charge rate.
- End-of-life threshold. Battery capacity decreases with inevitable degradation from day one. Whether “end of life” means 80%, 70%, or 60% of initial capacity changes the projected useful life count substantially, because the tail of a degradation curve is long.
Battery operating hours are an output of the operating conditions and not a specification. Convert hours to cycles for a specific application with a battery life expectancy calculator implementing the formula above.
Filed Under: Batteries, FAQs