How Long Do Custom Lithium Battery Packs Last? Cycle Life Explained

What counts as one cycle

A cycle is one full sequence of charge and discharge, counted from a defined starting point to a defined end point. In a lab test, that usually means charging to the specified full-charge voltage at a stated current, then discharging to the specified cut-off at a stated current, and repeating. The counter increments once per completed sequence, not once per charger connection.

This matters because real use rarely follows the lab script. A device that takes a partial charge during a shift and tops up again overnight may accumulate a fraction of a cycle per day, and the pack's cycle count will lag far behind the number of times it was plugged in. Make sure your supplier's cycle figure comes with the test conditions attached — charge rate, discharge rate, temperature and cut-off — because a cycle count without those conditions cannot be compared with another supplier's number.

Depth of discharge: the biggest lever

Depth of discharge (DoD) is how much of the pack's usable capacity you take out before recharging. It is the single largest influence on how many cycles a pack will deliver over its life. A shallow discharge puts less mechanical and chemical strain on the cell, so the pack tolerates many more shallow cycles than deep ones; a pack run from full to nearly empty every time will reach its end-of-life threshold far sooner.

Design decisions follow from this. If a device can be charged opportunistically — between tasks, during breaks, in a dock — the pack can be sized smaller and worked in a shallow band, which usually costs less than oversizing the pack and cycling it deeply. If the duty cycle forces a deep discharge every run, that has to be stated up front, because the cell selection, the protection limits and the thermal design all change. State the depth you expect in normal use and the worst case you must survive, and the pack can be matched to it.

C-rate, heat and calendar aging

C-rate is the current relative to capacity: a high-rate discharge drains the pack quickly and generates more heat inside the cells. Heat is the common accelerator behind most aging mechanisms, and it comes from three places — the load itself, the environment, and the charging process. A pack that runs warm in a sealed enclosure ages faster than the same pack in open air, even if the electrical duty is identical.

Calendar aging is the other clock. A lithium pack loses capacity over time even when it is not cycled, and the rate depends on the state of charge it is held at and the temperature it sits in. Storage at a high state of charge in a warm room is harder on the pack than storage at a moderate level in a cool one. For equipment that spends long periods idle — backup units, seasonal tools, spares on a shelf — this is often the dominant loss, not cycling.

Both effects are design inputs, not afterthoughts. Enclosure ventilation, charge-profile limits and a storage recommendation belong in the project specification, and they should be written against the temperatures and currents your device actually sees. Generic advice cannot capture that; your duty cycle can.

End-of-life capacity: reading a specification

A pack is not rated to "fail" at a fixed point. Its specification defines an end-of-life capacity threshold — the fraction of the original rated capacity the pack must still deliver at the rated conditions for the application to keep working. Cycle life is then stated as the number of cycles the pack completes before its capacity falls to that threshold under the stated test conditions.

Two suppliers can quote very different cycle life for cells that behave similarly, simply because one used a looser end-of-life threshold or gentler test conditions. Read the specification for four things: the threshold, the charge and discharge rates used in the test, the temperature, and the cut-off voltages. If any of these is missing, ask for them. Then compare against what your device actually needs: a product that only needs a modest runtime may be perfectly served past a strict threshold, while a product that must hold full runtime until replacement needs a conservative one. The threshold should be chosen from the application, and written into the agreed specification.

Specifying for service life: tell your maker the duty cycle

The most useful thing you can send a pack manufacturer is not a capacity figure — it is a description of how the pack will be used. Duty cycle means the pattern of charge and discharge over a typical day or week: how deep the discharge goes, how often, at what current, in what ambient temperature, and how the pack is stored between runs.

With that description, the design can be matched rather than guessed. Cells, protection limits, thermal path and enclosure are chosen against the real load profile, and the cycle-life expectation in the specification is set against your conditions. For long-life stationary and motive applications, LiFePO4 batteries are often considered because their chemistry suits deep, regular cycling; for compact devices, other chemistries may fit the space better. Either way, the choice follows the duty cycle.

If you are still defining the requirement, custom battery packs are built from the device outward — enclosure, connector, BMS and cell selection are agreed in the project specification, and options are confirmed for the model rather than assumed. Buyers comparing approaches can read the common questions on our FAQ page, and warranty terms are confirmed for the specific model and negotiated terms rather than stated as a single figure across the range.

Frequently Asked Questions

Does a partial charge count as a full cycle?

No. A cycle counter increments when a defined charge and discharge sequence completes. Taking a partial charge and using part of it before recharging accumulates only a fraction of a cycle. This is why a device that is topped up frequently can run for a long time before its pack reaches the cycle figure in its specification — the pack simply is not completing full cycles.

Does fast charging shorten pack life?

It can, because higher charge current generates more heat inside the cells, and sustained heat accelerates aging. Whether it matters for your product depends on the charge rate relative to the pack's capacity, the thermal path around the cells, and the ambient temperature. If fast charging is a product requirement, state it in the project specification so the charge profile and thermal design are set against it.

Is a bigger pack always longer-lived?

Not automatically. A larger pack worked in a shallow band of discharge will generally see less strain per cycle than a smaller pack worked deeply, which helps service life. But a larger pack also sits at a higher state of charge for longer periods if it is rarely used, and calendar aging then becomes the dominant loss. Pack size should follow the duty cycle, not be used as a substitute for one.

Can cycle life be confirmed for my model?

It is confirmed in the project specification for the specific model, against the test conditions written there. Cell-level cycle data is available for the cells selected, and the specification sets the end-of-life capacity threshold the application needs. Tell us the model you are considering and the duty cycle it will see, and the relevant figures can be confirmed for that configuration.

How should packs be stored between uses?

Store at a moderate state of charge rather than full, in a cool, dry place, and avoid leaving the pack at a high state of charge in a warm environment for long periods. The exact storage level and temperature range belong in the specification for your model, since they depend on the chemistry and the application. For equipment that idles for long stretches, storage conditions often matter more than cycling.

What information do I need to give for an accurate life estimate?

The duty cycle: depth of discharge in normal and worst-case use, discharge current, charge current, ambient temperature range, how often the pack is cycled, and how it is stored between runs. The device's end-of-life capacity threshold — what runtime the product must still deliver before replacement — completes the picture. With those, the pack and its specification can be matched to the application.