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Batteries are rated for a number of cycles to a given depth of discharge (DoD) — for example 3000 cycles at 80% DoD before capacity falls to 80% of new. Shallow cycles are gentler, so using less of the battery each cycle gives many more cycles. A robust, transparent way to estimate this is energy-throughput equivalence: the battery can deliver roughly the same total lifetime energy regardless of how you slice it, so halving the DoD roughly doubles the cycle count.
| Quantity | Formula |
|---|---|
| Lifetime throughput | Elife = Nrated × (DoDrated/100) × Capacity |
| Cycles at your DoD | N = Nrated × DoDrated / DoD |
| Calendar life | Years = N / (cycles/day × 365) |
| Equivalent full cycles | N × DoD / 100 |
This throughput model is a conservative estimate; real manufacturer curves often show even more cycles at low DoD, because shallow cycling stresses the electrodes less than throughput alone predicts. Calendar ageing (time, temperature, and average state of charge) also limits life independently of cycling — a battery cycled very gently will still age over years.
Cycle life is not a fixed number for a chemistry — it depends overwhelmingly on how deeply each cycle discharges the pack. Shallow cycling is dramatically gentler, and the effect is large enough to change how a system should be sized.
| Depth of discharge | Typical Li-ion cycles | Lifetime throughput, 60 kWh pack |
|---|---|---|
| 100% | ~800 | ~48,000 kWh |
| 80% | ~1,200 | ~57,600 kWh |
| 50% | ~3,000 | ~90,000 kWh |
| 20% | ~10,000 | ~120,000 kWh |
Read the right-hand column rather than the middle one. Cycling at 50% depth gives far fewer kilowatt-hours per cycle, yet delivers roughly twice the total lifetime energy of full-depth cycling. This is why an oversized pack cycled shallowly often outlasts and outperforms a smaller pack worked hard, despite costing more up front.
Cycle count is only half the story. Lithium cells degrade over time regardless of use, and that calendar ageing accelerates sharply with both temperature and state of charge. A pack held at 100% charge in a hot environment can lose meaningful capacity in a year having done almost no cycles at all.
This is the reasoning behind the common advice to store lithium batteries at around 40–60% charge, and behind the "daily charge limit" setting in electric cars: holding the cells at a lower average state of charge slows the chemistry down. For a system that sits idle most of the time — backup power, seasonal equipment — calendar ageing, not cycle count, is usually what ends the pack's life.
By convention a battery reaches end of life when it retains 80% of its original capacity. It does not stop working at that point; it simply holds less. Many packs retiring from vehicle use at 80% remain perfectly serviceable for stationary storage, where the weight and volume penalty of the lost capacity does not matter.
Cycle life is the number of charge-discharge cycles a battery can perform before its usable capacity falls to a defined level, commonly 80% of the original. It is always quoted alongside a depth of discharge.
Shallower cycles are far gentler, so using less of the battery each cycle gives many more cycles. As a rule of thumb, halving the depth of discharge roughly doubles the cycle count.
It is the total energy that passes through a battery over its life: cycles × depth of discharge × capacity. Throughput stays roughly constant regardless of how deeply you cycle, which is why shallow cycling yields more cycles.
No, it is a conservative estimate based on constant energy throughput. Real manufacturer data often shows even more cycles at low DoD, because shallow cycling stresses the electrodes less than throughput alone would suggest.
Typical Li-ion gives 1000–3000 cycles and LiFePO4 3000–6000+ cycles to 80% capacity at moderate DoD, versus only 300–800 for lead-acid. Exact numbers depend on chemistry, DoD and temperature.
Limiting DoD to about 50–80% greatly extends cycle life for lithium, and 50% or less is common for lead-acid. The trade-off is that you use less of the battery each cycle, so you may need a larger pack.
Batteries also degrade with time, temperature and average state of charge, independent of cycling. A lightly-cycled battery will still lose capacity over years, so calendar life can limit lifespan before cycle life does.
Divide the estimated cycles by how many cycles you do per day and by 365: Years = N / (cycles per day × 365). One cycle a day of a 4800-cycle pack is about 13 years, if calendar ageing allows.
Strongly. High temperatures accelerate degradation, roughly halving life for each 10 °C above the ideal range, while very cold charging can cause lithium plating. Keeping the battery cool and moderate extends life.
It normalises partial cycles to full ones: two 50% discharges count as one equivalent full cycle. It lets you compare mixed shallow and deep usage against a full-cycle rating.
Yes, somewhat. High charge and discharge rates generate heat and stress that accelerate ageing, so frequent fast charging typically reduces the number of usable cycles compared with slow charging.
Use a moderate depth of discharge, avoid extreme states of charge (keep between ~20–80%), keep the battery cool, and avoid unnecessary fast charging. Together these can multiply the usable lifespan.
The fraction of the pack's capacity used before recharging. Discharging a 100 Ah battery by 40 Ah is a 40% depth of discharge. Shallower cycles cause far less degradation per cycle.
Substantially. Roughly 800 cycles at 100% depth versus about 3,000 at 50% is typical for lithium-ion, and because total energy throughput is what matters, the shallow case delivers around twice the lifetime energy overall.
By convention, the point at which the pack retains 80% of its original capacity. The battery still works; it simply stores less, which is why retired vehicle packs are often reused for stationary storage.
Capacity loss that happens with time rather than use, driven by temperature and state of charge. A pack stored hot and fully charged can lose noticeable capacity in a year without being cycled at all.
Around 40–60%, and as cool as practical. Storing at full charge accelerates calendar ageing, and storing fully discharged risks the cell self-discharging below its safe minimum voltage.
To lower the average state of charge during daily use. Charging to 80% rather than 100% except before long trips measurably slows calendar ageing, at the cost of some everyday range.
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