If you’re comparing lithium batteries, you’ll often see specifications such as “4,000 cycles at 80% depth of discharge”. It’s a useful indication of how long a battery is designed to last, but where does that 4,000-cycle figure actually come from? And what counts as one battery cycle in the first place?
It’s not simply a matter of counting every time you plug your battery in to charge. Battery cycle life is determined through controlled testing, with factors such as depth of discharge, charge and discharge rate, temperature and remaining battery capacity all playing a part. Understanding how these ratings are calculated makes it much easier to compare batteries properly and get a realistic idea of what their cycle life means in everyday use.
If you’re looking for a refresher on what terms such as cycle life and depth of discharge mean, check out our guide to battery cycle life and 80% depth of discharge.
What Counts as One Battery Cycle?
The simplest way to think about a battery cycle is as the use and restoration of a defined amount of the battery's capacity. A cycle isn't just a discharge, and it isn't simply every time the battery is recharged. In cycle life testing, a complete cycle generally involves discharging a defined proportion of the battery's capacity and then recharging it.
For example, if a 100Ah battery is being tested at 80% depth of discharge (DoD), it could be discharged from 100% state of charge to 20%, then recharged back to 100%. That represents one complete 80% DoD cycle. But importantly, you don't necessarily have to use that 80% all at once.
Do Partial Discharges Count Towards a Battery Cycle?
Yes. In everyday use, batteries rarely follow a neat pattern of being charged to 100%, discharged to a particular level and then immediately charged back to 100%. Instead, you might use 20% of your battery capacity, recharge it, use another 20% the following day and so on.
The BMS can keep track of this cumulative energy throughout, rather than treating every small discharge and recharge as a completely separate cycle. For KickAss batteries with an 80% DoD cycle basis, once an accumulated 80% of the battery's capacity has been discharged and recharged, the BMS can record the equivalent of one cycle.*
Take a 100Ah battery as an example. You could discharge and recharge 80Ah in one larger cycle. Alternatively, you could discharge and recharge 20% of the battery's capacity five separate times. Once the programmed cumulative threshold has been reached, the BMS records the corresponding cycle. This gives you a much more meaningful cycle count than simply recording how many times a charger has been connected.
How Do Manufacturers Determine Battery Cycle Life?
Cycle life ratings are established by repeatedly charging and discharging battery cells under controlled laboratory conditions. Manufacturers define the conditions of the test so that each cycle is performed consistently. Depending on the battery and test methodology, these conditions can include:
- Depth of discharge
- Charge current
- Discharge current
- Temperature
- Upper and lower voltage limits
- Rest periods between charging and discharging
- The remaining-capacity threshold used to determine end of life
The battery is then repeatedly cycled under those conditions while its capacity is monitored. This is important because changing the testing conditions can significantly affect the resulting cycle life. A battery discharged gently to 50% DoD, for example, will generally achieve more cycles than the same battery repeatedly discharged to 80% or 100% DoD.
So, the headline cycle number should never be considered in isolation.
What Does a 0.5C Charge and Discharge Rate Mean?
Another important part of cycle testing is the C-rate. This describes the charge or discharge current relative to the battery's capacity. A rate of 1C means a current equivalent to the battery's rated Ah capacity. For a 100Ah battery, 1C is 100A. A rate of 0.5C is half of that, or 50A.
Our typical 4,000-cycle rating for KickAss lithium batteries is based on continuous charging and discharging at 0.5C. Using a 100Ah battery as an example, the battery can discharge 80% of its capacity at 50A and then recharge at the same rate. Under the specified test conditions, that represents an 80% DoD cycle.
C-rate matters because higher charge and discharge currents generally place more stress on battery cells. Lower current draw, provided the battery is otherwise being operated within its specifications, can be gentler on the cells.
Touring applications often involve loads considerably lower than a continuous 0.5C discharge. As a result, a battery regularly cycled at lower rates may achieve more cycles before its capacity declines to the specified end-of-life threshold.
Does 4,000 Cycles Mean the Battery Dies After Cycle 4,000?
No. This is one of the most important things to understand about battery cycle ratings. Cycle life isn't normally measured until a battery completely stops functioning. Instead, the manufacturer defines an end-of-life capacity threshold.
For KickAss lithium batteries rated for 4,000 cycles, this rating is typically based on the battery retaining 80% of its original capacity after those cycles under the specified test conditions. So, if a 100Ah battery reaches its rated cycle life and has 80% of its original capacity remaining, it effectively has around 80Ah of capacity available.
Therefore, it hasn't suddenly stopped working. It simply no longer stores as much energy as it did when new. The battery may continue operating for many more cycles after reaching this point, although its available capacity will continue to decline as the cells age.
Why Depth of Discharge Makes Such a Difference
Depth of discharge is one of the biggest factors affecting a battery's cycle life. Repeatedly using a smaller proportion of the battery's total capacity generally places less stress on the cells than regularly discharging them very deeply. This means a manufacturer might be able to advertise a much larger cycle figure if the battery has been tested at a shallower DoD.
For example, a battery tested at 50% DoD may achieve considerably more cycles than the same battery tested at 80% DoD. But each individual cycle also represents less energy delivered. This is why comparing a “6,000 cycle” battery with a “4,000 cycle” battery based purely on those two numbers can be misleading. You need to know the depth of discharge used for each test.
Total Energy Throughput Can Tell You More Than Cycle Count
Another useful way of thinking about battery longevity is the total amount of energy the battery can deliver over its usable life. Imagine two batteries of the same capacity. One offers a very high cycle count, but that figure is based on relatively shallow discharges. The other has a lower cycle count but allows considerably more of its capacity to be used during every cycle.
The battery with the higher cycle count isn't automatically going to deliver more energy over its lifetime. That's why cycle count, DoD and usable capacity need to be considered together.
Laboratory Cycle Life vs Real-World Battery Life
Controlled testing gives us a consistent way to measure and compare battery performance, but your battery doesn't live in a laboratory. In a caravan, camper trailer, 4WD or off-grid setup, its usage will be much more variable. You might run a fridge continuously, use an inverter for short periods, charge from solar during the day, top up from your DCDC charger while driving and rarely discharge the battery by exactly the same amount twice.
Environmental and usage factors can also influence how quickly a battery ages, including:
- High temperatures
- Very high charge or discharge currents
- Charging habits
- How the battery is stored
- How deeply it is regularly discharged
- Extended periods at very high or very low states of charge
- Vibration and environmental conditions
Some real-world conditions may shorten battery life compared with laboratory testing. Others, such as consistently lower discharge rates and shallower cycling, can be gentler than the conditions used to establish the battery's published cycle rating.
How Should You Compare Battery Cycle Ratings?
When you're comparing lithium batteries, don't stop at the biggest cycle number on the spec sheet. Look at the conditions attached to that figure. In particular, check:
- How many cycles is the battery rated for?
- At what depth of discharge?
- At what charge and discharge rate?
- What remaining capacity defines the end of the rated cycle life?
- Are the testing conditions clearly stated?
Two batteries might both advertise “4,000 cycles”, but those figures aren't necessarily equivalent if one was tested at 80% DoD and the other at 50%, or if different charge and discharge rates or end-of-life thresholds were used. The cycle count is useful, but it's the conditions behind that number that tell you what you're really getting.
The Bottom Line
A battery cycle rating is more than a simple count of how many times a battery can be charged. It represents repeated use of a defined amount of battery capacity under specific testing conditions. Depth of discharge, charge and discharge rate, temperature and the manufacturer's end-of-life threshold all influence the final cycle life figure.
For KickAss lithium batteries carrying our typical 4,000-cycle rating, testing at 80% DoD and a continuous 0.5C charge and discharge rate provides a demanding, consistent benchmark. In many touring applications, batteries experience lower current draws and less uniform cycling than they do under these test conditions.
So next time you're comparing two lithium batteries, look beyond the headline cycle count. A figure such as “4,000 cycles” only tells part of the story. The DoD, C-rate and remaining capacity behind it are what give that number meaning.
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* Footnote: Not all BMSs record energy throughput or cycle count. This functionality is generally found in smart BMSs with software-based monitoring, such as those used in the KickAss Ultra-X range. Basic BMSs without communication or monitoring capabilities may not record cycle data.
