Cycle life

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The 2 most important parameters impacting the performance of a battery | Cycle life | Self discharge | Technology comparison | Battery pack configurations

Cycle life

A battery does have a limited life. This life is expressed of number of cycles that the battery has gone through. One cycle means one discharge followed by a charge. Each cycle the battery is loosing a bit of its initial capacity. This loss of capacity is initially a linear process, but the closer the battery gets to its end of life the bigger the loss per cycle will be. At one stage the loss of capacity is getting so big that the battery becomes technically dead. This depends very much on the technology used.

The technical end-of-life is reached when the discharged capacity is at one of the following levels:

Ni-Cd: 60% of initial capacity
Ni-MH: 75% of initial capacity
Li-Ion: 80% of initial capacity

The following parameters have a direct impact on the cycle life of the battery:

The number of cells per charge string
The number of cells in discharge (e.g don't put to many strings in parallel)
The degree of optimisation of the charger on the battery
Depth of Discharge per cycle
Extreme temperatures
The number of internal connections as this increases the battery impedance
The application of the battery, meaning does the battery have a cycling function or a standby function

In rated capacity conditions a good quality cell will give 500 cycles.

The last remark in the above listing is crucial. This is because it makes the difference between a back up battery, that normally only has a very limited number of discharges, and a cycling battery. In the last type of application the battery is the primary power source of the device. Think of a mobile telephone.

Cycling batteries:

The cycle life of the battery is defined by the number of discharges a battery can make. Hence if the battery has to provide all the stored energy it will suffer more from that than when the battery has only been discharged for 50% for example. The conclusion is that battery life heavily depends on what we call the Depth of Discharge ( the percentage of discharged energy compared to the initially stored capacity).

Standby batteries:

Batteries also suffer from overcharge, especially in temperatures other then room temperature. This means the charger continues to put energy in the battery although the battery is already fully charged. Every time the charger has tried to put the equivalent of Ah of the capacity of battery after it has been fully charged, we talk about one overcharge cycle. In contrary of cycling applications where the number of discharge cycles defines the life of the battery, the expected life in a standby application is defined by the number of overcharge cycles.

Basically there is for each temperature a theoretical number of overcharge cycles before the battery is technically dead. Once you know this number, the life then become function of the charge current. With a high charge current you reach faster one full overcharge cycle, meaning the technical battery life expressed in time will be shorter.

Example:

Suppose a battery can sustain 1500 overcharge cycles at a certain temperature. One battery is charged with a current of 1C. That means that every hour the battery has had a new overcharge cycle. The battery life in that case is 1500 hrs, meaning less than 2.5 months.

The second battery is charged with a current of 0.05C, meaning it reaches an overcharge cycle every 20 hrs. That means that the battery life becomes 20*1500=30000 hrs, which is almost 3.5 years.

One overcharge cycle is when after the battery has been fully charged the charger has tried to put a number of Wh into the cell that is equal to the Wh's that is needed to fully charged a discharged cell.