One of the most important battery operational factors besides energy and power density is the cycle life. It determines how many times a battery will be able to deliver charge–discharge cycles without its capacity dropping below a certain level, typically 80% of the original capacity. The manufacturers specify the cycle life for certain conditions of charge and discharge such as C-rate, temperature, and the depth of discharge. Other factors may affect cycle life as well.
The cycle life is clearly different from the calendar life, which simply defines the time during which battery components age and affect the performance so that it fails below 80%. The calendar life may include time in operation or storage.
The table provides typical values for both calendar and cycle life for a number of battery systems (Table2.4).
The cycle life for secondary batteries can vary widely, from less than a hundred to even 10,000 (cycles) for some special nickel–cadmium batteries. From the
% capacity loss per year
0 10 20 30 40
5 10 20 50 100 200 500
Li-ion Zn-Ag0
Ni Cd Lead acid Fig. 2.19 Capacity loss per
year as a function of temperature for selected batteries. Data in this graph are conceptual and not based on actual batteries
Table 2.4 Calendar and cycle life for selected battery systems
Battery Calendar Cycle life
Lead–acid, SLI 3–6 months 200–700 Sealed NiCd (FNC) 5–20 months 500–10,000 Nickel metal hydride 2–5 months 300–600
Nickel iron 8–25 months 2000–4000
Zinc–silver oxide 2 months 50–100
Lithium cobalt oxide / 300–1000
2.11 Calendar and Cycle Life 39
information in the table it is clear that nickel–cadmium batteries are characterized by the highest cycle life. Lithium–ion battery chemistry shown in the table has the most common chemistry of cobalt oxide used in many portable devices. This particular battery chemistry is superior for its energy density, but a different lithium–ion chemistry, with iron phosphate cathode has a much longer cycle life of up to 3000.
The problem with battery cycle life is that it can only be estimated based on general empirical information and not absolutely determined. The actual battery cycle life will be known only at the end of its lifetime when a battery fails, which means when its capacity drops below specified value. Experimental determination of the battery cycle life is obviously complicated, as it would require a very long test time and a large number of batteries would need to be tested to failure. The accelerated lifetime testing methods often used for other electronic products would not be accurate since they would involve higher C-rates, which is known to change the cycle life. Requirements for accelerated tests are that the main aging parameter is not affected by the acceleration factor (i.e., stress parameter), which is usually the elevated temperature. The interconnectedness of numerous battery design parameters and operational factors such as voltage, current, self-discharge, internal resistance, and temperature makes the lifetime modeling and prediction extremely difficult. Some methods that estimate battery state of health have been shown to provide enough insight to reasonably predict the lifetime.
In addition, batteries employed in actual applications could be operating in very different conditions of temperature and actual depth of discharge, which would make prediction of lifetime even more difficult. The general reliability of batteries can be assessed similarly to other devices and is shown in Fig.2.20.
Failure rate is displayed versus time and there are three distinct regions in the typical shape called “bathtub” curve. Some batteries, just like other electronic devices, can have early failures due to manufacturing imperfections. After the initial period, failure rate becomes low and steady until battery materials start wearing out and reach end of life or wear out.
Besides the battery chemistry and construction, the two most critical operational factors for cycle life are depth of discharge and temperature. The depth of discharge impacts cycle life very directly for most batteries. The greater the depth of discharge, the shorter or lower the cycle life. This simply means that there is a certain battery
Failure rate
Time or number of cycles
“Bathtub curve”
Useful life
Early failures
Wearout
Fig. 2.20 General battery failure rate versus time
40 2 Operational Factors of Battery Systems
cycle life that depends on the battery chemistry, construction, and materials used;
and a nominal cycle life will be determined assuming that all active electrode masses are going to be engaged in charge and discharge reactions in every cycle. But, if a battery is not fully discharged on every cycle some of the active masses are not involved in the reaction and this has a positive effect on the battery life.
Cycle life also depends on temperature. Battery lifetime or cycle life testing can be done in a laboratory environment, at a constant and controlled temperature; and most cycle life specifications are given for conditions including a nominal tempera- ture of 20C. But clearly, over the lifetime of a battery the temperature will not be constant for most types of applications. This means that some average temperature will be taken into consideration when predicting the cycle life. The changes in temperature also have effects on battery components and particularly on the interfaces between the battery components. In general, the higher the temperature during battery cycling and storage, the lower the cycle life. As discussed before, higher temperature contributes to higher rates of reaction, but also faster aging and degradation of materials. The unwanted side reactions are also faster at higher temperature. All these factors contribute to decrease in cycle life with increased temperature.
The graph in Fig.2.21shows general trends for the cycle life as a function of the depth of discharge for three temperatures.
The graph underscores the previously discussed trends where cycle life decreases at higher depth of discharge and higher temperature. It is also obvious from the graph, by observing thefirst part of the graph, that the cycle life can be much higher than for shallow cycling or for cycling that involves low depth of discharge.
Effectively, a battery can last longer, in some cases much longer, if it is only partially discharged on every cycle.
However, there are problems with shallow cycling with nickel-based batteries, especially nickel–cadmium batteries. During repeated shallow cycling some of the active mass of cadmium and to a smaller extent nickel oxyhydroxide becomes inactive and that causes growth in crystal, converting active mass from smallfine
Cycle life
0 20 40 60 80 100
Depth of discharge, % T1< T2< T3
T1
T2
T3 Fig. 2.21 Cycle life as a
function of depth of discharge for a hypothetical battery at three different temperatures
2.11 Calendar and Cycle Life 41
particles with large surface area to larger lumps with lower surface area that are difficult to break. The reduction in surface area results in capacity loss. This is called a“memory effect,”as if a battery electrode remembers that it doesn’t need to keep all parts of the electrode active. The memory effect can be simply prevented by regularly fully discharging a battery. This discharge cycle can take place even only once every 3 months and that would prevent the growth of cadmium crystals while preservingfinely dispersed particles with high surface area. Therefore, the memory effect is not an irrepressible fundamentalflaw of nickel–cadmium batteries, it is a myth based on poor understanding of the basic principles and battery materials.