The rate at which a battery is discharged and its operating temperature have a profound effect on its capacity and life. An example of the effect of discharge rate on battery capacity is shown in Figure 9 for traction batteries. This figure shows that batteries discharged at a low rate will be able to deliver a higher capacity than those discharged at a high rate.

The depth of discharge also affects the life of a battery. As shown in Figure 10 for a typical traction battery, discharges beyond about 80% of capacity can be expected to shorten battery life.

Colder operating temperatures will yield a little extra life, but also lower the capacity of leadacid cells. High temperatures yield higher capacity (see Figure 11) but have a detrimental effect on life.

Figure 9. Typical effects of discharge rate on battery capacity.

Figure 10. Typical effects of depth of discharge on traction battery life.

Figure 11. Typical effects of operating temperatures on traction battery capacity.

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APPLICATIONS

There are numerous applications for the use of lead-acid storage batteries. They range from the extremely large battery systems used in load leveling by electrical utility companies to the relatively small batteries used in hand tools. Batteries may need to undergo deep and frequent cycling such as those used for electric vehicle power or they may remain on "float" as in an emergency lighting application and only rarely be discharged. Clearly, these applications cannot all use the same battery. Restraints on parameters such as operating temperature, desired capacity, voltage and power requirements, etc., affect the type of battery chosen.

All the above considerations are taken into account by determining the battery's duty cycle. The duty cycle is the required operating parameters of a cell or battery including factors such as charge and discharge rates, depth of discharge, cycle length, and length of time in the standby mode. The duty cycle must be known and included in the battery procurement specification. The duty cycle and battery chosen will also determine the type of charger used. More details on matching chargers to batteries and applications are included in later paragraphs. More information on duty cycles is included in the section entitled "Sizing and Selection."

The major categories of lead-acid battery applications are starting, lighting, and ignition (SLI); industrial, including traction and stationary applications; and small portable equipment. A brief description of each type is included below along with example uses of each type.