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5. Discharge Methods
The purpose of a battery is to store energy and release it at the appropriate time in a controlled manner. Being capable of storing a large amount of energy is one thing; the ability to satisfy the load demands is another. The third criterion is being able to deliver all available energy without leaving precious energy behind when the equipment cuts off.
In this chapter, we examine how different discharge methods can affect the deliverance of power. Further, we look at the load requirements of various portable devices and evaluate the performance of each battery chemistry in terms of discharge.
5.1 C-rate
The charge and discharge current of a battery is measured in C-rate. Most portable batteries, with the exception of the lead acid, are rated at 1C. A discharge of 1C draws a current equal to the rated capacity. For example, a battery rated at 1000mAh provides 1000mA for one hour if discharged at 1C rate. The same battery discharged at 0.5C provides 500mA for two hours. At 2C, the same battery delivers 2000mA for 30 minutes. 1C is often referred to as a one-hour discharge; a 0.5C would be a two-hour, and a 0.1C a 10 hour discharge.
The capacity of a battery is commonly measured with a battery analyzer. If the analyzer’s capacity readout is displayed in percentage of the nominal rating, 100 percent is shown if 1000mA can be drawn for one hour from a battery that is rated at 1000mAh. If the battery only lasts for 30 minutes before cut-off, 50 percent is indicated. A new battery sometimes provides more than 100 percent capacity. In such a case, the battery is conservatively rated and can endure a longer discharge time than specified by the manufacturer.
When discharging a battery with a battery analyzer that allows setting different discharge C-rates, a higher capacity reading is observed if the battery is discharged at a lower C-rate and vice versa. By discharging the 1000mAh battery at 2C, or 2000mA, the analyzer is scaled to derive the full capacity in 30 minutes. Theoretically, the capacity reading should be the same as a slower discharge, since the identical amount of energy is dispensed, only over a shorter time. Due to energy loss that occurs inside the battery and a drop in voltage that causes the battery to reach the low-end voltage cut-off sooner, the capacity reading is lower and may be 97 percent. Discharging the same battery at 0.5C, or 500mA over two hours would increase the capacity reading to about 103 percent.
The discrepancy in capacity readings with different C-rates largely depends on the internal resistance of the battery. On a new battery with a good load current characteristic or low internal resistance, the difference in the readings is only a few percentage points. On a battery exhibiting high internal resistance, the difference in capacity readings could swing plus/minus 10 percent or more.
One battery that does not perform well at a 1C discharge rate is the SLA. To obtain a practical capacity reading, manufacturers commonly rate these batteries at 0.05C or 20 hour discharge. Even at this slow discharge rate, it is often difficult to attain 100 percent capacity. By discharging the SLA at a more practical 5h discharge (0.2C), the capacity readings are correspondingly lower. To compensate for the different readings at various discharge currents, manufacturers offer a capacity offset.
Applying the capacity offset does not improve battery performance; it merely adjusts the capacity calculation if discharged at a higher or lower C-rate than specified. The battery manufacturer determines the amount of capacity offset recommended for a given battery type.
Li-ion/polymer batteries are electronically protected against high discharge currents. Depending on battery type, the discharge current is limited somewhere between 1C and 2C. This protection makes the Li-ion unsuitable for biomedical equipment, power tools and high-wattage transceivers. These applications are commonly reserved for the NiCd battery.