Observe how the charge and discharge rates are scaled and why it matters.
In the late 1700s, Charles-Augustin de Coulomb ruled that a battery receiving a charge current of one ampere (1A) passes one coulomb (1C) of charge per second. On discharge, the process reverses. Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.
C-rate is a measure that governs at what current a battery is charged and discharged. A 1C charges a battery that is rated at, say, 1,000mAh at 1,000mAh; a 1C discharge loads the battery at that same rate. The Ah rating is normally marked on the battery.
The capacity of a rechargeable battery is commonly rated at 1C, meaning that a 1,000mAh battery should provide a current of 1,000mA for one hour. The same battery discharging at 0.5C would provide 500mA for two hours, and at 2C, the 1,000mAh battery would deliver 2,000mA for 30 minutes. 1C is also known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is a half-hour discharge.
The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. (See BU-909: Battery Test Equipment.) The analyzer discharges the battery at a calibrated current while measuring the time until the end-of-discharge voltage is reached. An analyzer displaying the results in percentage of the nominal rating will show 100 percent if a 1,000mAh battery provides 1,000mA for one hour. If the discharge lasts 30 minutes before reaching the end-of-discharge cut-off voltage, then the battery has a capacity of 50 percent. A new battery is sometimes overrated and can produce more than 100 percent capacity; others are underrated and never reach 100 percent even after priming.
When discharging a battery with a battery analyzer capable of applying different C rates, a higher C rate will produce a lower capacity reading and vice versa. By discharging the 1,000mAh battery at the faster 2C, or 2,000mA, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same than with a slower discharge since the identical amount of energy is being dispensed, only over a shorter time. In reality, internal resistance turns some of the energy into heat and lowers the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over two hours, will likely increase the capacity to above 100 percent.
To obtain a reasonably good capacity reading, manufacturers commonly rate consumer alkaline battery as well as lead acid packs at a very low 0.05C, or a 20-hour discharge. Even at this slow discharge rate, lead acid seldom attains a 100 percent capacity. Manufacturers provide capacity offsets to adjust for the discrepancies in capacity if discharged at a higher C rate than specified. (See also BU-501a: Calculating the Battery Runtime.) Figure 1 illustrates the discharge times of a lead acid battery at various loads as expressed in C-rate.
Figure 1: Typical discharge curves of lead acid as a function of C-rate
Smaller batteries are rated at a 1C discharge rate. Due to sluggish behavior, lead acid is rated at 0.2C (5h) and 0.05C (20h).
While lead- and nickel-based batteries can be discharged at a high rate, the protection circuit prevents the Li-ion Energy Cell from discharging above 1C. The Power Cell with nickel, manganese and/or phosphate active material can tolerate discharge rates of up to 10C and the current threshold is set higher accordingly.
Last updated 2015-06-23
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