A battery is a type of voltaic cell used to store electric energy with a wide range of application in virtually all branches of engineering and industry. Regardless of the type, design and dimensions, each battery has a rated voltage and capacity in ampere-hours. Battery capacity in technical terms, it is a rated capacity referring to a fully working and charged battery discharged with direct current for 20 hours at 20°C until the voltage of a single cell reaches 1.7 V (10.2 V for 12 V battery). Discharging the battery below 10 V may lead to sulfation and as a result damage or severely reduce its capacity. The rated capacity is directly related with the definition of the rated current that specifies the amount of current discharging the battery in 20 hours. It is calculated by dividing the battery capacity by 20. For example, the rated current for a 40 Ah battery is 2 A. The higher the value, the higher the battery capacity, and the longer the time the device can operate until fully discharged.
Fig. 1. Two popular batteries available in Delta offer. Left: 12V, 7.2Ah and 12V, 18Ah (both are extended life batteries)
In practice, the battery capacity is the battery’s ability to accumulate the electric charge, and ampere-hour (Ah) is a product of discharge current (A) and discharge time (h) until a specific voltage is reached. For example, 18 Ah battery should, in theory, supply an electric circuit with 1A current for 18 hours. The battery life will be extended to 36 hours, if the current input of the supplied circuit is 0.5A. In practice, the values can vary depending on the discharge mode, operation and battery conditions.
The capacity specified by the manufacturers is based on battery tests at 20°C. Depending on the technology used, the battery capacity can be affected by temperature to a varying extent. With the increase in temperature the battery is discharged at, its capacity is reduced. For example, at 0°C the acid-lead battery capacity is 85% of its rated capacity. A decrease in battery capacity can be observed at negative temperatures, and at –10°C it is 75% of its rated capacity. The batteries can also be affected by high temperatures. High battery temperature also affects its service life and may result in premature wear. For example, when installing batteries in UPS devices, free air flow around the battery must be ensured.
For smaller batteries used in electronic devices, portable computers or mobile phones, the capacity is specified in a submultiple Ah unit - mAh (milliampere-hour).
The battery capacity is also determined by its charging current. As a general rule, the charging current should not exceed 0.1 of the battery capacity. 18 Ah batteries should be discharged with 1.8A current. The battery capacity is reduced with each charge/discharge cycle. Correct operation, ambient conditions and avoiding complete discharge will extend the service life of a battery.