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A common battery is an electrical power source that provides direct current (DC) power for electronic circuits used in various applications like telephony and data communications. Available in many voltages from about 1.2 to 12 volts (V) or more, the common battery may or may not include a charger or rectifier for maintaining the charge or for replacing any lost charge when the alternating current (AC) mains power is interrupted. The charger/rectifier system for a common battery generally serves two purposes.
The system provides the power necessary to drive the electronic circuitry for the specific system and to maintain the common battery charge so that the charger-rectifier-battery system is able to provide DC power during AC power outages. Common battery systems for telecommunications use generator sets that are programmed to maintain the charger/rectifiers energized during AC power outages. In some cases, such as remote sites in hard-to-reach locations, solar panels together with AC mains and generator sets provide the needed AC power source redundancy.
Choices for the common battery are usually the rechargeable type. For instance, the telecommunications stations use a –48 volts direct current (VDC) common battery voltage. It is common to use a series branch of four 12 V batteries to make a 48 V leg. Several 48 V legs are connected in parallel to build a battery bank that provides the needed ampere-hour (A-h) capacity.
The A-h capacity of the common battery bank is related to the sizes of the batteries in the battery bank. A 12 V battery rated at 48 A-h can provide theoretically 4 amperes (A) for 12 hours (h). If the design requires that the equipment during AC power outage will disconnect at 10 V, the battery will provide 4 A for 2 h only instead of 12 h. A 48 V battery bank using four 12 V batteries as mentioned will have a peak capacity of 48 A-h, but given the constraint, the 48 V nominal will be allowed to drop to 40 V before the power system disconnects the load on a low-voltage disconnect condition.
For fixed stations DC power, the common practice is to connect the positive side of the battery bank to earth or ground. By keeping the live side of DC at negative potential, a natural cathodic protection of the equipment prevents additional chemical oxidation of exposed metal parts and electronic components. Cathodic protection is a way of preventing corrosion on any critical metallic infrastructure such as vehicles and pipelines for fuel.