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Why Do Chargers Need to Change Watts?

Thousands of electronic devices for personal and industrial use have batteries that need chargers to keep them powerful. All chargers produce a certain amount of electrical power expressed in watts that changes during the charging process. Most chargers decrease wattage as batteries charge. Some chargers have additional settings to charge different sizes and types of batteries.

Ampere Hours and Charging
- Batteries designed at a specific voltage have different ampere hour ratings, which equal the number of amps provided for a certain amount of time. For example, a 12-volt laptop battery may provide one of current for four hours, while a car battery may provide 1 for 40 hours to run accessories or 400 amps for six minutes to start a car before it dies. Charging a battery works in the opposite manner. The charger provides a combination of current and time to basically equal the battery's ampere hour rating.
Watts
- Watts equals voltage times current. If a battery charger provides 10 amps of current at 12 volts, it is producing 120 watts of electricity. However, the wattage rating of a charger usually tells the maximum capacity of the power it can produce. It does not mean that it will produce that amount all the time while it is charging. A more important specification of a charger is the voltage. The wattage rating of the charger results when combined with the current needed for a specific type of battery at the maximum charge rate.
Battery Charging
- Usually chargers work with a constant voltage applied to the charger. When the process starts, the charger produces the maximum power for the initiation of the charge. A 120 watt, 12-volt car battery charger for example, may start charging at 10 amps, the full capacity. As the charge proceeds, the current rate slows down. When it charges at 5 amps, it's only producing 60 watts of power, and at 2 amps, only 24. A trickle charge of one only produces 12 watts of power.
Adjustable Chargers
- An industrial charger used to keep the batteries constantly charged in a telephone or cellphone switching office has an equalizing charge setting. It periodically adjusts the voltage to maintain a constant current through a string of 24 2-volt batteries to keep their chemical characteristics the same. Depending on the battery type, an equalizing charge may last for three or four hours once a month. Because watts equal voltage times current, wattage changes as the voltage varies to keep the current constant while doing an equalizing charge.

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