Wednesday, November 27, 2013
How Do Battery Chargers Vary Amps
Battery chargers perform a fairly simple task: convert alternating-current power to direct-current and transfer that energy to a battery. But this is a multidimensional challenge, since household current is 115 volts and upward of 200 amps, while the battery itself only needs 12 to 14 volts and 2 to 10 amps. The chargers control system is similar to that of an automotive alternator, but the control strategy varies by manufacturer and unit quality.
Ohms Law
Ohms Law got its name from Georg Ohm, the 18th century German physicist who defined electrical resistance as we know it today. Using the electrochemical cell pioneered by Italian count Assandro Volta (guess what got his name), Ohm quantified the proportional difference between voltage applied to a conductor and the resultant electrical current. The difference (or loss) within the circuit is its resistance in Ohms. Ohms basic law states that current in amperes flowing through a conductor equals the voltage applied across the circuit divided by its resistance in Ohms. In mathematical shorthand, the formula is " I = V / R", where "I" equals current in amperes, "V" equals volts and "R" equals resistance in ohms.
Consequences of Varying Resistance
Installing a resistor in-line between a power source and an accessory does have consequences. The First Law of Thermodynamics states that energy can never appear or disappear; it can only change forms. If you install an electric motor into an electrical circuit, the electrical energy turns into kinetic energy (movement); install a light bulb and the electrical energy turns into light. But not all of that energy will convert completely; a certain amount of it will get "stuck" in the devices internal resistance and bounce around to produce heat. If you increase resistance in the circuit by installing a higher-resistance material such as graphite or ceramic, the excess current will vibrate around in the material and cause it to heat up.
Varying Load
The other way to limit amperage through a circuit is to limit its amp draw, or the load that its physically capable of pulling without a resistor in line. The tiny motor in your electric razor blade will only draw a few milli-amps regardless of whether you connect it to a triple-A battery or a nuclear reactor. Thats because the diameter of the wires in the motor will only pass a certain amount of current. You can think of the wiring diameter as a hole in a bucket of water; a bigger hole will pass more water and a smaller one will pass less. You can shove more water through by increasing the water pressure (similar to increasing voltage in the electrical circuit), but the big hole will always flow more at a given pressure.
Battery Charger Strategy
Battery chargers will typically use either a load-varying approach or a combination of load-varying and resistance-varying. A high-quality charger will use multiple circuits with varying load draws to transfer power from the input to the output. When you flip the switch from a 10-amp quick-charge to a 2-amp trickle-charge, the charger will switch to a lower-draw circuit. Cheap chargers will typically use a calibrated circuit to maintain the maximum 10-amp quick-charge; flipping the switch to a 2-amp trickle-charge will re-route power through a higher resistance diode or resistor that absorbs the additional 8 amps, which is why cheap chargers will typically get far hotter under trickle charge than more expensive ones. This heat both reduces component life and wastes electricity.
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