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Plus in newer cars, the power output from the lighter plug is so reliable that it can charge laptops, without any voltage problems. Standardized 12 volt DC automobile plugs are defined in the United States by UL Standard 2089 regarding vehicle battery adapters. This standard covers plugs and cord sets that insert into cigarette lighter receptacles.
The device typically contains several capacitors and charge and discharge circuitry. When the device is connected to a USB port, the capacitors are charged from the USB port's 5 volt supply. When they are fully charged, the device discharges them through step-up circuitry, which delivers a high voltage back into the USB port.
Often pins 1 and 2 will be negative, 3 and 4 positive for a higher current rating. Female connectors are used as supply and male connectors are used on loads. Most battery belts and power supplies output 13.2 V, but equipment can usually handle a range of 11–18 volts to accommodate battery packs of varying voltages and charging while operating.
A 20-volt level was added in a revision named "class B" of the specification. [12] [13] The voltage range of the successor Pump Express Plus 2.0 is between 5 volts and 20 volts, with 0.5 volts steps. The Quick Charge 3.0 protocol supports finer-grain voltage levels with 0.2 volts steps and has a lower minimum voltage of approximately 3.3 volt.
An intelligent charger may monitor the battery's voltage, temperature or charge time to determine the optimum charge current or terminate charging. For Ni–Cd and Ni–MH batteries , the voltage of the battery increases slowly during the charging process, until the battery is fully charged.
Quick Charge is a proprietary technology that can charge battery-powered devices, primarily mobile phones, at power levels exceeding the 7.5 watts (5 volts at 1.5 amps) supported by the USB BC 1.2 standard, using existing USB cables. The higher voltage available allows more power (watts) to be supplied through wires without excessive heating.
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A charger and either no device or an A-device that is not asserting V BUS (not providing power) are attached. The OTG device is allowed to charge and initiate SRP but not connect. [7] A charger and an A-device that is asserting V BUS (is providing power) are attached. The OTG device is allowed to charge and connect but not initiate SRP. [7]