Battery Charging on USB （1）

Published:2011/8/14 23:02:00 Author:Phyllis From:SeekIC

By Harry Baggen

Besides being handy for connecting and disconnecting equipment while the computer is running, USB includes a built-in supply line that can provide a certain amount of power to connected devices. That can be handy for quickly charging a battery when you can’t find a real charger.

Nowadays every computer has several USB ports, which have taken over the task formerly handled by the RS232 bus. USB cables have different connectors at each end to prevent incorrect connections. Figure 1 shows types of device receptacles, which are used in peripheral equipment.





A USB connection has only four lines, consisting of a data line in each direction, a ground line, and a supply line. The supply line can be used to power peripheral devices. The host device (a PC or notebook computer) can source a maximum of 500 mA from each USB port. In USB terms, this is called five unit loads. A hub with its own power supply can provide the same amount, but a bus-powered hub can provide only one unit load per port. However, the host will only provide a current of 500 mA after communication with the connected USB device. Immediately after a connection to a USB device has been established, the device cannot draw more than 100 mA from the bus. The host does not decide whether to allow the device to draw a maximum current of 500 mA until the identification phase (’enumeration’) has been completed.

Simple devices actually do not need this enumeration, as long as they ensure that they don’t draw more than 100 mA from the bus. This can be exploited to simply tap off a bit of current from a USB port without causing the computer any problems. Some examples are mini-lamps and small fans with USB cables, which are currently sold for use with notebook computers. They ’misuse’ the USB port in exactly this manner.

This 100 mA can also be used to charge a couple of batteries when you don’t have a real , charger to hand. That’s very easy to do, as can be seen from Figure 2. A diode and a resistor are sufficient for charging two AA (penlight) or AAA cells connected in series, either NiCd or NiMH. It takes a while, since you’re limited to a current of 100 mA, but in an emergency it’s a perfectly good solution.





The component values for the circuit assume that the supply voltage on the USB line has a nominal value of 5 V and a minimum value of 4.5 V. The diode is for protection, to prevent damage to the USB port if the batteries are connected backwards.

Resistor Rl is dimensioned for a current of approximately 90 mA through the fwo NiCd or NiMH cells. They can handle this for days on end without becoming overcharged, but it also means that they can’t be fully charged in just a few hours. But once again, this is intended as a makeshift solution.

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