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Maxim > Design Support > Technical Documents > Application Notes > Battery Management > APP 3607
Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 3607
Keywords: USB, USB Charger, Li+ USB charger, Lithium Ion USB charger, NiMH USB charger, USB
battery charger, charging batteries from USB, charging from USB, charger, battery charger
APPLICATION NOTE 3607
Charging Batteries from USB
Sep 23, 2005
Abstract: USB offers great opportunities as a power source for all types of low power electronics, many
of which are battery operated. The widespread availability of USB presents unique opportunities as well
as challenges for battery-charging designs. This article describes how to interface a simple battery
charger to a USB power source. This review of USB power bus characteristics include an overview of
NiMH and Li+ battery technologies, charging methods, and charge termination techniques, and a
complete circuit example for smart-charging NiMH cells from a USB port.
Introduction
The Universal Serial Bus (USB) port is a bidirectional data port with power and ground. Peripherals of all
types can be connected to the USB, including external drives, memory devices, keyboards, mice,
wireless interfaces, video and still cameras, MP3 players, and countless other electronics. Many of these
devices are battery powered, some with internal batteries. The widespread availability of USB presents
unique opportunities, as well as challenges, for battery-charging designs. This paper describes how to
interface a simple battery charger to a USB power source. This review of USB power bus characteristics
includes voltage, current limits, inrush current, connectors, and cabling. An overview of nickel metal
hydride (NiMH) and lithium battery technologies, charging methods, and charge-termination techniques is
given. A complete example circuit for smart-charging NiMH cells from a USB port is presented, along
with charging data.
USB Characteristics
The USB bus can provide power for low-power electronics. It is isolated from power mains and is
relatively well regulated. However, there are limitations on available current and potential interactions
between the load and the host or power source.
The USB port consists of a 90Ω bidirectional differential shielded twisted pair, V
BUS
(+5V power), and
ground. These four wires are shielded with an inner shield of solid aluminum and a stranded outer shield.
Copies of the USB 2.0 specification are available free of charge from the USB organization. Full
compliance with the specification requires bidirectional communication between the device and the host
through a function controller. The specification defines a unit load as 100mA (max). The maximum
current that any device is allowed to draw is five unit loads.
USB ports are classified as either low-power ports, which supply up to one unit load, or high-power ports
which supply up to five unit loads. When devices are first connected to the USB port, an enumeration
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