Realization of intelligent charging of NiMH battery by USB port

Overview

The Universal Serial Bus (USB) port is a two-way data port with power and ground. USB can connect to all types of peripherals, including external drives, storage devices, keyboards, mice, wireless interfaces, cameras and cameras, MP3 players, and countless electronic devices. Many of these devices are battery powered, some with built-in batteries. For battery charging design, the wide application of USB brings both opportunities and challenges. This article explains how to interface a simple battery charger to a USB power source. The article reviews the characteristics of the USB power bus, including voltage, current limit, inrush current, connectors, and cable connections. At the same time, nickel-hydrogen battery (NiMH) and lithium battery technology, charging method and charging termination technology were introduced. A complete example circuit is presented for intelligent charging of the NiMH battery via the USB port and charging data is given.

USB features

The USB bus provides power to low-power electronic devices. The bus power is isolated from the grid and has good stability. However, the available current is limited and there are potential interoperability issues between the load and the host or power supply.

The USB port consists of a 90Ω bidirectional differential shielded twisted pair cable, VBUS (+5V power supply) and ground. These four wires are shielded by an inner foil shield and a braided outer shield. The latest USB specification is version 2.0 and is available for free from the USB organization. To fully comply with the specification, a two-function communication between the device and the host is required through a function controller. The specification defines a unit load of 100 mA (maximum). The maximum current that any device is allowed to draw is 5 unit loads.

The USB port can be divided into two types: low power port and high power port. The low power port can provide one unit load current, and the high power port can provide up to 5 unit load current. When the device is just connected to the USB port, the enumeration process identifies the device and determines its load requirements. During this process, only the device is allowed to draw up to 1 unit load of current from the host. After the enumeration process is completed, the high-power device can draw more current if the host's power management software allows it.

Some host systems (including downstream USB hubs) provide current limiting through fuses or active current detectors. If the USB device draws a large current (more than 1 unit load) from the USB port without enumeration, the host will detect an overcurrent condition and will shut down one or more USB ports in use. Many of the USB devices available on the market, including stand-alone battery chargers, do not have a function controller to handle the enumeration process, but draw more than 100mA. Under such inappropriate conditions, these devices may cause problems with the host. For example, if a device that draws 500mA is plugged into a bus-powered USB hub and the correct enumeration process is not performed, the hub port and host port may be overloaded at the same time.

The situation is even more complicated when the host operating system uses advanced power management, especially for laptops, which always want the port current to be as low as possible. In some power save modes, the computer issues a suspend command to the USB device, and then the device is considered to be in low power mode. It is always a good idea to include a function controller that can communicate with the host, even for low-power devices.

The USB 2.0 specification is very comprehensive and specifies the quality of the power supply, connector construction, cable material, allowable voltage drops, and inrush current. Low current and high current ports have different power supply specifications. This is primarily determined by the voltage drop across the connector and cable between the host and the load, and includes voltage drops on the USB powered hub. Hosts, including computers or self-powered USB hubs, have high current ports that provide up to 500mA. Passive, bus-powered USB hubs have low current ports. Table 1 lists the allowable voltage tolerances for the upstream (power) pins of the USB high current and low current ports.

Table 1. USB 2.0 Specification Power Quality Standards

* These metrics apply to the connector pins of the upstream host or hub port. I x R drops on cables and connectors require additional consideration.

In a USB 2.0 compliant host, the upstream end of the high power port has a 120μF, low ESR capacitor. The input capacitance of the connected USB device is limited to 10μF. During the initial load connection phase, the maximum amount of charge that the load is allowed to draw from the host (or self-powered hub) is 50μC. As a result, when the new device is connected to the USB port, the transient voltage drop of the upstream port is less than 0.5V. If a larger input capacitor is required for normal load operation, an inrush current limiter must be provided to ensure that the current does not exceed 100mA when charging a larger capacitor.

When the USB port has a bus-powered USB hub and the hub is connected to a low-power device, the DC voltage allowed on the USB port drops as shown in Figure 1. When a high-power load is connected to a bus-powered hub, the voltage drop will exceed the specifications given in Figure 1 and cause a bus overload.

Figure 1. A bus overload caused by a host-to-low-power load voltage drop greater than the allowable DC voltage drop shown in the figure.

Battery charging requirements

Single cell lithium ion and lithium polymer battery

Today's lithium batteries are typically charged between 4.1V and 4.2V after charging to their maximum rated capacity. The newer, larger, larger-capacity batteries currently on the market, with voltages ranging from 4.3V to 4.4V. Typical prismatic lithium ion (Li+) and lithium polymer (Li-Poly) batteries have capacities ranging from 600 mAh to 1400 mAh.

For Li+ and Li-Poly batteries, the preferred charging curve starts with constant current charging and continues until the battery voltage reaches the rated voltage. The charger then adjusts the voltage across the battery. These two adjustment methods constitute a constant current (CC) constant voltage (CV) charging method. Therefore, this type of charger is often referred to as a CCCV charger. After the CCCV charger enters the CV mode, the charging current of the battery begins to drop. If charging is performed at a typical charging rate of 0.5C to 1.5C, the charger is switched from CC mode to CV mode when the battery reaches 80% to 90% of its full capacity. Once the charger enters the CV charging mode, the battery current is monitored; when the current reaches the minimum threshold (a few milliamps or tens of milliamps), the charger terminates charging. A typical charging curve for a lithium battery is shown in Figure 2.

Figure 2. Typical curve when charging a Li+ battery using a CCCV charger

Capacitor Parts Forming Machine

SF-501C has High production efficiency, 15-20 tubes can be placed in one time,which is 2-3times that of manual work.