Choosing a USB Battery Charger

USB is a convenient power source for portable consumer devices, but it can be a tricky balancing act to optimize battery “care and feeding” for optimal performance.

The spec provides two types of charger ports for this purpose: dedicated charging port (DCP) and charging downstream port (CDP). DCPs can draw current up to 1.5A, while CDPs can pull in more from a hub to support faster recharge times.

Power Source

The USB port on a computer or laptop serves as a five-volt power source for personal handheld electronics like cell phones and digital audio players. It also provides the connection point for charging these devices with a cable.

Depending on the device you are charging, the type of cable and the charger itself, the voltage and current can affect the speed at which it charges the battery. For example, a thin cable can slow charging because it reduces the amount of current.

A higher quality cable can protect your device from overheating and harmful power surges, and ensure that your device charges as quickly as possible. Chargers that have a higher voltage and lower current allow for faster charging because they are able to charge more devices at the same time without running out of power.

Some devices can be charged by multiple batteries, such as a camera or a usb battery charger smartphone and tablet. These devices will often have more than one USB port, and will share the power provided by each of them. This is known as shared power and is common in larger batteries, such as a power bank.

These types of batteries can be recharging in multiple stages, which helps prevent damage to cells and maximizes the life span of the battery. This is a major benefit of Li+ batteries in portable consumer devices.

However, the low voltage of a Li+ battery can cause issues if the charger is not designed properly. For example, excess charging current, long overcharging, and cell reversal can damage cells and limit the lifespan of the battery.

To avoid these problems, many manufacturers rely on a combination of high-current and low-voltage charging circuitry. This combines the ability to sense a battery’s condition and detect charging needs with the low voltage of a Li+ battery, which makes it easier to control power and ensure that the charger maintains a stable battery voltage over time.

For example, the MAX8814 (Figure 6), a small USB peripheral device, incorporates simple Li+ charging functionality. The circuit uses an internal microprocessor to detect the USB port type and enumerate, and limits input current accordingly. The system then sends appropriate commands to the charger, which handles the hardware and safety aspects of charging.

Types of Batteries

Using the right battery for your USB powered device will make all the difference in its performance and longevity. However, there are several factors that play into your decision making process. Whether it is the size of the battery or the chemical construction, it is important to select a power source that best matches your needs and budget.

There are many types of batteries that you can choose from, including alkaline and lithium. Lithium batteries are a popular choice because of their superior energy density. This is because they can store a greater amount of energy per volt than standard alkaline batteries.

When selecting the best battery for your USB charger, it is also important to consider the type of application it will be used in. For example, if you are looking for a battery to power an LED penlight, you may be better off choosing a larger capacity AA or AAA battery than an ultra-low density cell.

For most users, a battery rated at 5,000 milliamp hours (mAh) is the gold standard. Batteries of this caliber will last for hours on end without needing a recharge.

One of the most exciting aspects of USB battery charging is that it has a wide range of applications. For example, a USB power source can be used to charge up a smartphone, or for portable electronic gadgets like cameras and Bluetooth speakers. Moreover, the latest version of the USB specification, BC1.1, features dedicated charger ports (DCPs) that can supply up to 1.5A of power in a compact package.

The AA and AAA batteries are the most common types of batteries used with a USB battery charger, but there are several other options available to meet your specific power needs. These include high-energy lithium batteries and high-performance rechargeables that have a long life span and are environmentally friendly.

Input Voltage

USB ports provide an ideal solution for powering small batteries and other electronic devices because of their flexibility, simplicity, and compatibility with existing electronics. However, if the USB device is not properly enumerated or a port has no current-limiting protection, it may not shut down and can cause system resets or damage.

For this reason, the original USB 1.0 and 2.0 specifications did not deal with battery charging. Instead, the document defined two types of USB power sources: a 5V 500mA source for small peripherals and a higher-current 1.5A charging source for PCs, laptops, and other hardware.

The USB spec is now complemented by the Battery Charging Specification, Rev 1.1, 4/15/2009 (BC1.1), which acknowledges USB battery charging and describes power sources that can supply up to 1.5A. This specification imposes order on the charging landscape and can improve interoperability between chargers and devices.

It also provides a simple way for battery-charging ICs to set charge parameters relevant to both USB and adapter power. The IC can also supply a complete set of status and fault signals to the system software that handles communication with a USB host and sends commands to the charger.

A charger circuit needs to have a minimum voltage drop across the bus supply and the battery cathode for optimal operation. This drop is typically 350 mV for the cable and connectors, but may be as low as 4.4V or less for a single-cell Li-ion battery.

To compensate for this voltage drop, the LM3525 switch isolates the USB charger circuit from the bus and provides over-current and under-voltage protection to the charging circuit. This allows the charger to be used at low USB power-bus voltages and enables the LM3525 switch to automatically turn off the charging circuit when a battery is fully charged or when the input current falls below a user-set maximum.

In a typical circuit, a transistor, Q1 (D45H8), and diode, D1 (MBRS130L), are chosen for their low-dropout properties and can handle a maximum charge current of 400 mA until the battery is fully charged. Because the charger circuit must be isolated from the USB bus during start-up, the LM3525 switches keep the charge current within the maximum USB load current limit of 500mA without drawing more than that amount from the USB bus.

Output Voltage

The output voltage of a usb battery charger is dependent on several factors. This includes the power source, battery type and the device that’s being charged. In addition, the charger must be able to charge a device’s batteries safely and efficiently.

This is especially important for Li+ batteries. Improper charging can shorten battery life and be a safety hazard. A well-designed charger optimizes user experience and lowers cost by reducing customer returns and warranty repairs.

A good charger takes into account the voltage thresholds and condition of the battery, which allows it to charge the battery faster than a standard charger. It also tries to prevent damage to the battery during pre-charge and fast charge.

To minimize the time it takes to charge a battery, an usb battery charger intelligent charger uses a three-stage charging scheme. First, it charges the battery at a low current, which helps to preserve the battery’s capacity until it reaches its maximum possible voltage.

Next, the charger switches to a constant current (CC) charge stage, during which it continues to hold the battery’s current high but limits it to a specific voltage threshold that is determined by the battery’s condition. Once the voltage reaches that threshold, the battery FET is considered fully on and the charger’s constant current is used to continue charging the battery.

The resulting battery is safely charged and will be ready for use when the charger is re-connected. In some cases, a third, constant charge stage may be applied to keep the battery’s voltage at a safe level while it’s being charged.

Another way to improve the charging process is to use a boost converter. This approach takes the battery’s input voltage, such as 25V from a USB adapter, and boosts it to a higher voltage.

Typically, this boost circuit is incorporated into the battery pack’s charge controller. It also takes into consideration thermal regulation and battery temperature to ensure the battery is safely charged.

Aside from the balancing of power requirements, USB PD also reduces e-waste by eliminating the need for proprietary ports and connectors. It also enables manufacturers to create one charging standard for a wide range of devices.