From：CTECHI GROUP Limited Release time：2019-11-21
Overview：Lithium battery charging methods are different for different numbers of cells, cell configurations, and power types. At present, lithium battery charging mainly includes three methods: linear, Buck (buck) switch and SEPIC (boost and buck switch). The specific operations are as follows:
What are the lithium battery charging methods?
Lithium battery charging methods are different for different numbers of cells, cell configurations, and power types. At present, lithium battery charging mainly includes three methods: linear, Buck (buck) switch and SEPIC (boost and buck switch). The specific operations are as follows:
1. Linear charging method for lithium battery
When the charger input voltage is greater than the open circuit voltage after fully charged cells plus sufficient headroom, it is best to use a linear scheme, especially if the 1.0C fast charge current is not much larger than 1A.
For example, an MP3 player usually has only one lithium battery cell, with a capacity ranging from 700 to 1500 mAh, and a full charge open circuit voltage of 4.2V. The MP3 player's power supply is usually an AC/DC adapter or a USB interface, and its output is a regular 5V; at this time, the linear solution charger is the simplest and most efficient solution. Example of a linear solution charger application - Dual Input Li+ Charger and Smart Power Selector is a dual input USB/AC adapter linear charger with built-in Smart Power Selector for portable devices powered by a rechargeable single-cell Li+ battery. The charger integrates all the power switches required for battery and external power charging and switching loads, eliminating the need for an external MOSFET: the MAX8677A is ideal for use
Portable devices such as smartphones, PDAs, portable multimedia players, GPS navigation devices, digital cameras, and digital video cameras MAX8677A can work with independent USB and AC adapter power inputs or any of two inputs: When connected to an external power source, the Smart Power Selector allows the system to be disconnected from the battery or can be connected to a deep discharge battery. The Smart Power Selector automatically switches the battery to system load and uses the unused input power section of the system to charge the battery, taking advantage of limited USB and adapter input power. All required current sensing
The circuit, including the integrated power switch, is integrated on the chip. The DC input current limit is adjustable up to 2A, while the DC and USB inputs support 100mA, 500mA and USB suspend modes. The charge current can be adjusted up to 1.5A to support a wide range of battery capacitance. Other features of the MAX8677A include thermal regulation, overvoltage protection, charge status and fault conditions, power good monitoring, battery thermistor monitoring, and charging timers. The MAX8677A is available in a space-saving, thermally enhanced, 4mm x 4mm, 24-pin TQFN package and is specified over the extended temperature range (-40 to +85°C).
2. Buck (buck) switch charging method for lithium battery
Buck or buck is a better choice when the 1.0C charge current is greater than 1A, or if the input voltage is much higher than the cell's full-fill open-circuit voltage. For example, in a hard disk-based PMP, a single-cell lithium-ion battery is usually used, and the full-fill open-circuit voltage is 4.2V, and the capacity ranges from 1200 to 2400 mAh. Now PMP is usually charged with a car kit, and its output voltage is between 9V and 16V. A relatively high voltage difference (minimum 4.8V) between the input voltage and the lithium battery voltage will reduce the efficiency of the linear scheme. This inefficiency, coupled with a 1C fast charge current greater than 1.2A, can cause serious thermal issues. In order to avoid this situation, it is necessary to adopt the Buck scheme.
3. SEPIC (boost and step-down) switch charging method for lithium battery
In some devices that use three or even four lithium-ion/polymer cells in series, the input voltage to the charger is not always greater than the battery voltage. For example, the laptop uses a 3-cell lithium-ion battery pack with a full charge open circuit voltage of 12.6V (4.2V x 3) and a capacity from 1800mAh to 3600mAh. The input power is either an AC/DC adapter with an output voltage of 16V or a car kit with an output voltage between 9V and 16V. Obviously, neither the linear nor the Buck solution can charge this battery pack. This requires the SEPIC scheme, which works when the output voltage is higher than the battery voltage and also when the output voltage is lower than the battery.