What are the charging methods for lithium batteries?
The charging method of lithium batteries is different for different numbers of cells, cell configuration, and power source types. At present, lithium battery pack 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 batteries
When the charger input voltage is greater than the open circuit voltage of a fully charged cell plus sufficient headroom, it is best to use a linear solution, especially the 1.0C fast charging current is not greater than 1A Too much time.
For example: MP3 players usually have only one lithium battery cell, with a capacity ranging from 700 to 1500mAh, and the fully charged open circuit voltage is 4.2V. The power supply of the MP3 player 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. Examples of charger applications for linear solutions-dual-input Li+ chargers and smart power selectors are dual-input USB/AC adapter linear chargers with built-in Smart Power Selector for portable devices powered by rechargeable single-cell Li+ batteries. The charger integrates all the power switches needed to charge the battery and external power supply and switch the load, so no external MOSFET is required: MAX8677A is ideal for portable devices, such as smart phones, PDAs, and portable multimedia players. The MAX8677A can work with independent USB and AC adapter power input or either of the two inputs: when an external power supply is connected, the smart power selector allows the system to be disconnected The battery may be connected to a deeply discharged battery. The intelligent power selector automatically switches the battery to the system load, uses the unused input power part of the system to charge the battery, and makes full use of the limited USB and adapter input power. All required current detection circuits, including integrated power switches, are integrated on the chip. The DC input current limit can be adjusted up to 2A, and both DC and USB inputs can support 100mA, 500mA and USB suspend modes. The charging current can be adjusted up to 1.5A to support a wide range of battery capacity. Other features of the MAX8677A include thermal regulation, overvoltage protection, charging status and fault output, power supply monitoring, battery thermistor monitoring, and charging timer. MAX8677A adopts space-saving, thermally enhanced, 4mmx4mm, 24-pin TQFN package, and is specified to work in the extended temperature range (-40~+85℃)
2. Buck (step-down) of lithium battery ) Switch charging method
When the 1.0C charging current is greater than 1A, or the input voltage is much higher than the fully charged open circuit voltage of the cell, Buck or a step-down solution is a better choice. For example, in the hard disk-based PMP, a single-cell lithium-ion battery is usually used, the open circuit voltage is 4.2V when fully charged, and the capacity ranges from 1200 to 2400mAh. And now PMP is usually charged with a car kit, and its output voltage is between 9V and 16V. The 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 low efficiency, coupled with a 1C fast charging current greater than 1.2A, will cause serious heat dissipation problems. In order to avoid this situation, it is necessary to adopt the Buck scheme.
3, the SEPIC (boost and buck) switch charging method of lithium batteries
In some cases, 3 or even 4 In devices with lithium-ion/polymer batteries in series, the input voltage of the charger is not always greater than the battery voltage. For example, a laptop computer uses a 3-cell lithium battery pack pack with a fully charged open circuit voltage of 12.6V (4.2V×3) and a capacity from 1800mAh to 3600mAh. The input power supply 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 linear nor Buck solutions can charge this battery pack. This requires the use of the SEPIC solution, which can work when the output voltage is higher than the battery voltage, and can also work when the output voltage is lower than the battery.
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