Lithium batteries are very popular because they are rechargeable and have high energy density. This paper discusses how to charge lithium batteries using the method called the lithium battery charging algorithm. This article also gives of examples of two highly integrated charging ICs, Microchip’s MCP73827 and Linear Technology’s LT3650.
Charging algorithm overview
The lithium battery charging algorithm consists of constant current and constant voltage stages. After the constant voltage stage, the battery should be disconnected to prevent overcharging. Periodically, the battery can receive small charges to keep it full. Figure 1 provides a visual overview of how a lithium battery is charged. Different stages of the charging algorithm are discussed below.
Figure 1: Voltage and current profile of charging a lithium battery versus time. This figure also labels the different stages of the algorithm.
During the constant current charge, the lithium cell is discharged. The cell will sink as much current as it is given, although providing too much current may be dangerous. Stay at or below the limit specified by the datasheet. A standard charge on a datasheet is typically defined as 0.5 C, where C stands for capacity. This means that the charge current should be half the battery capacity. For a 2500 mAh cell, the standard charge current would be 1250 mA.
The battery cell will have most of its charge when the battery voltage reaches 4.1 V or 4.2 V. At this point, the current going into the battery gradually decreases.
When the current drops below a datasheet value, charging should be terminated. C/10 and C/30 are common charge termination current limits. When the battery is fully charged, the battery should be disconnected from the charger. Leaving the battery connected to the charger will cause the battery to overcharge and will damage the battery.
Example: The 18650
The 18650 is popular cylindrical lithium cell, with a capacity of 2500 mAh. The datasheet recommends a 1250 mA constant current charge, then 4.2 V constant voltage charge, and charge termination when the current drops to 50 mA. The datasheet specifies a fast charge, which is 4000 mA constant current, then 4.2 V constant voltage, then cut off at 100 mA, which is a C/25 charge termination.
An easy way to charge a lithium battery is to use Microchip’s MCP73827 lithium charger IC. The MCP73827 biases an external p-channel MOSFET to provide power to the lithium cell. The MCP73827 senses voltage across a low-ohm sense resistor sensed to regulate the charge current for constant current charging and charge termination. The MCP73827 directly senses the battery voltage to control it during constant voltage charging. One of the cool things about using an external p-channel MOSFET is that you can adjust it for different-size battery packs. See figure 2 for the typical application schematic, and see the datasheet for more detailed application information.
Figure 2: Typical application circuit for the MCP73827
Another easy way to charge a lithium battery is to use Linear Technology’s LT3650, a DC-DC converter with integrated lithium-ion battery charger intelligence. A designer adds a few passive components to implement a complete lithium battery charging solution. See figure 3 for the typical application schematic, and see the datasheet for more information.
Figure 3: Typical application circuit for the LT3650
This paper has discussed the lithium battery charging algorithm in more detail and introduced two highly integrated charging solutions, the MCP73827 and the LT3650.