LIBRARYBattery charging.

A lithium cell charges in two phases: constant current until it nears full, then constant voltage while the current tapers off. A single-cell charger chip runs that profile for you, and using one is the only safe way to charge a lithium cell. Do not improvise it.

Constant current, then constant voltage

In the first phase the charger pushes a fixed current into the cell and the voltage climbs. Once the cell reaches its 4.2 V ceiling, the charger holds that voltage steady and lets the current fall on its own as the cell fills. This constant-current then constant-voltage profile, CC/CV, is how every lithium cell is charged.

Charge current and termination

The charge current is set as a C-rate, often 0.5 C to 1 C for a standard cell, and the specific cell's datasheet is the authority. Charging ends not at a fixed time but when the constant-voltage current falls below a small termination threshold, a few percent of the cell's rated capacity, which signals the cell is full.

A single-cell charger IC

A dedicated charger chip, such as the Microchip MCP73831 or the widely used TP4056, contains the whole CC/CV state machine, the 4.2 V reference, and the termination logic in one part. You set the charge current with a single resistor and feed the chip 5 V from USB. It handles the rest, safely, which is exactly why you never charge a lithium cell from a bare bench supply.