1S Lithium Cell BMS Circuit Working Explanation

Lithium cells are the most efficient type used in smart phones, laptops, and electric cars. They are compact, high-energy, and don't suffer from memory effects. However, they need careful monitoring during charging as discharging too low can lead to failure. Fully charged lithium batteries reach 4.2 volts, and charging improperly can cause blistering or even explosions due to overcharging, short circuits, or overheating. Some have safety valves to prevent explosions, but most powerful polymer batteries lack this feature. Hence, it's essential to use lithium batteries with additional protection.

You might have seen the tiny PCB board in mobile phone batteries. It's called a Battery Management System (BMS) protection board. It safeguards against deep discharge, overcharging, short circuits, and excessive current.

1S Lithium Cell BMS Circuit Diagram

The schematic of a 1s lithium cell battery management system circuit is shown below.

This circuit can easily detect overcharge voltages within the range of 4.25±0.05V, over-discharge voltages within the range of 2.54±0.1V, and over-currents ranging from 1 to 3A.

Working Principle of 1S Lithium Cell BMS Circuit

Here, I've explained the basic principle of a simple BMS protection board for a single lithium cell unit (1S). While more complex circuits, like those in laptops, may have smart chips controlling each cell, they can vary in appearance, short circuit current handling, and topology depending on battery capacity. However, their main function remains consistent: to protect the cell or battery from overcharging, deep discharge, and excessive currents.

The main components in the circuit are a DW01A chip and a N-type MOSFET FS8205A chip. The FS8205A consists of two integrated transistors, VT1 and VT2. VT1 handles the discharge, while VT2 oversees the charging process. The cell's positive (B+) wire is common, and control is managed through the negative (B-) wire.

Charging Monitoring System

The BMS circuit charges a Lithium cell when the DW01 chip detects the charger. When a constant voltage current source (usually 4.2V, 1A for an 18650 cell) is supplied to the input, the transistors in the 8205A microchip turn on. However, the second transistor (VT2) monitors the battery's charge. If the battery voltage reaches 4.2 volts, it shuts down the system and disconnects the lithium battery from the charger.

Short Circuit & Overload Protection System

A Lithium cell can be discharged with a current that depends on its capacity. This current is typically slightly more or less than 1 Amp for the Smartphone cells. The DW01 chip monitors the voltage drops across the transistors. Inside the transistors, there's a certain resistance (RX). In simpler terms, it works like a current shunt or a current sensor.

When the current increases, there's a greater voltage drop across the transistor junctions. The DW01 microchip monitors the voltage drop across the transistors via the CS pin. If it hits around 150 to 200 millivolts, the chip shuts off the transistors, disconnecting the battery from the load. The circuit resets in less than a second after the user removes the short circuit.