Lithium Battery BMS Protection Board Circuit Working Explanation

Lithium batteries are now the most efficient type of battery used everywhere in smartphones, laptops, and even electric cars. These are compact, have high energy intensity, lack of parasitic memory effect, but have a significant drawback.

The fact is their operation. The process of charging or overcharging should be carefully monitored. If the battery discharges below a certain limit, it has a high chance of failing.

The voltage of a fully charged lithium battery is 4.2 volts. Recharging can cause blisters, and even the battery to explode. The same thing happens with overloads, short circuits, and overheating.

Some lithium batteries are equipped with a safety valve. It will not allow the battery to explode. But, most of the powerful polymer batteries haven't such valves. So, lithium batteries should not be used without additional protection.

What is a BMS Protection board?

You've probably noticed the small PCB board used in mobile phone batteries. It's a battery management system (BMS) protection board. It protects from a deep discharge from overcharging, and from short circuits or over current.

TP4056 3.7V Lithium Battery Charging/Discharging Module

Lithium battery BMS Protection board Circuit diagram

Schematic of the Lithium battery BMS Protection board (1S) circuit shown below.

Working principle of Lithium battery BMS Protection board circuit

Inside the circuit, DW01A, and N-type MOSFET FS8205A are used. The first field-effect is responsible for transistor discharge. The second transistor (VT2) monitors the charge, positive (+Ve) wire is common, and the control is done by negative (-Ve) wire.

The first transistor (VT1) controls the discharge process. If the battery voltage is below the critical voltage (about 2.7 to 3 volts), the signal from the DW01 chip closes the first transistor and shutting-off the battery from the load.

How can we recharge lithium battery if Transistor is closed? - The DW01 chip is equipped with a system to detect the charger. If a charger is connected to the input, the 8205A microchip will be open the transistor. However, the second transistor responds to the battery charge. If the battery voltage reaches 4.2 volts, it will shut down the system. Then disconnect the lithium battery from the charger.

Short circuit & Overload protection

Lithium batteries can be discharged with a certain current depending on their capacity. For smartphone batteries, this current is a little more or less about 1 Amp.

The DW01 chip monitors the voltage drops across the transistors. The internal structure of the transistors has a certain resistance. In other words, We have something like a current shunt or a current sensor.

The greater current is then the greater voltage drop on transistor transitions. If it reaches a critical value of about 150 to 200 millivolts the chip closes the transistors and disconnecting the battery from the load. The circuit is restored in less than a second after you remove the short circuit. Microchip DW01 tracks the voltage drop across the transistors through the pin-2.

Here, I explain the principle of the simplest BMS protection board for one unit (1S) of a lithium battery. There are more complicated circuits for example in laptops are smart chips that control each battery.

There are also those which can control 10 to 12 cans at once and provide huge discharge currents up to 100 Ampers and higher. In particular, such controllers are used for small solar power stations or electric transport.

Depending on the battery capacity these controllers can be radically different in appearance short circuit current and second topology. But, they always have the same function to protect the battery from overcharging, deeply charged, and all the current.

Many controllers also provide thermal protection or protection against overheating of the cans. The temperature is controlled by a temperature sensor. Some boards can balance the voltage of the cans thereby providing the uniform charge.