Basic knowledge of lithium battery protection board

May 22, 2019

The lithium battery protection board is a charge and discharge protection for the series lithium battery pack; when the battery is fully charged, the voltage difference between the individual cells can be ensured to be less than the set value (generally ±20 mV), and the battery cells of the battery pack can be uniformly charged. , effectively improving the charging effect in the series charging mode; simultaneously detecting the overvoltage, undervoltage, overcurrent, short circuit, and overtemperature conditions of each single cell in the battery pack, protecting and extending the battery life; undervoltage protection enables each A single battery prevents damage to the battery due to overdischarge during discharge.

The finished lithium battery consists mainly of two major parts, a lithium battery core and a protective plate. The lithium battery core is mainly composed of a positive electrode plate, a separator, a negative electrode plate and an electrolyte; the positive electrode plate, the separator and the negative electrode plate are wound or laminated, packaged, and poured into the electrolyte. After the package is made into a battery core, the role of the lithium battery protection board is not known to many people. The lithium battery protection board, as the name implies, is used to protect the lithium battery. The role of the lithium battery protection board is to protect the battery but not charge it. Stream, there is output short circuit protection.


Lithium battery protection board

1, control ic, 2, switch tube, in addition to add some micro-capacitance and micro-resistance. The control ic function is to protect the battery. If the protection condition is reached, the mos is controlled to be disconnected or closed (for example, the battery reaches overcharge, over discharge, short circuit, over current, etc.), wherein the mos tube functions as a switch. Controlled by the control ic.

The reason why lithium batteries (fillable) need protection is determined by its own characteristics. Because the material of the lithium battery itself can not be overcharged, over-discharged, over-current, short-circuited and ultra-high temperature charge and discharge, the lithium battery lithium battery assembly will always follow an exquisite protection board and a current fuse. The protection function of the lithium battery is usually completed by the protection circuit board and the PTC. The protection board is composed of electronic circuits, and accurately monitors the voltage of the battery core and the current of the charging and discharging circuit in an environment of -40 ° C to +85 ° C.


How the protection board works

1. Overcharge protection and overcharge protection recovery

When the battery is charged so that the voltage exceeds the set value VC (4.25-4.35V, the specific overcharge protection voltage depends on the IC), VD1 flips to make Cout low, T1 is off, charging stops. When the battery voltage falls back to the VCR (3.8-4.1V, the specific overcharge protection recovery voltage depends on the IC), Cout becomes high level, T1 conduction is continued, VCR must be less than a fixed value of VC to prevent frequent jumps.

2. Over-discharge protection and over-discharge protection recovery

When the battery voltage drops to the set value VD (2.3-2.5V, the specific overcharge protection voltage depends on the IC) due to discharge, VD2 flips, after a short time delay, Dout becomes low level, T2 is cut off. The discharge stops and when the battery is placed in charge, the internal OR door is flipped and T2 is turned back on to prepare for the next discharge.

3, overcurrent, short circuit protection

When the circuit charge and discharge loop current exceeds the set value or is short-circuited, the short-circuit detection circuit operates to turn off the MOS transistor and the current is cut off.


Function introduction of the main parts of the protection board

R1: reference power supply resistor; and IC internal resistor constitute a voltage divider circuit, control the internal overcharge, over-discharge voltage comparator level flip; generally in the resistance value of 330Ω, 470Ω more; when the package form (ie with standard components Length and width indicate the component size. If the length and width of the 0402 package are 1.0mm and 0.5mm respectively, the resistance value will be numerically identified. For example, the digital resistance 473 on the chip resistor indicates that it The resistance is 47000Ω or 47KΩ (the third digit indicates the number of digits after the first two digits plus 0).

R2: Overcurrent and short-circuit detection resistors; control the current of the protection board by detecting the voltage of the VM terminal. Poor soldering and damage will cause the battery to over-current and short-circuit without protection. Generally, the resistance is 1KΩ and 2KΩ.

R3: ID identification resistor or NTC resistor (described above) or both.

Summary: The resistor is a black patch in the protection board. The resistance can be measured with a multimeter. When the package is large, its resistance value will be represented by a number. The method is as described above. Of course, the resistance values ​​generally have deviations. The resistors are all accurate. If the 10KΩ resistor is +/-5%, the resistance is 9.5KΩ -10.5KΩ.

C1, C2: Since the voltage across the capacitor cannot be abrupt, it acts as an instant regulator and filter. Summary: The capacitor is yellow patch in the protection board, the package form is more than 0402, there are also a few 0603 packages (1.6mm long, 0.8mm wide); the multimeter is used to detect its resistance is generally infinite or MΩ level; capacitor leakage will generate The power consumption is large, and the short circuit has no self-recovery. FUSE: Ordinary FUSE or PTC (abbreviation of Positive Temperature Coefficient, meaning positive temperature coefficient); prevent unsafe high current and high temperature discharge, wherein PTC has self-recovery function.

Summary: FUSE is generally a white patch in the protection board. LITTE provides FUSE to mark the character DT on FUSE. The character indicates the rated current that FUSE can withstand. For example, the rated current of D is 0.25A, and S is 4A, T. For 5A and so on.

U1: Control IC; all functions of the protection board are realized by the IC controlling the C-MOS to perform the switching operation by monitoring the voltage difference between VDD and VSS and the voltage difference between VM and VSS.

Cout: Overcharge control terminal; control the MOS tube switch through the gate voltage of the MOS transistor T2.

Dout: over-discharge, over-current, short-circuit control terminal; control the MOS tube switch through the gate voltage of MOSFET T1.

VM: overcurrent and short circuit protection voltage detection terminal; overcurrent and short circuit protection of the circuit by detecting the voltage of the VM terminal


Summary: IC is generally a 6-pin package in the protection board. The method of distinguishing the pins is as follows: the first pin is marked near the black dot on the package, and then rotated counterclockwise to be the second and third. 4, 5, 6 pins; if there is no black dot mark on the package, the character on the lower left side of the package is the first pin, and the other pins are counterclockwise analogous) C-MOS: field effect switch tube; protection function The achiever; continuous welding, virtual welding, false welding, breakdown will cause the battery without protection, no display, low output voltage and other undesirable phenomena.

Summary: CMOS is generally a package of 8 pins in the protection board. It consists of two MOS tubes, which are equivalent to two switches, which respectively control overcharge protection and over-discharge, over-current and short-circuit protection; The method of differentiation is the same as IC.

Under the normal condition of the protection board, Vdd is high level, Vss and VM are low level, Dout and Cout are high level; when any parameter of Vdd, Vss, VM is changed, the level of Dout or Cout will occur. Change, at this time, the MOSFET performs the corresponding action (on and off circuit), thereby realizing the protection and recovery function of the circuit.