This article mainly introduces the protection circuit of mobile phone
batteries. Various misoperations will always occur when we use batteries, and
the cells of mobile phone batteries are actually relatively fragile, so complete
protection measures are very important for a qualified mobile phone battery.
Speaking is essential. The following is the text:
1. Protection of nickel-metal hydride batteries
The protection device for mobile phone nickel-metal hydride batteries is
very simple. It is the flat ribbon-like thing in the picture that spans between
two cells. It is called a recoverable fuse, also known as pTC, which means
positive temperature coefficient thermal. The English abbreviation of resistor.
In the circuit, it is connected in series in the power supply circuit. Once a
large current (such as a short circuit) occurs, its own resistance value will be
rapidly increased due to its pTC effect, which will act as a circuit
breaker.
2. Introduction to pTC
pTC positive temperature coefficient thermistor is also called polyswitch
polymer resettable fuse. Polymer resettable fuse is composed of a polymer matrix
and carbon black particles that make it conductive. Because polymer resettable
fuses are conductors, current will flow through them. When an overcurrent passes
through a polymer resettable fuse, the heat generated (which is I2R) will cause
it to expand. As a result, the carbon black particles will separate and the
resistance of the polymer resettable fuse will increase. This will cause the
polymer resettable fuse to generate heat faster and expand more, further
increasing the resistance. When the temperature reaches 125°C, the resistance
changes significantly, causing the current to decrease significantly. The small
current flowing through the polymer resettable fuse at this time is enough to
keep it at this temperature and in a high resistance state. When the fault is
cleared, the polymer resettable fuse shrinks back to its original shape and
reconnects the carbon black particles, thereby reducing resistance to a level
with a specified holding current. The above process can be cycled multiple
times. The illustration illustrates the protection principle of pTC as
follows:
The cold pTC resistance value is only tens of milliohms, while the hot
resistance value can reach hundreds of kiloohms.
3. Lithium-ion battery protection circuit
For a detailed description of the principle of lithium-ion battery
protection circuits, please refer to <>
This article mainly explains with illustrations
The left side of the picture shows the schematic diagram of a typical
lithium-ion protection circuit. B+ and B- represent the typical positive and
negative electrodes, while p+ and p- represent the positive and negative outputs
of the finished mobile phone battery (see the right side of the picture).
4. Protection parameters of lithium-ion protection circuits
In addition, complex and advanced lithium-ion batteries (such as smart
lithium-ion batteries) will also include temperature protection, fuel metering,
real-time clock and electronic range identification code.
5. Detailed explanation of lithium-ion battery protection principles
1. Overcharge protection
When the charging voltage exceeds the protection value, the protection
circuit is triggered and the switch Q1 is turned off.
2. Over-discharge protection
When the load reduces the battery voltage below the protection voltage, the
protection circuit is triggered and Q1 is turned off.
3. Short circuit protection
There are two types of short-circuit protection circuits. One is that once
triggered, the short-circuit fault is eliminated and Q1 conducts itself. The
other is that it will not recover by itself after being triggered and requires
forced external charging before Q1 is released. These two types of protection
are currently on the market. circuit coexistence.
6. Definition and explanation of other battery pins
On the battery output terminals of many mobile phones, in addition to the
positive and negative poles, there are usually one or several other output
terminals, typically two as shown in the picture, NTC resistor and ID
resistor.
1.Principle of ID resistor
ID (abbreviated spelling of identification)
To put it simply, the mobile phone learns the type of battery by reading
the resistance value of this pin resistor (distinguish between nickel-metal
hydride batteries and lithium-ion batteries, and between large-capacity
batteries and ordinary-capacity batteries based on the resistance value)
2.NTC resistor
The NTC resistor is just the opposite of the previous pTC resistor. NTC is
the abbreviation of negative temperature coefficient thermistor.
To put it simply, the mobile phone obtains the temperature value of the
battery by reading the resistance value of the resistor. This can be used to
perform corresponding protective actions. For example, in an environment outside
of 0 to 45 degrees, the mobile phone does not charge, and outside the range of
-20 to 60 degrees The phone is forced to shut down to protect it from dangerous
operations in non-battery tolerant environments.
3.NTC principle
The corresponding relationship between the resistance and temperature of
the NTC resistor is shown in the figure above. It can be seen from the figure
that the resistor has a very clear corresponding relationship with the
temperature. The resistor can also be used for negative temperature compensation
in some circuits.
7. Integrated circuit
A typical lithium-ion battery comprehensive protection circuit, from left
to right is the fuel gauge circuit/protection circuit/real-time clock.
This is very interesting. One advantage of placing the real-time clock
circuit in the battery pack is that even if the battery is unplugged from the
mobile phone, the clock will not be lost, as long as the battery voltage does
not drop below 3.0V. Now most real-time clock circuits are The battery placed
inside the mobile phone only relies on a small-capacity backup battery. Once the
mobile phone loses power, the maintenance time is very limited.
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