Due to the high energy density of lithium batteries, in the overcharged
state, the energy will be excess when the battery temperature rises, so the
electrolyte decomposes to produce gas, which can easily increase the internal
pressure and cause the risk of spontaneous combustion or rupture; conversely, in
the overdischarged state, The decomposition of the electrolyte causes the
battery characteristics and durability to deteriorate, reducing the number of
recharges and shortening the battery life. Therefore, the protection of lithium
batteries is very important. Lithium battery applications must have battery
protection chips to prevent battery overcharge, overdischarge and
overcurrent.
lithium battery
The design of lithium battery protection circuit is very important.
However, the lithium battery protection circuit will increase the additional
loss of battery energy and reduce the battery's application time. This requires
the lithium battery protection circuit to achieve low power consumption with
high precision. In addition to completing basic functions such as overcharge
protection, over-discharge protection and over-current protection, a lithium
battery protection chip must also meet the following requirements - this is also
the goal of the chip designed in this article.
(1) Ultra-low power consumption. When the lithium battery protection
circuit is working, the power consumption it consumes is the loss of the
battery. Therefore, we need to minimize the power consumption of the lithium
battery protection circuit.
(2) Highly accurate voltage detection. In order for the lithium battery
protection circuit to respond correctly to different working states of the
battery, the protection circuit must be able to accurately detect voltage
parameters such as overcharge protection voltage and over-discharge protection
voltage.
(3) Work correctly under a wide voltage range. Since the power supply
voltage of the lithium battery protection circuit is the battery voltage, and
the battery voltage can float within a wide range, the lithium battery
protection circuit is required to work correctly within this voltage range.
Lithium battery protection circuit scheme
Among the various types of batteries currently in use, lithium batteries
(also known as lithium-ion secondary batteries or lithium-ion batteries) are a
new type of power source that have only been developed in the past decade.
Lithium batteries are different from general chemical power sources. The
charging and discharging process is achieved through the insertion and
deintercalation of lithium ions in the positive and negative electrodes of the
battery. The negative electrode of a lithium battery is a carbon material, such
as graphite; the positive electrode is a lithium-containing transition metal
oxide, such as lithium cobalt oxide (LiC002). Moreover, both the positive and
negative electrode materials of lithium batteries use lithium ion intercalation
compounds with a layered structure in which lithium ions can be freely inserted
and detached. The lithium ions between the layers will undergo an
electrochemical reaction in an appropriate electrolyte. During charging, lithium
ions are extracted from the positive electrode lattice driven by the external
electric field, pass through the electrolyte, and are embedded in the negative
electrode lattice. The process during discharge is exactly the opposite. Lithium
ions return to the positive electrode, and electrons reach the positive
electrode through the external circuit to recombine with the lithium ions.
Compared with commonly used nickel-cadmium and nickel-hydrogen batteries,
lithium batteries have many superior characteristics, mainly in the following
aspects:
(1) The power supply voltage of lithium batteries is high, generally 3.6V,
which is about three times the voltage of nickel-cadmium batteries and
nickel-metal hydride batteries. For electronic equipment with higher power
supply voltage requirements, the number of series cells required in the battery
pack can also be greatly reduced. Therefore, the lithium battery used in
combination can easily obtain higher voltage.
(2) High specific energy, that is, lithium batteries of the same weight
provide higher energy than other batteries. The specific energy of lithium
batteries is generally 2 to 3 times that of nickel-cadmium batteries and
nickel-metal hydride batteries. Therefore, it is beneficial to reduce the size
and weight of portable electronic devices.
(3) No memory effect. Nickel-cadmium batteries and nickel-metal hydride
batteries have memory effects and must be discharged regularly, otherwise the
batteries will fail due to the memory effect. Lithium batteries have no memory
effect and can be charged directly regardless of the remaining power. This
allows the lithium battery performance to be fully utilized.
(4) Long service life. Lithium batteries use carbon negative electrodes.
The carbon negative electrodes do not generate metallic lithium during the
charge and discharge process, thus preventing the battery from being damaged by
a short circuit of internal metallic lithium. At present, the cycle life of
lithium batteries can reach more than 5,000 times, which is much higher than
other types of batteries.
(5) The working environment temperature range is wide, and it can generally
work between -30℃ and 0℃, and has excellent high and low temperature discharge
performance.
(6) Low self-discharge rate. Self-discharge rate, also known as charge
retention rate, refers to how much the battery automatically discharges when it
is not in use. The self-discharge rate of lithium batteries is 2% to 5%, that of
nickel-cadmium batteries is between 25% and 30%, and that of nickel-metal
hydride batteries is between 30% and 35%. Therefore, lithium batteries retain
charge for the longest time under the same environment.
(7) Lithium batteries do not contain any toxic elements such as mercury and
cadmium and are truly green and environmentally friendly batteries.
Based on the above advantages, lithium batteries are widely used in
portable electronic devices. On the other hand, lithium batteries have high
energy density, making it difficult to ensure battery safety. Specifically, in
the overcharge state, the electrolyte will be decomposed, causing the
temperature and pressure inside the battery to rise; in the overdischarge state,
the electrolytic material in the negative electrode, copper, will melt and cause
an internal short circuit, causing the temperature to rise: When the external
circuit is short-circuited or the discharge current Chapter 1 Introduction 3 is
too large, due to the characteristics of high internal resistance, the internal
power consumption of the battery will increase and the temperature will also
rise, which may cause oxidation or decomposition of the electrolyte, resulting
in shortened lithium battery life. In addition, if the lithium battery is
excessively discharged, the electrolyte in the battery will change, and the
number of recharge cycles will be reduced, thereby affecting the service life of
the lithium battery.
Since lithium batteries have the shortcomings analyzed above, a protection
circuit must be added to the application of lithium batteries. The basic
functions of the protection circuit must also correspond to the above
shortcomings, so we require the lithium battery power protection chip to be able
to achieve the following most basic functions: overcharge protection,
over-discharge protection, over-current protection and short-circuit protection.
From the above application requirements of lithium batteries, it can be seen
that in order to increase the service life of lithium batteries and ensure the
safe use of batteries, lithium battery protection circuits need to have the
following functions:
(1) If the charging voltage exceeds the maximum allowed value of the
battery, a battery discharge circuit can be provided.
(2) If the discharge voltage is lower than the minimum allowed value of the
battery, a battery charging circuit can be provided. then disconnect the battery
from the external circuit, and then disconnect the battery from the external
circuit, and
(3) If the charging and discharging current of the battery is greater than
the limit value, cut off the connection between the battery and the external
circuit.
(4) When the battery returns to normal state, the protection circuit should
be able to release the protection state accordingly so that the battery can
continue to work normally.
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