Because the aluminum-plastic film is light in weight and has high internal
space utilization, soft-pack batteries are suitable for the development of
batteries with larger energy density. However, metal casings are limited by
internal space and generally have slightly lower energy density than soft-pack
batteries. Safety: Because aluminum-shell batteries are protected by metal
casings, their safety is higher, while soft-package batteries can only rely on
the performance of the material itself to pass the safety test, which currently
seems to be more difficult. In terms of process difficulty, because soft-pack
batteries have many small pole pieces, they require high die-cutting equipment
and are prone to large self-discharge and local micro-short circuits. At the
same time, due to internal space limitations, there is less free electrolyte,
and the cycle performance may be slightly lower. Difference. Rolled batteries
are relatively better, with some margin, and are easy to realize automated
production. In terms of cost, because rolled batteries require high welding of
the shell, the cost is slightly higher, while soft-pack batteries do not involve
lasers. Welding focuses on packaging and equipment investment is low.
Calculate the number of positive and negative electrodes and separator
layers of the battery core based on the internal space of the battery. According
to the development status of the industry, the relevant parameters of the
material are all experimentally verified based on past testing experience. It is
necessary to verify the compaction density and material performance. The
performance of auxiliary materials (including verification of SBR, CMC, PVDF,
conductive agents, etc.) is based on the development of platform models. The
development of the final process also needs to be matched with the materials to
derive the final control plan and process flow chart. In order to shorten the
time, manufacturers now combine experimental verification and process
development, but the risks are often relatively high. After all, the material
system itself develops with the development of technology.
Each performance of each material has relevant testing standards. Certain
performance indicators of the positive and negative electrodes are directly
related to the performance indicators of the battery. However, there is
currently no suitable model for forward electrochemical performance simulation.
It is only based on existing Empirical data for patching.
Voltage (V) Open circuit voltage, as the name suggests, means that the
battery is not connected to any external load or power supply, and the potential
difference between the positive and negative electrodes of the battery is
measured, which is the open circuit voltage of the battery. The working voltage
corresponds to the open circuit voltage, that is, when the battery is connected
to an external load or power supply, current flows through the battery, and the
potential difference between the positive and negative electrodes is measured.
Due to the existence of the internal resistance of the battery, the operating
voltage is lower than the open circuit voltage when discharging (external load),
and the operating voltage is higher than the open circuit voltage when charging
(external power supply). Battery capacity (Ah) can hold or release the charge Q,
Q=It, that is, battery capacity (Ah) = current (A) x discharge time (h), the
unit is generally Ah (ampere hour) or mAh (milliamp hour) . For example, if the
battery in the car is marked 16Ah, then when the working current is 1A, it can
theoretically be used for 16 hours. Battery energy (Wh) The energy stored in the
battery, in Wh (Watt-hour), energy (Wh) = voltage (V) × battery capacity (Ah).
As shown in the picture below, it is a battery marked 3.7V/10000mAh, and its
energy is 37Wh. If we connect 4 such batteries in series, it will form a battery
pack with a voltage of 14.8V and a capacity of 10000mAh, although the battery
capacity is not increased. , but the total energy did increase by 4 times.
After reviewing our high school knowledge, let’s take a look at some useful
information... Energy density (Wh/L&Wh/kg) is the amount of energy released
by a battery per unit volume or unit mass. If it is unit volume, it is volume
energy density (Wh/L), which is simply called energy density in many places; if
it is unit mass, it is mass energy density (Wh/kg), which is also called
specific energy in many places. For example, if a lithium battery weighs 300g,
has a rated voltage of 3.7V, and a capacity of 10Ah, its specific energy is
123Wh/kg. According to the "Energy Saving and New Energy Vehicle Technology
Roadmap" released in 2016, we can have a rough idea of the development trend of
power batteries. As shown in the figure below, by 2020, the specific energy of a
pure electric vehicle battery cell will reach 350Wh/kg
Power density (W/L&W/kg) divides energy by time to obtain power, in W
or kW. In the same way, power density refers to the power output of the battery
per unit mass (also called specific power directly in some places) or unit
volume, and the unit is W/kg or W/L. Specific power is an important indicator to
evaluate whether the battery meets the acceleration performance of electric
vehicles. What is the difference between specific energy and specific power? To
give a vivid example: a power battery with a high specific energy is like a
tortoise in the tortoise and the hare. It has good endurance and can work for a
long time, ensuring a long cruising range of the car; a power battery with a
high specific power is like a hare in the tortoise and the hare. It is fast and
can provide high instantaneous current to ensure good acceleration performance
of the car;
The following parameters are a bit convoluted... Battery discharge rate (C)
Discharge rate refers to the current value required to discharge its rated
capacity (Q) within a specified time. It is numerically equal to a multiple of
the battery's rated capacity. That is: charge and discharge current (A) / rated
capacity (Ah), the unit is generally C (abbreviation for C-rate), such as 0.5C,
1C, 5C, etc. For example, for a battery with a capacity of 24Ah: use 48A When
discharging, the discharge rate is 2C. On the contrary, when discharging at 2C,
the discharge current is 48A, and the discharge is completed in 0.5 hours; when
charging at 12A, the charging rate is 0.5C. Conversely, when charging at 0.5C,
the charging current is 12A, 2 Hours of charging; the charge and discharge rate
of the battery determines how quickly we can store a certain amount of energy
into the battery, or how quickly we can release the energy in the battery. State
of charge (%) SOC, the full name is State of Charge, state of charge, also
called remaining capacity, represents the ratio of the remaining capacity of the
battery after discharge to its capacity in the fully charged state. Its value
range is 0~1. When SOC=0, it means the battery is completely discharged. When
SOC=1, it means the battery is fully charged. The battery management system
(BMS) mainly ensures the efficient operation of the battery by managing SOC and
making estimates, so it is the core of battery management. At present, SOC
estimation mainly includes open circuit voltage method, ampere-hour measurement
method, artificial neural network method, Kalman filter method, etc. We will
explain it in detail later. Internal resistance Internal resistance refers to
the resistance to current flowing through the interior of the battery when the
battery is working. Including ohmic internal resistance and polarization
internal resistance, among which: ohmic internal resistance includes the
resistance of electrode materials, electrolytes, diaphragm resistors and various
parts; polarization internal resistance includes electrochemical polarization
resistance and concentration polarization resistance. Let the data speak. The
figure below shows a battery discharge curve. The X-axis represents the
discharge amount and the Y-axis represents the battery open circuit voltage. The
ideal discharge state of the battery is the black curve, and the red curve is
the true state when the internal resistance of the battery is taken into
account.
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