What are the main technical parameters of power batteries

Aug 14, 2019   Pageview：102

Open circuit voltage, as the name suggests, that is, the battery is not connected to any external load or power supply, measurement of the potential difference between the positive and negative electrodes of the battery, that is, the battery open circuit voltage. Working voltage, corresponding to the open circuit voltage, that is, the battery is externally connected to the load or power supply, there is a current flow through the battery, the potential difference between the positive and negative poles measured.

Due to the existence of the internal resistance of the battery, when discharging (external load), the working voltage is lower than the open circuit voltage, and when charging (external power supply), the working voltage is higher than the open circuit voltage.

Battery capacity (Ah)

Q, Q=It, that is, battery capacity (Ah) = current (A) x discharge time (h), the unit is generally Ah (ampere hour) or mAh (milliampere hour).

For example, if the battery in the car is labeled with 16Ah, it can theoretically be used for 16 hours when the current is 1A.

Battery energy (Wh)

The energy stored by the battery in Wh (watt-hours), energy (Wh) = voltage (V) battery capacity (Ah).

As shown in the figure below, the battery is marked as 3.7v /10000mAh, and its energy is 37Wh. If four such batteries are connected in series, they form a battery pack with a voltage of 14.8v and a capacity of 10000mAh. Although the battery capacity is not improved, the total energy is increased by 4 times.

(here I have checked the logo of my charger and searched the civil aviation regulations that no one can carry more than 160Wh...)

Review the high school knowledge, we come to a little dry goods...

Energy density (Wh/L&Wh/kg)

The energy released by a battery per unit volume or mass.

If it is the unit volume, that is, the volume energy density (Wh/L), many places directly referred to as the energy density;

If it's a unit mass, it's the mass energy density in Wh/kg, which in many places is also called specific energy.

If a lithium battery weighs 300g, rated voltage is 3.7v, and capacity is 10Ah, its specific energy is 123Wh/kg.

According to the "energy conservation and new energy vehicle technology roadmap" issued in 2016, we can roughly have a concept of the development trend of power battery. As shown in the figure below, by 2020, the specific energy of battery unit of pure electric vehicle should reach 350Wh/kg

Power density (W/L&W/kg)

Divide energy by time, and you get power in either W or kW. Similarly, power density refers to the output per unit mass (or, in some places, specific power) or per unit volume of a battery, in W/kg or W/L.

Specific power is an important index to evaluate whether the battery meets the acceleration performance of electric vehicle.

What is the difference between specific energy and specific power?

Here's a graphic example:

The power battery with higher energy is just like the tortoise in the race between the tortoise and the hare.

Higher than the power of the battery is like the tortoise and rabbit in the race, fast, can provide a high instantaneous current, to ensure that the car acceleration performance is good;

The following parameters are slightly convoluted...

Battery discharge rate (C)

Discharge rate refers to the current required to discharge its rated capacity (Q) within a specified time, which is equal to the multiple of the rated capacity of the battery. Namely: charging and discharging current (A)/rated capacity (Ah), the unit is generally C(c-rate abbreviation), such as 0.5c, 1C, 5C, etc

For example, for a 24Ah battery:

Discharge with 48A, its discharge ratio is 2C, conversely, 2C discharge, discharge current is 48A, 0.5 hours discharge completed;

If 12A is used for charging, the charging ratio is 0.5c. Conversely, 0.5c is used for charging, and the charging current is 12A.

The charge and discharge rate of the battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery.

Soc (%)

SOC stands for StateofCharge, also known as residual charge. It represents the ratio of the residual capacity of a battery after discharging to the capacity of its full charging 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. Battery management system (BMS) is the core of battery management, which is to ensure the efficient work of battery mainly by managing SOC and making estimation.

At present, SOC estimation mainly includes open circuit voltage method, ampere hour measurement method, artificial neural network method, kalman filter method, etc., which will be explained in detail later.

Internal resistance

Internal resistance is the resistance to the flow of electric current through the battery during operation. Includes ohmic internal resistance and polarization internal resistance, wherein: ohmic internal resistance includes electrode material, electrolyte, diaphragm resistance and resistance of each part; Polarization resistance includes electrochemical polarization resistance and concentration polarization resistance.

In terms of data, the figure below represents a battery discharge curve, X axis represents the discharge, Y axis represents the open circuit voltage of the battery, the ideal discharge state of the battery is the black curve, and the red curve is the real state when considering the internal resistance of the battery.

Figure: Qmax is to maximize the battery capacity; Quse is the actual capacity of the battery; Rbat represents the internal resistance of the battery; EDV is the discharge termination voltage; I is the discharge current;

It can be seen from the figure that the actual capacity of the battery is Quse< the theoretical maximum capacity Qmax of the battery. Because of the resistance, the actual capacity of the battery is reduced. We can also see that the actual capacity of the battery Quse depends on two factors: the product of discharge current I and battery internal resistance Rbat, and the discharge termination voltage EDV. It should be pointed out that the battery internal resistance Rbat will gradually increase with the use of the battery.

The unit of resistance is generally milliohm Ω (m), the internal resistance of battery, at the time of charging and discharging, high internal power, serious fever, can lead to accelerated aging and longevity attenuation of battery at the same time also limit big ratio of charge and discharge applications. Therefore, the smaller the internal resistance, the better the battery life and multiplier performance. Usually the battery internal resistance measurement method has the alternating current and the direct current test method.

self-discharge

Battery self-discharge, refers to the phenomenon of voltage drop in the open circuit static process, also known as the battery's charge holding capacity.

Generally speaking, self-discharge of battery is mainly affected by manufacturing process, material and storage conditions. Self-discharge can be divided into two types according to whether the self-discharge is reversible after the loss of capacity. Irreversible loss of capacity means that capacity cannot be restored.

The cause of battery self-discharge more research theory, sum up can be divided into physical reasons (storage environment, manufacturing process, material, etc.) and chemical (electrode in the electrolyte instability, internal produces chemical reaction, active material is consumed, etc.), the battery self-discharge will directly reduce the capacity of the battery and storage performance.

life

Battery life is divided into cycle life and calendar life two parameters. Cycle life refers to the number of times the battery can be recharged and discharged. That is, under ideal temperature and humidity, the rated charging and discharging current is used for charging and discharging, and the cycle times when the battery capacity decays to 80% are calculated.

Calendar life refers to the time span of battery life ending condition (capacity decaying to 80%) after specific working conditions under operating conditions. Calendar life is closely combined with the specific requirements of the use, usually need to specify the specific working conditions, environmental conditions, storage interval and so on.

Cycle life is a theoretical parameter, but calendar life has more practical significance. However, the calculation of calendar life is complicated and time-consuming, so generally battery manufacturers only give the data of cycle life.

The figure above shows the charging-discharging characteristics of a ternary lithium battery. It can be seen that different charging-discharging modes have different influences on the battery life. As shown in the figure above, the life of 25%-75% charging-discharging can reach 2500 times, that is, the battery is charged and discharged shallowly. We'll talk more about battery life later.

Battery pack consistency

This parameter is quite interesting. Even if the batteries of the same specification and model are grouped together, the performance of the battery pack varies greatly in terms of voltage, capacity, internal resistance, life span, etc. When used in electric vehicles, the performance index often fails to reach the original level of the single battery.

Current reasonable explanation:

After the single battery is manufactured, due to technical problems, the internal structure and material are not completely the same, and there are certain performance differences in the battery itself. Initial inconsistent with the battery in the process of using continuous charging and discharging cycle and cumulative, coupled with the use environment of battery pack for each monomer battery is different, lead to a bigger difference, each monomer battery status in use process gradually amplifier, and in some cases made some monomer battery performance acceleration attenuation, and eventually lead to premature failure. The battery pack

It should be pointed out that the performance of a power battery depends on the performance of a single battery, but it is by no means a simple accumulation of the performance of a single battery. Due to the inconsistent performance of the single battery, the battery pack will have various problems in the repeated use of electric vehicles, resulting in shortened service life.

In addition to the requirements in the production and assembly process, strict control process and try to maintain the consistency of the single battery, the current industry generally use a balanced function of the battery management system to control the battery pack battery consistency, in order to extend the service life of the product.

into

Let's talk about the last parameter, which is related to the manufacturing process of the battery.

Made after the battery, need to small current charging of batteries, its internal anode material is activated, the anode surface to form a passivation layer, the SEI (solidelectrolyteinterface) membrane, the battery performance is more stable, the battery can reflect the real performance, after into a process known as into.

The sorting process in the formation process can improve the consistency of the battery pack and the performance of the final battery pack. The formation capacity is an important index for screening qualified batteries. The SEI film below looks like a black rose.

The page contains the contents of the machine translation.