Learn about the dangers of inconsistent lithium-ion batteries

Power lithium battery, has firmly occupied the position of the leader of the electric vehicle power market. It has long service life, high energy density, and great potential for improvement. Safety can be changed and energy density can continue to rise. Legend in the foreseeable period (about 2020 years or so), it can catch up with the endurance and cost-performance of fuel vehicles and enter the first mature stage of electric vehicles.

However, lithium batteries have the trouble of lithium battery.

1 Why are lithium batteries mostly small

Lithium batteries, cylindrical batteries, flexible pack batteries, square batteries, we see that are generally beautiful looking, do not find the traditional lead-acid batteries as large, why?

High energy density, lithium batteries are often not designed the large capacity. Lead-acid battery energy density in 40wh/kg, and lithium battery has more than 150 wh/kg. With the increase of energy concentration, the demand for safety is increasing.

First of all, a single lithium battery with too high energy will encounter an accident and cause heat runaway. The battery will react sharply inside. In a short time, there will be no place to release too much energy, which is very dangerous. Especially when the development of safety technology and control ability is insufficient, the capacity of each battery should be restrained.

Second, the lithium battery shell wrapped up the energy, once appear accidental, firefighters, fire extinguishing agent can't reach and can't help. Only in the event of an accident to isolate the scene, let the accident battery self-reaction, until the energy burn out.

Of course, for safety reasons, current lithium batteries have been designed with multiple safety measures .Take cylindrical battery for example.

Internal reactions beyond the normal range, relief valve, when the battery temperature rise, and accompanied by generation reaction gas, pressure to meet the design value, the relief valve automatically open, drain off the pressure. When the safety valve opens, the battery is completely dead.

Thermistor, some cells are equipped with thermistor, once there is a flow, after the resistance reaches a certain temperature, the resistance value increases sharply, the current in the loop drops, preventing the temperature from further rising.

Fuse, the cell is equipped with the fuse with the function of overcurrent fuse, once there is overcurrent risk, and the circuit is disconnected, to avoid the occurrence of malignant accidents.

2 consistency lithium-ion batteries

Lithium battery cannot be made into a big one, had to put a lot of small cells up, we work hard to make one place, sincere cooperation, also can take the electric car to fly. At this point, you need to face a problem: consistency.

Why need consistency

Our everyday experience is, two dry batteries, the positive and negative poles are connected, the flashlight can shine, who CARES if it is consistent or not. But the large-scale application of lithium battery, the situation is not so simple.

The inconsistency of lithium battery parameters mainly refers to the inconsistency of capacity, internal resistance and open circuit voltage. Inconsistent cell strings used together can cause the following problems:

1) the capacity loss, battery monomer composition, capacity in line with the principle of "bucket", the worst of the batteries capacity to determine the ability of the whole battery pack.

In order to prevent the battery overcharge discharge, battery management system of logic so Settings: discharge, when the lowest monomer voltage reach discharge cut-off voltage, discharge the battery to stop; When charging, when the supreme monomer voltage when touched by charging voltage, stop charging.

Take the example of two batteries in series. One battery has a capacity of C, and the other has a capacity of 0.9c. In series, two batteries pass the same amount of current.

Charging, small capacity battery must first fill, achieve charge cut-off condition, system no longer continue to charge. Discharge, small capacity battery must be first all the available energy, light system stop discharge immediately.

In this way, the capacity of the small batteries in full all the time, large capacity batteries has been using partial capacity. Some of the capacity of the entire battery pack is idle.

2) Life loss, similarly, the life of a battery pack is determined by the cell with the shortest life. Great possibility, the cell with the shortest life span is the one with the smallest capacity. Small capacity batteries, every time is filled, with output too hard, most likely the first to reach the focus of life. Batteries life end, a set of batteries, welded together with dies a natural death.

3) Internal resistance increases, different internal resistance, through the same current, the internal resistance big batteries calorific value is relatively more. Battery temperature is too high, resulting in accelerated deterioration speed, internal resistance will further increase. Internal resistance and temperature rise, form a negative feedback, make the high internal resistance batteries acceleration deterioration.

The above three parameters, are not completely independent, aging degree deep cell internal resistance is bigger, also more capacity attenuation. Separately, just want to clearly state their respective influence direction.

3 how to deal with inconsistencies

Batteries performance, are formed in the process of production, in use process. Within the same battery batteries, constant is weak, the weak and accelerate the weaker. Parameters of discrete degree between monomer batteries increase as the degree of aging.

At present, the engineer shall monomer batteries, mainly from three aspects to consider. Single battery sorting, heat management after grouping, battery management system provides balanced function when there is a small amount of inconsistency.

3.1 sorting

Different batches of the batteries, theoretically not used together. Even if the same batch of batteries, also need to be filtered, the parameters of relatively concentrated batteries put in a battery pack, the same battery pack.

The purpose of separation is similar batteries of the parameter is selected. Separation method, have been studied for many years, divided into two types: static and dynamic separation.

Static separation, in view of the open circuit voltage of batteries, internal resistance, capacity characteristic parameters, such as screening, selection of target parameters, the introduction of statistical algorithms, set the selection criteria, the final will be the same batch of batteries is divided into several groups.

Dynamic filter, it is in view of the batteries in the process of charging and discharging of features to filter, some choose to constant current constant voltage charging process. Some choose pulse impulse charging and discharging process, and some compare the relationship between their charging and discharging curves.

Movement in combination with separation, made preliminary static screening group, on the basis of the dynamic filtering, which divided the group more, screening accuracy is higher, but the cost will increase accordingly.

Here reflects the importance of the production scale of power lithium battery. Large-scale shipments allow manufacturers to conduct more detailed sorting, to get closer to the performance of the battery pack. If the production volume is too small and there are too many groups, each batch cannot be equipped with a battery pack, and even the best method cannot be applied.

3.2 Thermal management

For internal resistance inconsistent batteries, heat is not the same problem. The addition of thermal management system can adjust the temperature of the battery pack, make it keep in a smaller range. Temperature rise of the generated heat more batteries, still on the high side, but not with other batteries fall behind, degradation level will not appear obvious difference.

3.3 Equilibrium

Cell monomer is not consistent, some cell terminal voltage, always ahead of other cells, the first to reach the control threshold, resulting in a smaller capacity of the entire system. In order to solve this problem, battery management system (BMS balance function is designed.

A secondary batteries charge cut-off voltage first to arrive, and the rest of the batteries voltage obviously lag, the BMS starting charging balance function, or resistance, put off the high voltage part of batteries, battery, or the energy transfer and in low voltage batteries. So, charging by conditions were lifted, start charging process, battery pack into more power.

Until now, cell inconsistency is still an important field of research in the industry. No matter how high the energy density of the cell is, the battery pack capacity will be greatly reduced when it is disturbed by inconsistency.

The page contains the contents of the machine translation.

Reference

Automotive Power battery Many inner heat source heating model and the study of electrothermal inconsistencies

The electric car Lithium ion power battery Sorting method research

Electric vehicle power battery sorting method research

Based on the consistency of power battery research more spectrum method

Cylindrical lithium ion power battery sorting process optimization and application

lithium iron phosphate power battery Under the dynamic state of SOC sorting

The page contains the contents of the machine translation.