The harm of lithium battery inconsistency coping strategies

Polaris energy storage network: the performance of the cell is inconsistent, are formed in the production process, in the use of the process to deepen. In the same battery pack, the weak become weak, and accelerate to become weak. The dispersion degree of parameters between single cells increases with the increase of aging degree.

Power lithium battery, has firmly occupied the position of electric vehicle power market leader. Long service life, high energy density, and great potential have improved. Security can be changed and energy density can continue to rise. In the foreseeable future (around 2020, legend has it), we can catch up with the endurance and cost performance of fuel vehicles and step into the first mature stage of electric vehicles. However, lithium batteries also have lithium battery trouble.

Why are lithium batteries mostly small?

We see the lithium battery, cylindrical battery, Soft package battery, square batteries, generally looks handsome, completely can't find the traditional lead-acid battery so big, is this why?

High energy density, lithium batteries are often not designed the large capacity Lead-acid battery energy density in 40 wh/kg, and lithium battery, has more than 150 wh/kg. Increase of concentration of energy, the requirements for the safety of a rising tide lifts all boats.

First of all, only high energy lithium battery, met an accident, causing thermal runaway, sharp reaction inside the battery, a short period of time, too much energy is released, is very dangerous. Especially in security technology, control capability is not enough to fully, 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, powerless, can only at the scene of the accident when the isolation, President of accident battery self-reaction, energy burnt out.

Of course, current lithium batteries have been designed with multiple safety measures in mind. Take cylindrical batteries.

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. The moment the safety valve opens, the battery completely fails.

Thermistor, some batteries configuration thermistor, once appear, over current, resistance after reach a certain temperature, resistance, loop current decline, to prevent the further increase of temperature.

The fuse, batteries with which has the function of over-current fuse fuse, once appear, over-current risk, circuit disconnect, prevent malignant accidents.

Lithium battery consistency issues

Lithium battery can't make a big, only have to set up numerous small batteries, interest makes toward one place, sincere cooperation, also with the electric car fly up. At that time, you need to face a problem, consistency.

Our everyday experience is, two dry cell, is negative, the flashlight can shine, who CARES consistently inconsistent. While the large-scale application of lithium-ion batteries, the story is not so simple.

Lithium battery parameters of inconsistent mainly refer to the capacity, internal resistance, open circuit voltage of inconsistencies. Inconsistent batteries and used together, can appear the following questions.

1) the capacity loss, batteries, 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 1C and the other has a capacity of just 0.9c. In series, the two batteries flow through 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. The battery capacity is always part of the idle

2) Life lost, similarly, the service life of the battery pack, is decided by the shortest life that batteries. Great possibility, the shortest batteries, life is the capacity of small batteries. 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, causing degradation speed, resistance will rise further. Internal resistance and temperature rise, form a negative feedback, make the high internal resistance batteries acceleration deterioration.

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

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. Thermal management after monomer battery sorting, grouping, a small battery management system to provide the balanced function.

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 being 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 selection impact pulse charging and discharging process, some contrast of the relationship between the curve of charging and discharging.

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 is a small reflects the importance of a power lithium battery production scale. Large shipments, making factory can undertake more sophisticated sorting, get closer to the performance of the battery pack. Grouping if production is too small, too much, a batch cannot be equipped with a battery pack, it is a good method also is unable to put to good use.

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) Balance

Voltage of the batteries of the monomer, some batteries, always ahead of other batteries, first docked control threshold, causing the entire system capacity is smaller. 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, the inconsistency of batteries, is still the important domain of industry research. Batteries energy density high, meet inconsistencies to sweeten battery pack ability will also be discounted.

The page contains the contents of the machine translation.