The Danger of Inconsistency of Power Lithium Battery

BSLBATT power lithium battery, suitable for golf carts, solar energy storage, RV energy storage, electric forklift, a low-speed power car and other applications should be used, is a substitute for lead-acid batteries. Power lithium battery has a long service life, and high energy density, but also great potential for improvement. And safety can be constantly upgraded, energy density can continue to rise. In the foreseeable time, it can catch up with the range and cost performance of fuel cars, and step into the first mature stage of electric vehicles. However, lithium batteries also have the troubles of lithium batteries.

Why power lithium batteries are mostly small?

We see the power lithium batteries, using cylindrical batteries, soft pack batteries or square batteries, are generally clean-looking, completely unable to find the traditional lead-acid batteries as large, this is why?

With high energy density, power lithium batteries often do not dare to design for large capacity. The energy density of lead-acid batteries is about 40Wh/kg, while lithium batteries have more than 150Wh/kg. energy concentration increases, the requirements for safety go up.

First of all, a single lithium battery with too much energy, encounters an accident, triggering thermal runaway, the rapid reaction inside the battery, a short period of time, too much energy is nowhere to release, is very dangerous. Especially in the safety technology, control capacity is not fully developed, each battery capacity should be restrained.

Secondly, the energy wrapped up by the lithium battery shell, in the event of an accident, firefighters, fire extinguishing agents can not reach, can only isolate the scene in the event of an accident, let the accident battery react on its own, until the energy is burned out.

Of course, for safety reasons, the current lithium battery has been designed with multiple safety means. Take cylindrical batteries as an example.

Safety valve, when the internal reaction of the battery is beyond the normal range, the temperature rises, and with the generation of side reaction gases, the pressure reaches the design value, the safety valve automatically opens to drain the pressure. The moment the safety valve opens, the battery completely fails.

Thermistors, some cells are equipped with thermistors, once overcurrent occurs, the resistance value increases steeply after reaching a certain temperature, and the current in the circuit drops, preventing the temperature from rising further.

Fuse, the battery cell is equipped with a fuse with an overcurrent fuse function, once the risk of overcurrent, the circuit is broken to avoid malicious accidents.

What are the dangers of consistency of power lithium batteries?

Li-ion battery can not be made into a large, had to organize a large number of small cells, everyone’s strength to one place, sincere cooperation, but also with the cheesy power car to fly. This time, it is necessary to face a problem, consistency.

Our daily experience is that two dry cell batteries, positive and negative terminals connected, flashlights can light, who cares if it is a consistent or inconsistent thing. The situation is not so simple for the large-scale application of power lithium batteries.

The inconsistency in the parameters of power lithium batteries mainly refers to the inconsistency in capacity, internal resistance, and open-circuit voltage. When inconsistent cells are used in series and together, the following problems will occur.

(1) capacity loss, cell monomer battery pack, capacity in line with the “barrel principle”, the capacity of the worst cell determines the capacity of the entire battery pack.

In order to prevent the battery from overcharging and over-discharging, the logic of the battery management system is set as follows: when discharging, when the lowest cell voltage reaches the discharge cut-off voltage, the whole battery pack stops discharging; when charging, when the highest cell voltage touches the charge cut-off voltage, charging stops. 

Take two batteries connected in series as an example. One battery has a capacity of 1C and the other has a capacity of only 0.9C. In series, the two batteries pass the same size of the current.

When charging, the battery with the smaller capacity must be filled first, and the system will not continue charging when the charging cutoff condition is reached. When discharging, the smaller capacity battery is also bound to discharge all the available energy first, and the system stops discharging immediately.

In this way, the small-capacity battery is always in full charge, while the large-capacity battery is always using part of its capacity. The capacity of the whole battery pack is always partly in the idle state．

2) Loss of life, similarly, the life of the battery pack is determined by the cell with the shortest life. Most likely, the shortest life of the cell is the one with the smallest capacity. Small capacity battery cell, each time is full of full release, out of too much power, it is very likely to be the first to reach the focus of life. Has been the end of the life of the battery cell, a group of welded together battery cells, also followed by the end of life.

3) The internal resistance increases, different internal resistance, flowing through the same current, the internal resistance of the large cores generates relatively more heat. The battery temperature is too high, resulting in faster deterioration, and the internal resistance will further increase. Internal resistance and temperature rise, forming a pair of negative feedback, so that the high internal resistance of the cell accelerates the deterioration.

The above three parameters are not completely independent, the aging degree of the core is deeper internal resistance is larger, and capacity decay is also more. The three parameters are not completely independent.

How to balance the power lithium battery during assembly?

The inconsistency of cell performance is formed during the production process and deepened during the use process. The weaker cells within the same battery pack are constantly weaker and get weaker at an accelerated rate. The degree of dispersion of parameters between single cells increases with deepening aging.

Currently, BSLBATT engineers deal with the inconsistency of single cells in three main ways. Single-cell binning, thermal management after grouping, and battery management system to provide equalization function when a small amount of inconsistency occurs.

1）Sorting

Different batches of cells are theoretically not put together for use. Even if the same batch of cells, they need to be selected, and the cells with relatively concentrated parameters are put in a battery pack, the same battery pack.

The purpose of sorting is to pick out the cells with similar parameters. Sorting methods, which have been studied for many years, are mainly divided into two categories: static sorting and dynamic sorting.

Static sorting, for the open-circuit voltage, internal resistance, capacity and other characteristics of the cell parameters for screening, select the target parameters, the introduction of the statistical calculation method, set the screening criteria, and finally, the same batch of cells will be distinguished into several groups.

Dynamic screening is to screen the characteristics of the cores in the charging and discharging process, some choose constant current and voltage charging process, some choose pulse shock charging and discharging process, and some compare the relationship between their own charging and discharging curves.

The combination of static and dynamic sorting, with static screening to do the initial grouping, based on this dynamic screening, which divides out more groups, screening accuracy is higher, but the cost will also rise accordingly.

Here is a small reflection of the importance of a handful of power lithium battery production scales. Large-scale shipments allow lithium battery manufacturers to perform finer sorting and get the closing performance of the battery pack. If the production is too small, too many groups, a batch can not be equipped with a battery pack, and then a good method can not be performed. 

2) Thermal management

For the internal resistance of inconsistent cells, the generation of heat is not the same problem. The addition of a thermal management system can regulate the temperature difference of the whole battery pack to keep it in a small range. The cell that generates more heat will still have a high-temperature rise, but it will not pull away from the other cells, and the deterioration level will not be significantly different. 

3）Equalization

The inconsistency of the cell monomer, certain cell end voltage, always ahead of the other cells, is the first to reach the control threshold, resulting in the whole system capacity becoming smaller. In order to solve this problem, the battery management system BMS is designed with an equalization function.

When a certain cell reaches the charging cutoff voltage first, and the rest of the cells lag behind, the BMS starts the charging equalization function, either by accessing the resistor and discharging part of the power of the higher voltage cells or by transferring the energy away to the lower voltage cells. In this way, the charging cutoff condition is lifted, the charging process starts again, and the battery pack is charged with more power.

Until now, the inconsistency of the cells is still an important area of research in the industry. Even if the energy density of the cells is high, when it comes to inconsistency to stir up trouble, the lithium battery pack capacity will be greatly reduced. As a professional manufacturer of power lithium batteries, BSLBATT will do tests on the performance of lithium battery packs before and after production to minimize the hidden danger of inconsistency of power lithium batteries.