Battery failure – common failures and repair steps

The value of lithium battery repair

For a battery pack of battery failure, the first thing is to judge whether it is worth repairing, which is necessary for both battery users and maintenance personnel. For example, the lithium-ion batteries of the deliverers and drivers are usually used almost every day. When the capacity decays, the general situation is that the service life has reached the end, and the battery has no repair value and can only be recycled as waste.

On the contrary, if the battery is only used for transportation, the frequency of use is not high, when there is a sudden battery failure (such as undercharge, sudden drop in cruising range, sudden power failure, battery failure to start, etc.), this type of battery failure has the value to get repaired.

Causes of lithium battery failure

In addition to the damage of the protection board and the natural aging of the battery cell, the lithium battery failure is almost related to the pressure difference, and the main reasons for the pressure difference are:

1. The degree of aging of the cell is inconsistent, and one or several cells age faster than other cells to produce differential pressure;
2. Damage of a cell or a string of cells causes the voltage of the string to be lower than that of other strings resulting in a pressure difference. The battery pack follows the “barrel theory”, which means that the performance of the whole battery is determined by the string of worst performance.

Classification of lithium battery failure

Protection board failure

The methods for detecting whether the protection board is normal are:

1. Appearance inspection, to check whether the components and lithium ion battery structure have burning or blasting marks;
2. By testing whether the voltage of B- and the total cathode (voltage not over the board) and P and the total cathode (voltage over the board) is consistent, the voltage is consistent, indicating that the protection board is at normal status, if the pressure difference is large, the protection board is abnormal.

By measuring the resistance between B- and P-, if the resistance is zero, it is normal. If it is not conductive, the protection board is abnormal. Different protection board manufacturers have different circuit designs, and the detection method is based on the manufacturer’s designs. If there is no mark of the manufacturer, you can determine whether the protection board is of battery failure by replacing the protection board.

Generally, the protection board is replaced rather than repaired, because the protection board and the motorcycle battery pack are energized in real time, if a component is damaged, it will have a chain reaction, causing the performance of other components to decay or damage, and if the maintenance is not thorough, it will bury the hidden danger. The damage of the protection board is generally related to the mismatch of charging voltage and current, and it is necessary to pay attention to the model, voltage, current and other parameters of the matching charger when maintaining.

Abnormal capacity reduction

Undercharge:
The battery pack has a voltage, but not a full voltage, when charging, the charger will automatically jump the lamp even if it’s not fully charged, and it won’t be able to continue to fill the battery. If the detection protection board and the connection line are normal, you can unplug the protection board line, measure the voltage of each string of cells one by one.

There may be a string (or several strings) of cells in the other cells have not been fully charged when the highest working voltage has been reached, as long as the protection board detects a string of cells to reach the highest working voltage, so as to prevent it from overcharging, which will automatically cut off the charging circuit, resulting in battery failure to charge.

The repair method is to balance the pressure difference, you can use the repair device directly to connect the line to the repair device automatically, or use the single series charging method to balance the voltage of each series. The pressure difference is the largest at high and low voltage, and the low voltage repair is the best, the battery pack discharge and then the single series charge to balance.

The discharge test must be done after the completion of the pressure difference repair, because when there is a large pressure difference, most of it is caused by a battery damage in a string.

To measure the voltage of each string at the same time of discharge, if there is pressure difference of a string which is getting larger and larger, it means that the string cell is damaged, you can disconnect the nickel of that string, and leave it for more than ten minutes and then measure the cell voltage one by one, the cell of zero voltage or fastest self-discharge is the damaged cell, and it need to be replaced.

Battery failure to fully discharge
The battery pack has not reached the discharge cut-off voltage, the power protection can not continue to discharge, the battery pack capacity is significantly smaller, and the driving range is significantly reduced.

This kind of battery failure is also due to the pressure difference that is too large, although most of the cells are still in the effective working voltage range, but the protection board detects that a series of cells have reached the minimum working voltage, in order to prevent overdischarge which will automatically cut off the discharge circuit resulting in inability to continue discharging. The maintenance method is the same as charging, but the highest working voltage of the cell is changed to the lowest working voltage.

Battery failure to charge and discharge

The battery pack has voltage but cannot be charged and discharged. This kind of battery failure is also caused by pressure difference under the premise of excluding damage to the protection board, but the probability of damage to the internal battery core is greater.

When the normal cell voltage has been at the highest operating voltage, the protection board can not be charged, but the damaged cell voltage is already zero or has been lower than the minimum operating voltage, the protection board at the same time started overdischarge protection, so neither charge nor discharge repair method is through the series voltage measurement to find the string with zero voltage or the lowest voltage, and disconnect the nickel sheet. After leaving it for a while, find the damaged cell and replace it.

Methods to replace the battery cell

Since the repairs are all aimed at the batteries that have been used for a period, it is almost impossible to find batteries with the same consistency.

Therefore, it is generally not recommended to directly replace the cells of battery failure with new ones, because the internal resistance SOC and capacity of new and old batteries are not the same, the inconsistency of the pressure difference will gradually increase during use, resulting in battery failure such as insufficient charging or discharging. If only need to replace one battery cell, you should try to find a battery cell from the same manufacturer, the same model, or the same capacity, the same rate, and the same internal resistance.

Subtraction and combination:
In order to ensure the consistency of the battery cell and the stability of the battery pack, the subtraction method can be used under the premise that a part of the capacity is allowed to be sacrificed. Reducing the number of parallel connections of each series in the original battery pack by one, and replacing the cells of battery failure with the reduced normal cells is the method of subtraction and combination.

For example, 10S5P battery pack has been detected that 5 cells in 1 series are all damaged, and the cell with the same parameters cannot be replaced, you can remove one cell from each series out of the 10 strings and change it to 10S4P , except for the broken string of 5 batteries, there are still 9 batteries that are normal, and 4 of them can be used to replace the damaged string. The battery packs repaired in this way are still the original batteries, and the consistency is guaranteed, but the capacity is 20% less than the original.

Addition and combination:
It is to ensure the constant capacity on the basis of the subtraction method. After the completion of the reduction method, find another 10 cells with the same parameters as the original cell, each string increases by one, and can not be mixed with the remaining cells of the subtraction method, must make each string to 4 old cells with 1 new cell, so as to achieve the balance of each string to ensure the consistency of each string and the stability of the battery pack of battery failure.

String reduction method:
It is to remove the damaged string of batteries and change 10S to 9S. This method is the simplest and most effective method, but it must be clear whether the protection board supports the work of string reduction, and some highly integrated protection boards cannot be used with string reduction. It is also necessary to know whether the voltage after the series reduction can meet the voltage requirements of the original electrical appliances. After the series is closed, the voltage and current change, and the charger must be replaced accordingly.

Lithium battery repair steps

Firstly, carefully observe whether the appearance structure is intact, whether there is any damage, leakage, scorching, flooded, etc., to judge the operation risk, so as to avoid accidents during the disassembly and maintenance process. After confirming that there is no problem, measure the voltage of the charging and discharging port.

If the measured charge and discharge port voltage is 0, first measure whether there is voltage on the junction protection board (between B- and the main positive of the battery pack). If there is voltage, check whether the connecting wire is disconnected. If the circuit is normal , it can be judged that the battery failure point may be on the protection board, and the protection board needs to be replaced or the battery pack should be charged and tested.

Measure the voltage without the protection board (the total cathode and total anode poles of the battery pack). If it is 0 voltage, it may be caused by the battery’s long idle self-discharge. You can directly charge the cathode and anode poles of the battery pack without the protection board. Single-string charging activates the battery cell, and the charging voltage must be correct. The single-string charging voltage is the highest working voltage of the battery cell.

The charging voltage of the whole group: iron phosphate charging voltage = 3.65VXS (number of series), ternary potassium charging voltage = 4.2VXS (number of series). At the beginning, it should be charged with a small current of no more than 0.2C. After the voltage rises, the current should not exceed 0.2C.

When the voltage exceeds the minimum working voltage of the battery cell, it will be charged through the protection board. After the cell is activated, the voltage of each string should be measured. If the voltage difference is normal, it means that the battery failure has happened to the cell in the string and cannot be activated.

Find the string and disconnect the nickel sheet, and then measure each cell in the string one by one after leaving for more than ten minutes. The battery cell with zero voltage or the fastest self-discharge is the damaged cell, just replace it.