My friend Mr WEI is the engineer of electrical company, He fall into the trouble for the battery and inquiry my suggestion last week.
Below as the detail for case
There is ICR26650 7.4V 3000MAH battery module with protection board (built-in protection board) were used in new equipment. According to the performance of the lithium-iron battery, the voltage should reach 8.4V when the battery is fully charged. However, when the voltage meet to 7.2V the battery will stop charging, but normal discharge are workable.
They try to charge the battery three times, The voltage can only charge to 7.2V, he asked me: what is wrong with the battery ? I bought bad one?
So, what is the problem? How to check and What’s the solutions?
Indeed, it is really reject production for the battery pack can not be full charged,
In fact battery pack structure is actually simple, it can be mistake on two part,1) battery pack the 2)protection board , so we just screen out the reject reason One by one .
The battery problem:
1) Single Cell’s consistency for battery pack is not good
If the consistency is large difference between two single cell ,during in the charging process, One battery cell is charged to 4.2V, the other only is charged to 3.2V, so the whole battery pack shows 7.4V, However, since if the protection board setting the parameter 4.2V, the battery pack was protected and the battery pack can not be charged anymore.
So the first step is try to test the voltage and inner resistance of the battery pack , if the battery pack performance is ok , then test the voltage and inner resistance of each single cell’s , Comparative the different for the cell
2)One single cell for battery pack was damaged
If it is confirmed that the single cell have been sorted with same level ,check the self-discharge performance of single cell , if one of cell’s self-discharge rate beyond the standard, it will lead to the voltage is less than normal standard (U = IR)
The method for check the self discharge performance as below
Storage the battery with high temperature can speed up the chemical reaction process ,and the reject battery will show lower voltage than normal. Just sort out it .
3) the protection board was damaged
If the battery pack zero defect was confirmed, Need to consider the possible cause of the protection board
3.1), At first of all, check the appearance of the protective board, whether there are below defect as below a) faulty soldered joint b)the joint is bigger than normal c) rag
All of these defect will lead to the battery pack are rejected,
3.2) Second, check the performance for protection board by specially analysis equipment ,Test the key parameters with specification so that find out the reject reason.
A few days later , I got information form Mr Wei that they found what’s wrong with the battery pack, and
He said that the wire is very weak connection between the battery and the protection board, they just connect the wire again, the battery were amended.
So, for the battery pack manufacturer ,it is really important to pay attention to the details .The detail is king