The phenomenon of “charging high and discharging low” (too high voltage when charging, too low voltage when discharging) or “charging low and discharging high” (too low voltage when charging, too high voltage when discharging) of lithium battery packs is essentially the abnormalities of the state of the electric cell in the battery pack, the management system, or the external conditions resulting in the The essence of the phenomenon is that the state of the battery cells, management system or external conditions in the battery pack are abnormal, resulting in the charging and discharging voltage deviation from normal range. The following is an analysis of the core causes:
I. Definition of the two phenomena first
Fill high and put low:
The total battery pack voltage exceeds the rated charging cutoff voltage when charging (e.g., more than 4.2V / cell × number of cells in series), but the voltage drops rapidly to less than the rated discharging cutoff voltage when discharging (e.g., less than 2.75V / cell × number of cells in series).
Fill low and release high:
The total battery pack voltage stops (or rises slowly) before reaching the rated charging cutoff voltage during charging, but the voltage drops slowly during discharging, and is even higher than the normal discharge initial voltage.
II. Analysis of core causes
Battery management system (BMS) failure or abnormal parameters
The BMS is the “brain” of the battery pack, responsible for monitoring cell voltage, current, temperature, and performing charge/discharge protection (overcharge, overdischarge, overcurrent, etc.). Its abnormality directly leads to voltage control failure:
Resulting in high charging and low discharging:
The BMS overcharge protection threshold is set too high (e.g., mistakenly set to 4.3V / monobloc), or the precision of the voltage detection module deviates (e.g., mistakenly determining that the battery cell is not fully charged), resulting in a continuous voltage increase to “Charge High” during charging;
The BMS over-discharge protection threshold is set too low (e.g., below 2.5V / monobloc), or the current detection is wrong when discharging (failing to recognize the high current leading to over-discharge), resulting in “Discharge Low”.
Resulting in charging low and discharging high:
The BMS overcharge protection threshold is set too low (e.g. mistakenly set to 4.0V / monomer), or the voltage detection module mistakenly judges that the battery cell is full (actually not full), resulting in “charging low”;
Abnormal current detection when BMS discharges (e.g. misjudging small current as high current and limiting the depth of discharge in advance), or wrong internal resistance compensation algorithm, resulting in slow voltage drop during discharge, which is manifested as “Discharge High”.
Poor cell consistency (most common cause)
Battery pack is composed of multiple cells connected in series/parallel, if the difference in capacity, internal resistance and voltage between cells is too large (poor consistency), there will be a “barrel effect” when charging and discharging:
Leading to charging high and discharging low:
When charging, the cell with small capacity is filled first, and continued charging will lead to over-charging (voltage surge) of that cell, driving the total voltage of the whole battery pack to “Charge high”;
When discharging, the cell with small capacity is discharged first, and continued discharging will lead to over-discharge (voltage plunge) of that cell, driving the total voltage “discharge low”.
Lead to charging low and discharging high:
When charging, the core with large internal resistance has poor ability to accept current, voltage rises slowly, the BMS may continue to charge because the total voltage is not up to the standard, but the actual part of the core has been overcharged, and the total voltage is still lower than normal in the end (charging low);
When discharging, the core with small internal resistance has high discharging efficiency, the voltage falls slowly, while the core with large internal resistance has large voltage drop, and the total voltage may be higher than normal after the two are canceled out (discharging high), but the The total voltage may be higher than normal (high discharge), but the actual capacity has been seriously attenuated.
“Charge High Discharge Low” or “Charge Low Discharge High” is the result of the joint action of cell state, BMS control and external conditions, of which poor cell consistency and BMS failure are the main causes. These phenomena will not only shorten the battery life, but also may lead to overcharging fire, over-discharge leakage and other safety risks, which need to be avoided by regular testing of the BMS, screening of consistent battery cells, and matching of compliant charging and discharging equipment.
Post time: Jul-17-2025