Customer Inquiry:
The problem lies in battery self-discharge. There will always be some batteries with inherent defects that exhibit relatively high self-discharge rates. If such batteries are connected in parallel within a battery pack intended for long-term standby, they may cause severe lifespan issues, and even lead to over-discharge.The more batteries connected in parallel, the more severe the problem becomes. I generally try to avoid parallel connection as much as possible.
Battery self-discharge seems to be difficult to test, as the process is extremely time-consuming, and there is currently no good solution to this problem.
Batteries with high self-discharge rates (those with inherent defects), even if fully charged together with normal batteries, will develop voltage discrepancies due to “extra discharge” after long-term standby.
You can try using a MOSFET-based battery balancing circuit:
Balancing Controller
- Discrepancy Detection: Collect the voltage of each battery in real time to determine whether a voltage difference caused by self-discharge exists.
- Switch Control: If the voltage of a certain battery is too high, trigger the conduction of its corresponding MOSFET to activate the bypass path.
Bypass Branch
- MOSFET: Acts as a “switch” whose on/off state is controlled by the balancing controller.
- Current-Limiting Resistor: Restricts the bypass current (prevents excessive current from burning out the MOSFET or the battery).
Example: Suppose you have 2 parallel lithium-ion batteries (Battery A is normal, Battery B has a high self-discharge rate). After long-term standby, the SOC of Battery A is 90% (voltage: 3.8V), while the SOC of Battery B is 70% (voltage: 3.6V).
Working Principle Flow
- Discrepancy Detection: The balancing controller collects the voltages of A and B in real time and detects a 200mV difference between A and B (exceeding the preset threshold, e.g., 50mV). It then determines that the SOC of A is too high and requires current shunting.
- Bypass Activation: The controller outputs a signal to turn on the bypass MOSFET corresponding to Battery A.
- Current Shunting: Part of the charging current of Battery A (or the float current during standby) no longer flows into the cell, but is dissipated through the bypass branch formed by the MOSFET + current-limiting resistor (converted into heat with minimal power, which does not affect battery safety).
- Discrepancy Reduction: Since no current is shunted from Battery B, it can receive the full charging current, and its SOC gradually rises from 70%. Meanwhile, the SOC of Battery A is maintained at 90% without further increase until the voltage of B catches up with A (e.g., both reach 3.7V with an SOC of 80%).
- Bypass Deactivation: The controller detects that the voltage difference has disappeared, turns off the MOSFET of A, and the balancing process ends. This prevents over-shunting that would cause the SOC of A to drop too low.
Note: SOC is the abbreviation of State of Charge, which refers to the percentage of remaining battery capacity. It is a core parameter of the battery management system (BMS).
Precautions: Avoid “Worsening the Imbalance”
- Perform Balancing Only in the Late Stage of Charging: The charging current is small in the late charging phase, minimizing the impact of battery internal resistance (IR). At this stage, the voltage difference can more accurately reflect the SOC discrepancy caused by self-discharge (this avoids mistaking “low voltage due to high battery impedance” for “low SOC due to self-discharge” and blindly supplying additional charge to high-impedance batteries).
- Prohibit Balancing During Discharging: The discharge current is large, and the impact of internal resistance (IR) is significant. Voltage differences during discharging may be caused by impedance. Activating the bypass at this time will cause over-discharge of normal batteries, which will instead widen the self-discharge discrepancy.
Related part:TI bq20
Related Application Notes:TI Battery Balancing