Mapping Current Direction Through Battery Protection FET States

A battery protection circuit with one MOSFET commanded off may still permit current in the opposite direction through a body diode. That behavior can be intentional during recovery, but sustained diode conduction can create significant heating. Review current direction and the protector’s state logic together.

Draw every switch state

Show the actual MOSFET orientation, body diodes, charge and discharge connections, and any precharge or predischarge path. For each combination of gate states, trace both possible current directions. Avoid using a single label such as battery disconnected when only one direction is blocked.

The TI BQ76952 technical reference manual describes body diode protection behavior that can automatically turn on an otherwise off FET under defined current and configuration conditions. Such behavior must be considered when interpreting commanded and observed gate states.

Separate commands from autonomous actions

Record which functions control each gate: host commands, protection faults, recovery logic, and dedicated control pins. Check priority and interaction in the selected device documentation. A host write does not necessarily guarantee a gate remains in that state under every autonomous protection mode.

Read status and measured current alongside gate voltage. Timestamp these observations so the order of events is clear. If a gate changes unexpectedly, correlate the event with the documented state machine before assuming a hardware defect or firmware race.

Review diode conduction stress

Estimate the current and duration that can flow through an off device’s diode in each permitted recovery sequence. Check device thermal and pulse limits using the actual conditions. A lower current charger and a high current load can create very different stress even through the same physical path.

Verify that a permitted reverse-direction recovery does not bypass another required protection function. Use the supported circuit topology and configuration rather than disabling features simply to force a desired gate waveform.

Test bounded transitions

Use an appropriate battery simulator or controlled test arrangement to verify state changes before applying the full pack’s available energy. Observe terminal voltage, current direction, gate voltages, and protection status. Include attachment and removal of both charger and load.

Save the state table, configured protection behavior, component orientation, and synchronized measurements. The result should identify which current paths remain possible in every relevant state and how prolonged diode conduction is handled. A low terminal voltage or one low gate signal alone does not prove complete isolation. Clear state mapping also helps prevent a later firmware change from contradicting the intended hardware protection behavior.