What will cause an individual battery cell to reverse polarity.

Correct Answer: Option C — When discharging the battery string if a cell becomes weaker than the remaining cells, the discharge current will effectively charge the weaker cell in reverse polarity.

In a series-connected battery string, if one cell is significantly weaker or discharged, the remaining cells will force current through it during discharge. This reverse current flow causes the weaker cell to charge in the opposite polarity, leading to permanent damage, gassing, and potential failure of the entire bank.