Yes, a car battery can reverse polarity, but it is a relatively rare and serious electrical malfunction. This typically doesn't happen spontaneously in a healthy battery. It occurs under specific failure conditions, most commonly when a battery is deeply discharged and then jump-started or recharged incorrectly. The reversal happens when the battery's voltage drops so low that the remaining chemical energy is insufficient to maintain the correct charge on the plates. If power is applied from an external source like jumper cables, the current can force a reversal of the electrochemical cells, causing the positive terminal to become negative and vice-versa.
The most frequent cause is incorrect jump-starting. Connecting the jumper cables backwards—positive to negative—sends a massive surge of current that can permanently damage the battery's internal structure and cause polarity reversal. A battery that has reversed polarity will often show a negative voltage reading on a multimeter instead of the normal +12.6 volts. It can cause catastrophic damage to a vehicle's sensitive electronic components, including the Engine Control Unit (ECU), alternator, and infotainment system, leading to very expensive repairs.
Here is a comparison of a normal battery state versus one that has undergone reversal:
| Condition | Voltage Reading | Symptoms | Risk to Vehicle Electronics |
|---|
| Healthy, Fully Charged | +12.6V to +12.8V | Starts car normally, powers all electronics | None |
| Deeply Discharged | +3V to +10V | No crank, dim lights | Low risk if properly recharged |
| Reversed Polarity | -1V to -9V (Negative Voltage) | No power, possible strange electrical behavior | Extremely High |
If you suspect your battery has reversed polarity, the safest course of action is to disconnect it immediately and have it tested by a professional. In almost all cases, the battery will need to be replaced. To prevent this, always double-check jumper cable connections (red to positive, black to a ground point) and avoid letting your battery become completely dead.
