It's surprisingly common to hear stories about connecting a battery charger to the wrong terminals and the subsequent consequences that occur. In many cases, realizing the error quickly and removing the charging or jumper cables prevents damage. Also, safety triggers in some vehicles minimize the damage.
Charging a Battery Backwards
A battery consists of a negative and positive terminal; charging the battery backward is not possible. The cables connecting the battery to the charging source must match to charge the battery. The charging cables connect to another battery or a wall outlet with an intermediary charging device that manages voltage and prevents overcharging.
Charging from another battery is common, especially in the field where electricity is not immediately available. The cables connect to a battery on a running vehicle that is charging off its alternator. The other ends of the cables connect to the dead battery. The voltage from the running vehicle eventually charges the dead battery.
Battery charges that connect to a wall outlet are often designed to trickle charge. Rather than injecting a rush of voltage, they slowly raise the battery charge level in a safe manner until it reaches a full charge. These types of chargers are known as battery tenders, and they are particularly common on vehicles, boats and machinery that require a battery to function but do not operate regularly. These batteries gradually lose charge while sitting unused, and the tenders ensure they remain ready for action.
Correct Charging Procedures
Charging a battery requires that the positive cable connects to the positive terminal and the negative cable connects to the negative terminal. When charging a battery from another battery, the cables must meet this requirement on both ends. The battery terminals and cables are typically labeled or color-coded, making it simple to match the connections.
When the cables are connected backward, sparks fly, and the heat created may melt the cable insulation. Remove the cables immediately to prevent further damage. After the initial round of sparks and excitement, assessing the damage requires testing the vehicle and examining everything from basic fuses to the vehicle computer or ECU.
First, replace the battery with one that is fully charged. Test the vehicle, boat or machine by turning the key to start. Also, test electrical components such as blinkers, lights and the horn. If the vehicle remains dead, work through the fuses and links starting at the battery and working upstream.
In many cases, the first fuse blows, and that fuse protects the remainder of the vehicle. The first fuse or set of fuses is the bigger one in the fuse box. Check these along with all the other fuses in the box and replace any that are blown. Next, test the vehicle a second time. It should start and function properly. If the fuses are in good working order and problems persist, have the ECU checked by a mechanic.
Testing the Battery
After charging a battery backward, charge the battery in the correct manner. After charging, test the battery to ensure it operates properly. If the battery fails to charge or fails to hold a proper charge over time, have the battery tested by a professional. Many auto parts stores and battery-specific stores test batteries for free.
If the battery tests bad and fails to hold a charge, return or recycle the battery and purchase a new model.