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Battery Bombs

Explosives under your hood

All car batteries come with warning labels. Most people are familiar with the cautions about battery acid, which can melt skin and cause blindness if it contacts eyes. As such, the sulfuric acid gets top billing in the potential-battery-hazards department.

But batteries contain an even more heinous substance. In addition to acid-caused burns and vision problems, batteries' warning labels also mention well-ventilated areas away from open flames, sparks, cigarettes and other sources of ignition. These warnings are similar to those on a gas pump.

Invisible Enemy

Lead-acid batteries—the type used to start cars—produce electricity through a chemical reaction. Lead plates are submerged in an electrolyte (water and sulfuric acid) inside the battery's cells. These components react chemically to generate direct-current electricity.

The hazard is in the byproduct of this reaction: hydrogen gas. It's so flammable that this gas is used to fuel rocket engines. The Catch-22 in car batteries is that a spark can easily ignite hydrogen gas, and batteries themselves are capable of producing sparks. If the lead plates inside the battery touch, arcing and sparking can convert the battery into a bomb. The resulting gunshot-like explosion disperses the caustic acid, where it can eat through underhood components as well as human parts.

Maintenance

The best way to prevent battery explosions is through routine maintenance. On removable-cap batteries, keep the water level up so that the lead plates' tops are less likely to emerge above the electrolyte, warp and contact each other. Also, keep battery terminals clean so that the power isn't inclined to arc in search of a path of least resistance.

The two most likely explosion times are during start-up and improper jump-starting. Power demands are greatest when the battery must get the starter motor spinning, and the resulting heat increases the amount of hydrogen gas. Hot weather compounds the problem.

Concerning jump-starting, the last cable connection usually produces a spark. To minimize the danger, keep the spark as far away from the dead battery as possible. Always make the final connection on the good battery, and connect the negative jumper cable to an unpainted metal surface of the dead car (such as an alternator bracket), not to the drained battery's negative post. In extreme cold weather, the battery can basically become a chunk of ice. Jump-starting can cause hydrogen gas to blow apart a battery in its attempt to escape.

Modern Batteries

Technology has both increased and decreased the probability of battery explosions. Sealed "maintenance-free" batteries don't allow the electrolyte level to be filled, thus increasing the chances of internal sparking. However, maintenance-free cells normally hold more electrolytes than comparable fillable-cell models. Also, modern cars place more electrical demands on batteries than ever before. Because batteries are heavy and weight affects performance and fuel economy, battery manufacturers try to generate more amps from smaller packages by placing the lead plates closer together. Of course, this also makes the battery more susceptible to internal sparking.

The good news is that "suspended electrolyte" batteries are available. Although more expensive than traditional "wet-acid" batteries, these high-tech units use a gelatinous electrolyte or contain the acid on rolled-up matting inside the cells. Furthermore, "recombinant" batteries convert the hydrogen gas into water to keep the electrolyte at its proper level.

So just because you've never heard first-hand about an exploding car battery, don't assume that it only happens in third-world countries. Play it safe by learning proper jump-starting procedures. And never smoke near an open car hood.