It helps to know a little bit about 12-volt lead-acid batteries. They have six two-volt chambers, called cells, that contain a grid of lead plates submerged in sulfuric acid. Electricity is generated when the acid reacts with the lead plates and water.
One byproduct of the process is gaseous hydrogen, an element so highly flammable that it is used to power rocket engines. Charging a battery also generates hydrogen. And because heat drives up hydrogen output, you can expect more trouble generally in hot weather.
The danger is that hydrogen will explode if a spark occurs nearby. One source of sparks can be the battery itself.
As a battery ages, it loses water, leaving the top of the lead plates exposed to the air inside the battery case. Over time, this can lead to warpage of the plates.
When the driver starts the engine, the heavy demand for power can cause these already warped plates to flex, touch and thus spark, says Steve Mazor, head of engineering and safety for the Automobile Club of Southern California.
The most common cause of battery explosions upon start-up is dirty battery posts and cables, says Sam Memmolo, a master mechanic in Douglasville, Ga., and a nationally recognized automotive repair expert. The dirt prevents a good connection and allows electrical arcing. So it is a good idea to inspect and clean battery posts regularly.
Improper jump-starting is another leading cause of explosions. The mistake many motorists make is to connect the jumper cables to another car's good battery and then to the dead battery, a practice that causes sparking. Always connect jumper cables to the dead battery first, then to the good battery.
Another important safety precaution is to attach the negative jumper cable for the dead battery to an unpainted metal portion of the car frame, rather than to the negative battery post. That allows any sparking to occur far from the battery itself.
The potential for battery explosions may be greater than it was 10 years ago. Some newer batteries are sealed, preventing motorists from adding water to keep the electrolyte (the mixture of sulfuric acid and water) above the lead plates. But many manufacturers have gone back to batteries with tops that can be removed.
Another problem is that new cars have much larger demands for electrical current, so batteries must produce more juice but with the same physical volume. One method used by battery makers to increase current is to space the lead plates closer together, but that only makes it easier for them to short, Mazor says.
In any case, motorists should always exercise caution when the hood is up, avoiding smoking or anything else that can ignite hydrogen.