The Dangers of an Exploding Car Battery

How to safely care for your battery

Automotive electrical systems aren’t tremendously complicated, when you look at the big picture, and a lot of the technologies we use today — from alternators to lead-acid batteries — have been around for a long time, but there are still a lot of folks out there who look askance at a relatively simple task like hooking up jumper cables, possibly because they’ve heard that doing it wrong can cause a lot of damage, or even cause the battery to blow up. And while you’ll find that a lot of weird myths and rumors about automotive technology are just that — unsubstantiated myths and rumors — the dangers associated with hooking up jumper cables, or a battery charger, incorrectly can cause a lot of damage, or even result in an exploding battery. The good news is that if you take the time to understand why a car battery can explode, and take a few basic precautions, this isn’t a problem you’ll ever have to worry about.

Safely Connecting Jumper Cables or a Battery Charger

There are a few general rules of thumb that can help you safely connect jumper cables, but there are also a number of special cases that supersede those rules. So before you use your car to provide a jumpstart, accept a jump from someone else, or hook up a charger to your battery, the first thing you need to do is check your owner’s manual to make sure your car doesn’t have designation connection points other than your battery. If your car has a battery that’s buried somewhere weird, like a wheel well or the trunk, then there’s a good chance that you’re supposed to use a junction block or some other kind of remote connection.

Jumper wires sparking when pressed together.
 J. Ronald Lee / Moment / Getty Images

Regardless of the vehicles in question, the basic idea behind safely connecting jumper cables is to connect the electrical system of a donor vehicle, which has a good battery, to the electrical system of a vehicle with a dead battery. Positive should be connected to positive, and negative should be connected to negative, as connecting backward can damage both vehicles and create potentially hazardous sparks, but more on that later.

The Best Procedure for Safely Hooking up Jumper Cables

  1. Ensure that the keys of both vehicles are in the Off position.

  2. Connect one jumper cable to the positive (+) terminal of the donor battery.

  3. Connect the same cable to the positive (+) terminal of the dead battery.

  4. Connect the other jumper cable to the negative (-) terminal of the donor battery.

  5. Connect the other end of that cable to bare metal on the engine or frame of the vehicle with the dead battery.

Connecting a battery charger is done in much the same way, except instead of a donor battery, you’re using a charger. The positive charger cable should be connected to battery positive (+), after which the negative charger cable should be connected to bare metal on the engine or frame of the vehicle.

There are some exceptions where positive is ground, but in most automotive electrical systems, negative is ground. That’s why you can connect a charger, or a jumper cable, to bare metal on the frame or engine of a vehicle with a dead battery and have current flow into the battery. Of course, it’s technically possible to connect directly to battery negative, and it may even be easier in some cases. So if it’s possible, and it’s essentially the same thing as connecting to some other ground, why go through the trouble? Because you don’t want your battery to explode.

The Science of Exploding Car Batteries

Car batteries are referred to as “lead acid” because they utilize plates of lead submerged in sulfuric acid to store and release electrical energy. This technology has actually been around since the 18th century, and it isn’t terribly efficient from either an energy-to-weight or energy-to-volume standpoint. However, they do have an excellent power-to-weight ratio, which essentially just means that they are good at providing the high levels of on-demand current required by automotive starters.

The downside of lead-acid batteries, other than the fact that they aren’t a terribly efficient way to store energy, is that they’re made up of fairly hazardous materials, and those hazardous materials can interact in dangerous ways. The presence of lead is the primary reason that car batteries have to be carefully and properly disposed of, and the presence of sulfuric acid is why you have to take care when handling them unless you want holes eaten in your clothes or chemical burns on your skin.

Of course, the danger that we’re particularly concerned with here is a sudden and catastrophic explosion, and the source of that particular hazard flows from the interaction between the lead and sulfuric acid in a battery. Small amounts of hydrogen gas are produced during both the process of discharge and during charging, and hydrogen is highly flammable. So when a battery has discharged to the point where it can no longer power a starter motor, there’s always a chance that some amount of hydrogen gas is still lingering inside the battery, or leaking out of the battery, just waiting for an ignition source. The same is true of a battery that has just been charged, as charging — and particularly overcharging — with a high voltage leads to the formation of both oxygen and hydrogen.

Preventing Car Battery Explosions

There are two primary ignition sources that you have to worry about, and they can both be avoided with careful charging, jumping, and maintenance practices. The first ignition source is a spark created when connecting or disconnecting a jumper or charging cable. This is why it’s so important to connect to bare metal on the engine or frame instead of the battery. If you hook up the negative jumper cable to the battery itself, any lingering hydrogen may be ignited by the ensuing spark. This is also why it’s a good idea to wait to turn on or plugin, your charger until after it’s connected.

The other type of car battery explosion still involves hydrogen gas, but the ignition source is inside the battery. The issue is that if a battery isn’t properly maintained, and the electrolyte level is allowed to drop, the lead plates will be exposed to oxygen and may warp. This can lead to the plates flexing and touching during the extreme current drain initiated whenever you crank the starter motor, which can result in a spark inside the battery. That, in turn, can ignite any hydrogen present in the cell, causing the battery to explode.

What About Sealed Car Batteries?

There are two main types of sealed car batteries: traditional lead-acid batteries that just aren’t serviceable, and VRLA (valve-regulated lead-acid) batteries that actually don’t need to be serviced. In the case of VRLA batteries, the electrolyte is contained in a saturated glass mat or gelled, so evaporation isn’t really an issue, and there really is no need to ever add more electrolyte, and it is little or no danger of the plates ever becoming exposed to the air. Sealed batteries that use a liquid electrolyte, however, can cause issues later in life.

If you have a VRLA battery, be it absorbed glass mat or gel cell, then the chances of the battery ever exploding are exceedingly low. However, it’s still a good idea to follow jumpstart and charging best practices just so that you don’t get out of the habit. However, maintenance of these batteries is actually impossible, so you don’t have to worry about checking the charge or electrolyte level regularly.

Special care should be taken with non-VRLA sealed and “maintenance-free” batteries, since at least some level of evaporation will take place over time, and the situation will only get worse if the battery is allowed to fully discharge repeatedly, or overcharged with a high voltage. So while it’s a good idea to be careful around any battery when jump starting or charging it, it’s an even better idea to be extra careful when dealing with old, discharged, or recently charged non-VRLA sealed batteries.

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