The Science of Automotive Battery Technology

How does car battery technology work?

Lead and acid are two things that most people know well enough to avoid. Lead is a heavy metal that can cause a whole laundry list of health problems, and acid is, well, acid. The mere mention of the word conjures up images of bubbling green liquids and cackling-mad scientists bent on world domination.

But like chocolate and peanut butter, lead and acid wouldn't seem to go together, but they do. Without lead and acid, we wouldn’t have car batteries, and without car batteries, we wouldn’t have any of the modern accessories—or basic necessities, like headlights—that require an electrical system to function. So how, exactly, did these two deadly substances come together to form the rock-solid foundation of automotive electronic systems? The answer, to borrow a turn of phrase, is elementary.

The Science of Storing Electrical Energy

Electrical batteries are simply storage vessels that are capable of holding an electrical charge and then discharging it into a load. Some batteries are capable of producing an electrical current from their base components as soon as they are assembled. These batteries are called primary batteries, and they are typically disposed of once the charge has been depleted. Car batteries fit into a different category of electrical battery that can be charged, discharged, and recharged again and again. These secondary batteries utilize a reversible chemical reaction that differs from one type of rechargeable battery to another.

In terms that most people can readily understand, the AA or AAA batteries that you buy at the store, stick in your remote control, and then throw away when they die are primary batteries. They are assembled, typically from either zinc-carbon or zinc and manganese dioxide cells, and they are capable of providing current without being charged. When they die, you throw them away—or dispose of them properly, if you prefer.

Of course, you can purchase those same AA or AAA batteries in a “rechargeable” form that costs more. These rechargeable batteries typically use nickel-cadmium or nickel-metal hydride cells. Unlike traditional “alkaline” batteries, NiCd and NiMH batteries are not capable of providing current to a load upon assembly. Instead, an electrical current is applied to the cells, which causes a chemical reaction within the battery. You then stick the battery in your remote control, and when it dies, you place it in a charger and the application of a current reverses the chemical process that occurred during discharge.

Car batteries, which utilize lead and sulfuric acid instead of nickel oxyhydroxide and a hydrogen-absorbing alloy, are similar to NiMH batteries in function. When an electrical current is applied to the battery, a chemical reaction occurs, and an electrical charge is stored. When a load is connected to the battery, that reaction reverses, and a current is provided to the load.

Storing Energy With Lead and Acid

If using lead and acid to store an electrical charge sounds archaic, it is. The first lead-acid battery was invented in the 1850s, and the battery in your car uses the same basic principles. The designs and materials have evolved over the years, but the same basic idea is in play.

When a lead-acid battery is discharged, the electrolyte becomes a very dilute solution of sulfuric acid— meaning it is mostly plain-old H20 with some H2SO4 floating around in it. The lead plates, having absorbed the sulfuric acid, become primarily lead sulfate. When an electrical current is applied to the battery, this process reverses. The lead sulfate plates turn (mostly) back into the lead, and the diluted solution of sulfuric acid becomes more concentrated.

This isn’t a terribly efficient way of storing electrical energy, in terms of how heavy and large the cells are compared to the amount of energy they store, but lead-acid batteries are still in use today for two reasons. The first is a matter of economics; lead-acid batteries are much cheaper to manufacture than any other option. The other reason is that lead-acid batteries are capable of providing tremendous amounts of on-demand current at once, which makes them uniquely suited to use as starting batteries.

How Shallow Is Your Cycle?

Traditional car batteries are sometimes referred to as SLI batteries, where "SLI" stands for starting, lighting, and ignition. This abbreviation illustrates the main purposes of a car battery pretty well, as the main job of any car battery is to run the starter motor, the lights, and the ignition before the engine is running. After the engine is running, the alternator provides all the necessary electrical energy, and the battery is recharged.

This type of usage is a shallow type of duty cycle, in that it provides a short burst of a large amount of current, and that’s what car batteries are specifically designed to do. With that in mind, modern car batteries contain very thin plates of lead, which allows for a maximum amount of exposure to the electrolyte, and provides the most possible amperage for short periods. This design is necessary due to the huge current requirements of starter motors.

In contrast to starting batteries, deep cycle batteries are another type of lead-acid battery that is designed for a “deeper” cycle. The configuration of the plates is different, so they aren’t well-suited to providing large amounts of on-demand current. Instead, they are designed to provide less power for longer amounts of time. The cycle is “deeper” because it is longer, rather than due to the overall discharge being larger. Unlike starting batteries, which are automatically recharged after every use, deep cycle batteries can be slowly discharged—to a safe level—before being recharged again. Like starting batteries, deep cycle lead acid batteries shouldn’t be discharged below the recommended level to avoid permanent damage.

Different Package, Same Technology

Although the basic technology behind lead-acid batteries has remained more or less the same, advances in materials and techniques have resulted in a number of variations. Deep cycle batteries, of course, use a different plate configuration to allow for a deeper duty cycle. Other variations take things even further.

The biggest advance in lead-acid battery technology has probably been valve-regulated lead-acid (VRLA) batteries. They still use lead and sulfuric acid, but they don’t have “flooded,” wet cells. Instead, they use either gel cells or an absorbed glass mat (AGM) for the electrolyte. The chemical process is the same at a basic level, but these batteries aren’t subject to off-gassing like flooded cell batteries are, nor are they vulnerable to leakage if tipped.

Although VRLA batteries have a number of advantages, they are much more expensive to produce than traditional flooded cell batteries. So while technology continues to march ever forward, chances are you’ll still be driving around with cutting-edge 1860s technology under your hood for some time yet—unless you go electric. But that’s a whole different matter in terms of batteries.

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