What happens to a car battery internally, that causes it to “go bad?”

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What happens to a car battery internally, that causes it to “go bad?”

In: Chemistry

Car batteries are two thin sheets of lead alloy, separated by a solution. As the battery charges, lead is plated from one plate to the other. As the battery discharges, the process is reversed.

In an ideal world, the lead ions would be perfectly uniformly distributed in the solution, and the plates would remain completely flat. So, that’s not how it really works. One plate gets a little bump from random concentration imbalances. That makes the gap between the plates a little smaller. The smaller gap makes that point a more active part of the battery. Over time, a little stalagmite starts to grow toward the other plate. One day, it touches, and the battery has an internal short. Lots of heat in generated, and the plate might be damaged, or even crack and come loose. Not to worry, there are a couple dozen plates in each cell, and the other ones are working fine, for now.

Over time, more plates are damaged. that’s why it’s bad to run a battery dead, this is the stage where the maximum plating effect occurs, shortening the overall life.

When the battery is initially built, it consists of a sheet of corrugated lead metal. The lead is treated with sulfuric acid in such a way that it reacts to form a thick layer of lead(+2) sulfate or PbSO4. Two sheets are then sandwiched between an absorbent glass mat with a gel of fairly dilute sulfuric acid.

This forms the battery in it’s discharged state. Charging the battery involves applying a positive voltage to one terminal and a negative to the other. At the positive plate, the layer of lead sulfate reacts with water to form lead oxide, additional sulfuric acid (H2SO4), and hydronium ions (H3O+) while at the negative plate the sulfate is converted into additional metallic lead, which “plates out” on the positive electrode, as well as bisulfate ions(HSO3-). This reacts with the hydronium ions producing more sulfuric acid. This consumes water in the glass mat while producing more acid.

In the charged state, one electrode consists mostly of lead oxide with a modest amount of lead and the other of thick metallic lead, while the electrolyte consists of concentrated acid. The acid is more reactive towards lead oxide, producting a voltage difference between the two. Discharging consumes the lead oxide, the lead metal, and the acid in the electrolyte. Generally the battery is designed such that the lead oxide runs out first.

The battery is fully discharged when either the lead oxide or the acid runs out.

However, discharging deeply several times can cause bits of lead oxide or lead sulfate to flake off the electrodes, for several reasons. If these materials are not in direct electrical contact with the electrodes, they cannot contribute to the reaction, leading to a loss of capacity. As in most batteries, use leads to damage amd or degradation of the electrodes materials.

When fully charged, some lead sulfate becomes dissolved in the electrolyte, this can be exacerbated by a lack of sufficient water in the cell. When the cell is discharged, this precipitates out as lead sulfate crystals that form in the bottom of the cell. Over time this leads to erosion of positive plate, while the lead sulfate crystals in the bottom grow in size. This is known as sulfation. Storing the battery in a discharged state for long periods exacerbates this and consumes the acid. Eventually not much acid is available as the sulfate crystals formed are not in contact with either electrode. This can be remedied somewhat by adding more acid and water.

Certain conditions can also cause large, needle-like lead crystals to grow during charging, called dendrites. These can bridge between the two plates, causing a short circuit. This makes further charging and discharging impossible. You have a dead cell that needs to be recycled.