• How Does a Lead Acid Car Battery Work

    Lead oxide plates interacting with sulfuric acid in a car battery is what creates the electrical current used by the car’s starter and electrical system. That’s the basic and simple description. In order to gain a better understanding of how a car battery works, you need a little more than just the bare basics. The following paragraphs will describe some construction features of a lead acid twelve volt car battery and then give a more complete description of the chemical processes your battery undergoes during the charge and discharge cycles.


    Your car’s battery has a specialized hard plastic shell which contains the sulfuric acid electrolyte and the lead oxide plates. The top cover is made from a slightly thicker and heavier piece of plastic. From this top plate is a heavy lead oxide suspension matrix which supports the lead oxide plates and the separator plates. If the separator plates weaken or fail, the battery can short out internally. The material most often used for separator construction these days is rubber. In a lead acid car battery, the lead oxide plates alternate positive and negative, or cathode and anode. The greater the surface area of the plates, the more current the lead acid battery will be capable of supplying. Typical methods of increasing the surface area of the lead oxide plates and thus increasing output is by intentionally pitting the plates and drilling holes in them. Other chemicals and metals are added to the lead oxide of the plates in order to stabilize them and to slow down their rate of degradation under extreme usage. Many manufacturers use a red lead and sulfuric acid dilute paste that is pressed into the holes in the plates. The use of this mixture greatly enhances electron transfer as well as multiplies the usable surface area of the plates.

    The Chemical Process

    In a lead acid car battery, electrical current is created during the discharge cycle as the lead oxide plates absorb sulfate from the diluted sulfuric acid solution. The amount of current produced depends on how much sulfate the electrolyte is able to give up during discharge. When the battery is charged, the process is reversed. Current is pushed through the positive terminal into the plate pack, causing it to shed the sulfate it absorbed during discharge. During the charge and discharge cycle, the lead oxide plates increase in size, as they absorb sulfate from the sulfuric acid electrolyte. Conversely, during the charging cycle, the lead oxide plates shed the sulfate, which precipitates back into the sulfuric acid electrolyte.

    Keep it Up

    One of the best ways to ensure your battery will continue to produce an acceptable level of current is to regularly check the specific gravity or density of the electrolyte with a hydrometer. This testing device is very simple to use. Open the cell vents carefully, squeeze the bulb on the end, and insert the tube completely into the cell. Release the bulb drawing electrolyte into the tool. There is a small floater of a known density. There is a graduated scale that is calibrated into two or three sections, green, orange and red. Green signifies that the electrolyte has an acceptable amount of sulfuric acid, while red signifies that you should draw off some of the current electrolyte and refill the cell with fresh electrolyte. Orange means the cell is almost to where the electrolyte needs to be replaced.

    Lead acid car batteries are very complex devices that operate on a very simple process of chemical transfer. Hopefully the preceding paragraphs have given you a better idea of how these batteries are constructed and how they work.