How lead acid battery works

Lead batteries when they are shipped, they are dry. No electrolyte in them. Instead electrolyte is supplied in plastic containers separate with batteries. When battery is needed to be installed in the place where they are needed, electrolyte is filled in it. Most batteries are dry charged and they are ready to use when electrolyte is filled in. But a slow starting charge is always recommended. The electrolyte starts chemical reaction in the battery. The reaction is done between the positive plate lead peroxide and the negative spongy lead. The electrolyte works as ion carriers. It reacts with plates to produce electricity and also it carries ions produced.

The fully charged automotive battery has specific gravity 1.266 at 81 degree F or 28 degree Celsius. The plane or distilled water specific gravity is considered to be 1. So the battery electrolyte is 1.266 times heavier than water. When battery is used, the specific gravity for it reduces as the acid in the water reacts with plate material. This way specific gravity is a measure of battery charging state. Following detail shows how battery charge relates to the battery specific gravity.

Fully charged Battery: 1.266

75% charged Battery: 1.226

50% charged Battery: 1.191

25% charged Battery: 1.156

Discharged Battery: 1.121 and lower

Specific gravity figures could vary little with design and manufacturing parameters of battery. But one thing is common regardless of battery manufacturer and design, the battery electrolyte specific gravity reduces with battery discharging. Temperature also effects the specific gravity of electrolyte. When the temperature increased beyond the standard temp rapture 81 degree F, the specific gravity of electrolyte decreases and when the temperature reduces the stated limit, the specific gravity of the electrolyte increases. The electrolyte specific gravity is temperature sensitive and the reading drastically changed with the change in temperature. A general rule of thumb is to minus 0.004 to specific gravity reading at each 10 degree F increase beyond 81 degree F or 5.5 degree C above 28 degree Celsius and add below this temperature limit.

During the operation of a lead acid battery the battery is never discharged completely instead partially discharged and then charged again to compensate for the discharge. This cycle is repeated for the entire life of the battery. During this charging and discharging cycles the battery plates active materials reacts with the acid and slowly wears away from the cell plates. This sulfates the battery plates. Also the active materials are removed from the plate and accumulated in the bottom of the battery. These materials are conductive to electricity. Sometimes these accumulated materials shorting the positive and negative plates in battery cells. And also when the materials removed from the cell plates they become basic lead grid plates which are no longer able to produce electricity. In such a condition the battery must be replaced with a new one. The older battery should be sent to manufacturer for recycling. Some people try to repair these worn out batteries by putting different items like Epsom salt or Coca Cola. Such techniques don’t work and this is just waste of time and resource.