In the automobile, the battery serves three main functions. First is the obvious. It supplies electrical power for the ignition system, starter, computer system, and every other electrical component while the engine is not running (or any other instance when the alternator is not charging). Second, it serves as a reserve power source, should the electrical demand in the vehicle exceed that of what the alternator can produce.
Finally, it serves as a damper or stabilizer for any electrical pulses. It absorbs high voltage spikes, and fills in between moments of low voltage. That action helps to smooth out the electrical "noise" in the system. Without that electrical dampening action, delicate electronic components could be at risk of damage from electrical spikes.
Electricity 101
Before we get in to talking about the battery, we need cover a few basics. First, we must define some of the terms that will be used later. These terms are "Electricity", "Amps", "Volts" and "Resistance.
The first term to identify is, ironically, the most difficult one; Electricity. That term actually has more than one definition. For the scope of this article, my definition will be that "electricity" is a supply of atoms with excessive electrons. Therefore, the flow of electricity is the exchange of electrons from one atom to the next. The terms "amperage" and "voltage" are used to measure that exchange. Amperage is a measurement of quantity, whereas voltage is a measurement of pressure. Some atoms are more willing to share their electrons than others. Materials that are made out of atoms that will freely pass along electrons are referred to as "conductors".
These are materials such as metal and water. Some materials are made up of atoms that don't share their electrons as easily, like rubber, air, and plastic. Those materials, for the scope of this article, are referred to as "insulators." Conductors have a lower resistance than insulators.
Amps, volts, and resistance are connected to each other in terms of effect. For illustration of this effect, picture a generic roadway. The speed limit will represent voltage. The number of vehicles on the road at the same time will represent amperage. The number of potholes on the road's surface will represent resistance.
If you were to stand at the edge of the road and count the cars as they passed by, as long as there was a steady stream of vehicles, you would count more over a minute's time if they all sped up; right? In most cases, if you increase the volts (speed limit), you will increase the amps (number of cars) that pass through the wire (roadway). If you add resistance (potholes), the volts (speed) and amps (quantity) will be reduced. However, there are some exceptions. Just because the cars are moving faster doesn't mean there will always be more. They can have gaps between them. The same is true for electricity. Just because you have 12 volts, doesn't mean there will automatically be a lot of amps available. Keep that in mind as much electrical testing depends on that knowledge. Since 12 volt car batteries all have the same "speed limit," they are rated by CCAs. The number of amps that a battery can deliver at 70°F/26°C is called the battery's CCA rating (Cold Cranking Amps).
How and Why It Works
I'm sure most of us have explained a battery to our customers as being simply a storage unit, much like a glass of water. That may be fine for our customers, however, as technicians, a more in-depth explanation may be required. That is what I hope to provide.
A car battery is actually more of a little electric generator than a "cup." It stores energy in a chemical form, then transforms that stored chemical energy into electrical energy on demand. A lead/acid battery has two different types of lead plates submersed in an electrolyte to make this possible. Lead Dioxide is the material used to make the positive (+) plates. The negative (-) plates are made from Sponge Lead. The "plate" is a lead alloy mesh grid (used as a framework) with Lead Oxide "pasted" onto the grid. The paste used for the negative plate has "expanders" added to it.
The two types of plates are later formed into sponge lead (-) and lead dioxide (+) during the initial charging process after complete assembly of the battery. The two types of plates are stacked together with "separators" between them. The separators are porous and allow ionic current flow between the plates, but don't allow the plates to directly touch each other.
The assembled plates and separators form an "element." Each element has two connectors on top of it, one for positive, and one for negative. The elements are lowered into the battery case where they will have there own divided cell space and submersed in the electrolyte. They don't rest directly on the floor of the case, rather they rest on a honey comb of plastic walls molded in the bottom of the case. The elements are "wired" to each other by their appropriate polarity connector using straps. The straps are joined with the appropriate terminal post of polarity. The electrolyte is made from a mixture of water and sulfuric acid. All of these elements are necessary for the battery to function. All of these materials are depleted with time and usage, even the water used in the electrolyte.
During the discharge half of the cycle, sulfate from the electrolyte bonds with the positive and negative plates to form a layer of lead sulfate on them. Also the oxygen in the positive plate combines with hydrogen in the sulfuric acid to form water. It is during this chemical reaction that electricity is produced out of the chemicals. As both plates become lead sulfate, the performance of the battery is reduced. It is also important to understand that the chemical reaction can only take space on the surface area of the plates where the electrolyte comes in direct contact with it. Since the acid is replaced by water during the discharge, the performance of the battery will decline as that occurs. However, you will notice that the battery bounces back after the load is removed and the stronger electrolyte can circulate and come in contact with plates again.
Recharging the battery causes the sulfate to leave the lead and reform with the electrolyte. One discharge and recharge together form one cycle. The plates and the electrolyte undergo some permanent changes during the cycle, not everything can be restored by simply recharging it. One thing that happens during the recharge is that hydrogen gas is being released at the positive plates, and oxygen is being released at the negative plates. These gasses were once elements used to make up the parts of the battery, and some amounts of them are vented out of it. Obviously, such a cycle can only occur a limited number of times before some of these elements are depleted.
Another thing that is important to note is that both, hydrogen and oxygen are highly explosive. Care must be taken when handling a battery not to create sparks around it. Never remove the vents for charging a modern battery. Modern batteries have special vent designs that reduce the chance of explosion.