In a discharged lead-acid cell, the active material of both plates is lead sulphate. The electrolyte is a weak sulphuric acid solution.
The cell is connected to a DC source with electrical pressure higher than that of the cell, since it must act like an electron pump, forcing electrons from the positive plates, to the negative plates.
At the negative plates, sulphate is discharged. More sulphuric acid forms, and the plate changes into sponge lead. At the same time, lead peroxide is formed at the positive plates, which restores the cell’s electrical potential.
The charging process increases the amount of acid in the electrolyte, making the electrolyte stronger. When further charging no longer makes the electrolyte stronger, charging is complete.
Now let’s examine discharging. Connecting a lead acid battery to a load causes chemical changes.
At the positive plate, sulphate from the electrolyte joins with lead to form lead sulphate. Oxygen from the plate joins hydrogen to form water. Lead sulphate also forms at the negative plate, as sponge lead forms with sulphate from the electrolyte.
Overall, the percentage of acid in the electrolyte falls, and the percentage of water rises, which reduces the strength of the electrolyte. As the cell discharges, the plates develop the same composition, which reduces the potential of the cell.
Car battery
Most car batteries are lead-acid batteries. The lead acid battery is made up of plates, lead, and lead oxide (various other elements are used to change density, hardness, porosity, etc.) with a 35% sulfuric acid and 65% water solution. This solution is called electrolyte which causes a chemical reaction that produce electrons. When you test a battery with a hydrometer you are measuring the amount of sulfuric acid in the electrolyte. If your reading is low, that means the chemistry that makes electrons is lacking. So where did the sulfur go? It is stuck to the battery plates and when you recharge the battery the sulfur returns to the electrolyte.
Basically there are two types of batteries; starting (cranking), and deep cycle( marine/golf cart). The starting battery is designed to deliver quick bursts of energy (such as starting engines) and have a greater plate count. The plates will also be thinner and have somewhat different material composition. The deep cycle battery has less instant energy but greater long-term energy delivery. Deep cycle batteries have thicker plates and can survive a number of discharge cycles. Starting batteries should not be used for deep cycle applications. The so-called Dual Purpose Battery is only a compromise between the 2 types of batteries.
Galvanic cell
The Galvanic cell consists of two metals connected by an electrolyte which forms a salt bridge between the metals. It is also known as a voltaic cell and an electrochemical cell. Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this "animal electricity". The Voltaic pile, invented by Alessandro Volta in the 1800s, is a similar concept. These discoveries paved the way for all electrical batteries.
Description
The Galvanic cell's metals dissolve in the electrolyte at two different rates, leaving some electrons in the rest of the metal, which charges it negative with respect to the electrolyte. Each metal undergoes a different half-reaction, giving different dissolving rates, which causes an unequal number of electrons in the two metals. This results in a different electrode potential between the electrolyte and each metal. If an electrical connection, such as a wire or direct contact, is formed between the two, an electric current appears in the metal. At the same time, ions of the more active metal, which forms the anode, are transferred through the electrolyte to the less active metal, the cathode, and deposited there as a plating. In this way the anode is consumed or corroded. When the anode material corrodes away, the potential drops and the current halts. The metal may be regarded as the fuel which powers the device. A similar process is used in electroplating. The electric current in the electrolyte is equal to the current in the external circuit, but opposite in direction, so a complete circuit is formed with a path through the electrolyte.
There is a flow of electrons from the oxidized ion at the anode to the reduced atom (formerly an ion) at the cathode. It is this flow, due to this redox reaction which constitutes the current.
Electric potential of a Galvanic cell
The electric potential of a cell can be easily determined by use of a standard reduction potential table. An oxidation potential table could also be used, but the reduction table is more common. The first step is to identify the two metals reacting in the cell. Then one looks up the Eo (standard electrode potential, in volts) for each of the two half reactions. The electric potential for the cell is equal to the more positive Eo value plus the opposite of the more negative Eo value. The reason you add the opposite of the more negative Eo value is that that reaction is not going to be a reduction reaction, but instead an oxidation reaction.
Source: CDX Global & Wikipedia - en.wikipedia.org
