A modern car battery is a device that converts chemical into electrical energy using a lead-acid design. It is made of six cells connected in series, each producing 2 volts of charge for a total of 12 volts. The cells are filled with sulfuric acid which is an electrolyte that conducts a charge and generates an electrical current.
Each cell contains positive plates made of lead dioxide (PbO2) and negative ones made of the element, Lead (Pb). Non-conductive sheets or separators are placed between the plate groups and all this is placed in a battery case constructed out of polypropylene.
The heavy-duty car batteries usually have thicker, more durable materials that prevent fluid loss and make the system maintenance-free. If the plate grids contain calcium, cadmium or strontium, it is called a lead-calcium battery.
Car batteries, also called SLI (starting, lighting, ignition) batteries, are rechargeable and are used to provide an automobile with electrical energy that powers the motor and the ignition system. As the battery runs down, the surface of the plates become coated with lead sulfate. This is reversed when the system is recharged, transforming the chemicals once more to their original components.
Most batteries today use a liquid electrolyte in flooded cells. The ones that have no free liquid electrolyte are the absorbed glass mat (AGM) type, now becoming popular with customers.
The starting (cranking) or shallow cycle type is designed to deliver large bursts of energy, usually to start an engine. The SLI batteries usually have a greater plate count in order to have a larger surface area that provides high electric current for short period of time. Once the engine is started, they are recharged by the engine driven charging system. See Jump start (vehicle).
The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for example in a trolling motor for a small boat, auxiliary power for a recreational vehicle, or traction power for a golf cart or other battery electric vehicle). They can also be used to store energy from a photo voltaic array or a small wind turbine. They usually have thicker plates in order to have a greater capacity and survive a higher number of charge/discharge cycles. The energy to weight ratio, or specific energy, is in the range of 30 Wh/kg (108 kJ/kg).
Batteries intended for SLI systems are intended to deliver a heavy current for a short time, and to have a relatively low degree of discharge on each use. They have many thin plates, thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance. Deep-cycle batteries have fewer, thicker plates and are intended to have a greater depth of discharge on each cycle, but will not provide as high a current on heavy loads. [3]
Car batteries using lead-antimony plates would require regular watering top-up to replace water lost due to electrolysis on each charging cycle. By changing the alloying element to calcium, more recent designs have lower water loss unless overcharged. Modern car batteries have reduced maintenance requirements, and may not provide caps for addition of water to the cells. Such batteries include extra electrolyte above the plates to allow for losses during the battery life. If the battery has easily detachable caps then a top-up with distilled water may be required from time to time. Prolonged overcharging or charging at excessively high voltage causes some of the water in the electrolyte to be broken up into hydrogen and oxygen gases, which escape from the cells. If the electrolyte liquid level drops too low, the plates are exposed to air, lose capacity, and are damaged. The sulfuric acid in the battery normally does not require replacement since it is not consumed even on overcharging.
Impurities in the water will reduce the life and performance of the battery. Manufacturers usually recommend use of dematerialized or distilled water since even potable tap water can contain high levels of minerals.
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