Automotive lead acid batteries used to start car engines, are one of the oldest design of rechargeable battery in existence. The lead acid battery was primarily used for the storage of DC power and used in early commercial vehicles due to the weight and size. These batteries have literally dominated the marketplace for many years ever since their invention back in the early 1800’s and have continued to be an important automotive component to the point were there is one in every vehicle on the roads.
Today, automotive batteries used to start gasoline and diesel engines, are smaller, cheaper and more cost effective than their earlier cousins, supplying greater starting and surge currents than ever before at a fraction of their original price.
Not only does the lead acid battery provide the correct amount of electricity needed to start engines, but they also supply electricity for the ignition system, lights, indication, stereos and other such electronic features found on today’s modern vehicles.
The energy produced by a lead acid battery occurs via an internal chemical process which involves lead (hence their name), lead oxide and a liquid acid solution called the electrolyte. Solid lead and lead oxide plates are submerged within an electrolytic solution that consists of a very small percentage of sulphuric acid (H2SO4) mixed with distilled water. When electrical power is drawn from the battery, a chemical reaction takes place between the plates and liquid electrolyte releasing electrons. These free electrons in the form of an electrical current flow through electrical conductors mounted on the battery and out via lead terminals providing the electricity required to start the car.
As the lead acid battery becomes discharged, the sulphuric acid forms deposits onto the lead plates, but when the lead acid battery is recharged again, the sulphuric acid breaks down and returns back into their separate lead and lead oxide components. Heat is generated by this constant charging and discharging of the battery evaporating the water inside.
This evaporation meant that older lead acid batteries needed to be “topped-up” with more distilled water on a monthly basis but the modern maintenance free lead acid battery is fully sealed against leakage in which the electrolyte in the form of a gel is contained in separate compartments are now used. The result is that these fully sealed batteries can be rotated upside down or positioned sideways in the event of an accident without any risk of an acid leakage. Also these modern batteries have safety valves fitted which allow the venting of fumes during the charging, the discharging and changes in atmospheric pressure when driving at altitude.
The lead acid battery is the only batteries suitable to be used in alternative energy systems but the continuous cycle of discharging a battery into a load (night time hours) and then recharging the battery (sunlight hours) many times over requires a different type of battery as not all lead acid batteries are the same. The most important requirement of a battery is whether it is a Deep Cycle Battery or a Shallow Cycle Battery.
Consider automotive starting batteries. These batteries are cheap to buy but are designed to provide high amperes of current for very short periods of time (less than 10 seconds) to operate the starter motor and turn over the engine. After the car has started, the battery is then trickle charged by the cars alternator. Even on cold frosty mornings the cars battery is only discharged to less than 10% of its rated capacity at startup so automotive batteries are designed for this very shallow cycle service, (100% to 90% state of charge).
As a car battery is designed to deliver high currents for very short periods of time it is therefore made of many thin lead plates giving a large surface area for the chemical reaction to occur. These thin lead plates do not have the necessary mechanical strength for repeated cycling over a period of many years and wear out very quickly after only 200 to 400 cycles. Therefore shallow cycle car batteries which although they work, are not designed for a long term solar power or wind power system which requires a much deeper cycling service.
Deep Cycle Batteries on the other hand are designed to be repeatedly charged and discharged by as much as 80% of their full capacity (100% to 20% state of charge) without sustaining any serious damage to the cells before recharging, making the deep cycle battery an ideal choice for solar photovoltaic and wind power systems, as well as marine applications, golf buggies, fork lift trucks and other such electric vehicles. Although the deep cycle battery uses the same chemical reactions to store and generate energy as their automotive battery cousins, deep cycle batteries are made very differently.
The physical size of a deep cycle battery is much larger than a regular car battery due to the construction and size of their lead plates (electrodes). These plates are made of solid lead usually doped with Antimony (Sb) and are many times thicker than the thinner plates of a car battery. This means that deep cycle batteries can be repeatedly discharged almost all the way down to a very low charge and it is not uncommon for deep cycle batteries to be emptied (discharged) to as much as 20% of their total capacity before energy ceases flowing from the battery.
So to summarise, the lead-acid battery is the perfect choice for automotive cars, vans and motorcyles as it is reliable, inexpensive, and delivers high starting currents but for alternative energy systems, the deep cycle battery is the way to go as it can be charged and discharged many times over to a much lower value.