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The evolution of battery technology

As a result of the increasing demand for improved capacity and smaller/lighter packages, batteries are undergoing a radical development. So, what are the different chemical forms used in some of these batteries and what are their advantages and disadvantages?

Traditional wet lead acid batteries 

Lead Acid batteries have been in use for over 100 years and are an established battery technology for emergency supplies such as Uninterruptible Power Supplies (UPS).
However, they can have their drawbacks in that they have limited energy density (typically 300Wh/kg) and they can also be bulky and heavy. In addition, they can be considered environmentally unfriendly if improperly disposed of at the end of their life, and they also have a relatively short cycle life (typically 250-300 cycles).

Traditional lead acid batteries are a 'wet' design and as such, they are prone to gassing (the release of bubbles of hydrogen and oxygen from the electrolyte during excessive charging). This limits their usefulness as they can only be used vertically, and ventilation has to be provided and precautions taken when handling and transporting the batteries. 
However, the lead acid battery has recently undergone some significant changes. New variants of this traditional design include the Valve Regulated Lead Acid Battery (VRLA), which offer significant advantages over older technologies. The two types of VRLA batteries are AGM and Gelled Electrolyte.

1. Absorbed Glass Mat (AGM)  
The electrolyte (acid) is absorbed into the glass mat; this means a reduced risk of spillage, making shipping and transport easier and safer. These batteries are also recombinant - the oxygen and hydrogen recombine inside the battery, and this recombining is typically over 99% efficient, so almost no water is lost through electrolysis. 
AGM batteries also have a very low self-discharge rate, making them ideal for more long-term usage where reliability after long periods of dormancy is needed, such as for use with UPS. 
2. Gelled Electrolyte
The electrolyte is a form of gel (silica additive), so there is no risk of spillage. They are also often used in UPS applications due to their slower charge rate compared to other lead acid batteries, which is necessary to prevent excess gas.

Early re-chargeable batteries contained lithium based electrodes, but in the 1980s it was discovered that re-charging resulted in changes to the electrodes that reduced thermal stability. Thermal runaway led to a rapid increase in temperature, with the cell reaching the melting point of lithium, resulting in violent venting and flaming. So, today's Li-ion batteries do not actually contain lithium metal and the electrodes are made from alternative materials such as lithium cobaltate (for the cathode) and graphite (for the anode). 
Education of the consumer also plays a significant part by promoting the safe use of batteries and discouraging the purchase of non-genuine or counterfeit batteries and chargers, where they are tempted by the lower prices without realising the potential safety implications. 
It is true that the increase in capacity of today's batteries, and end-user misuse, is part of the reason for the Li-ion battery's bad publicity, but the manufacturing process has also been questioned. Many battery safety incidents have been linked to inadequate procedures relating to the avoidance of contaminates in the production process. In such cases, it is suspected that metal particles penetrated the battery separator and caused a short circuit between the cathode and the anode resulting in thermal runaway. 

As a result of the number of incidents involving Li-ion batteries, there was an international move to improve the testing and quality control of the cell and battery packs. This is now addressed by the application of standards including UL 1642 "Lithium Batteries", IEEE 1725 "IEEE Standard for Rechargeable Batteries for Cellular Telephones" and "UN Recommendations on the Transport of Dangerous Goods Manual of Tests and Criteria" (ST/SG/AC.10/11).
Another key catalyst in the improvement of Li-ion battery safety was the introduction of the CTIA (Cellular Telecommunications and Internet Association) certification scheme. This is an industry led initiative in the USA for mobile handsets and is based on the IEEE 1725 standard.
Manufacturers of Li-ion batteries also include internal protection devices in addition to the protection circuits within the overall battery pack to guard against excessive heat and pressure. Typical protection devices are:

1. Vent Plate / Vent Tear Away Tab
Excessive build up of pressure within battery cells is caused primarily from excessive abnormal heat generation or over-charging. The vent allows the safe release of gas.
2. Positive Temperature Coefficient (PTC)
PTCs act as both a current fuse and a thermal fuse so that, when excessive current is drawn, the resistance of the PTC increases resulting in increased heat generation. The resistance of the PTC is selected so that it trips at the pre-determined current.
3. Separator
When the separator reaches its defined temperature (typically 130ºC), the pores are blocked by the melting of the material, preventing electrical current to flow between the electrodes. The separator is also sometimes known as a 'shut-down separator'.
4. Thermal Fuse
Some prismatic batteries have an additional feature, a thermal fuse which limits the current under fault conditions.

A protection circuit is also usually fitted within the battery pack consisting of a custom designed integrated circuit that monitors the cell and prevents over-charge, over-discharge, and over-current. This in combination with two Field Effect Transistor (FET) devices control the charge and discharging. Also present is a temperature sensing device (thermistor) designed to invoke protective action via the control IC in the event of an over-temperature scenario. While Li-ion batteries still have some disadvantages, their progressive development over the last few years has meant that these are far outweighed by the advantages.
The advantages of Li-ion batteries are:
1. High energy density
Compared to other battery technologies such as nickel-cadmium (Ni-Cd) the energy density of the Li-ion battery is greater with the opportunity to increase capacity, for example by adding more nickel to the cathode.
2. Small package size and weight
The Li-ion battery is ideal for portable consumer products. Designers have the option of using the prismatic package, which is typically thinner than 19mm, or the Li-ion polymer pouch, which is typically thinner than 5mm. In addition to the size advantage, there is also a reduction in weight due to the chemistry (e.g. solid /gel electrolytes rather than liquid electrolytes) and the packaging used (e.g. foil).
3. Memory effect
Unlike nickel-cadmium batteries (Ni-Cd) Li-ion (Li-Ion) batteries do not suffer from "memory effect". Memory effect occurs when, over time, a battery has been consistently partly used and then fully recharged, resulting in the appearance of rapid discharge. In modern batteries, this is more likely to be caused by voltage depression as a result of repeated overcharging leading to clogged plates, which increases internal resistance thus lowering the voltage of the battery.
5. Low discharge rate
Compared with other rechargeable batteries Li-ion have a low self-discharge rate which means they can be left unused for longer.
The disadvantages of Li-ion batteries are:
1. Protection
Li-ion batteries are sensitive to temperature and the chemistry is complex, therefore circuitry is required to protect the battery against over-charge, over-discharge, and over-temperature.
2. Premature ageing 
Li-ion batteries are susceptible to capacity deterioration over time. However, storage of the battery in a cool environment can reduce the effects. Once the battery is shipped by the manufacturer, it is important that it is used it as soon as practical in order to provide the end-user with the longest possible battery life.
3. Chemistry
Due to the nature of lithium, severe temperature or mechanical impact can result in venting and possible thermal runaway. This requires more extensive testing than other forms of battery technology to demonstrate stability in the final battery product and safeguard against potential foreseeable misuse
4. Production costs
Compared to other types of rechargeable battery production costs can be high.

Where is the future?
While Li-ion batteries still have some disadvantages, their progressive development over the last few years has meant that these are far outweighed by the advantages. An improvement in manufacturing processes through the introduction of more robust standards, as well as increasing consumer understanding of how to respect these batteries, means that the safety of Li-ion has dramatically improved. 

However, the most exciting developments being made in power technology are in fuel cells, which we anticipate will revolutionise power sources for both static and portable power applications. Fuel cells are highly efficient, have modular construction and produce low emissions. The main difference with a conventional battery is that methane-based gas is passed through the cell. As heat and electricity are produced, fuel cells are ideal for applications such as home heating and power, as well as industrial sites. Some manufacturers are even producing fuel cells small enough to be used in hand-held devices such as mobile phones and MP3 players. Despite their history, batteries are moving to a greener, safer and more sustainable future. 

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