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Battery considerations

Battery considerations

As a result of the increasing demand for improved capacity and smaller/lighter packages, batteries are undergoing a radical development. However, when charging or discharging, a chemical reaction occurs. If this is not effectively controlled the reaction can 'run away' and cause excessive heat, fire or even explosion. As the size and weight of batteries decrease, so tolerances for these components and the production control become even more critical. So let's examine the different common battery technologies, and look at 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.

Due to the current demand for higher energy capacities and a more user and environmentally friendly package, 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 offers significant advantages over older technologies. The two types of VRLA batteries are AGM and gelled electrolyte.

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.

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.

Lithium-ion: In the 1970s, non-rechargeable lithium-ion batteries first became commercially available, followed 20 years later by rechargeable batteries. Nowadays, lithium-ion batteries do not actually contain lithium metal due to its inherent instability that can lead to a rapid increase in temperature and violent venting and flaming. The electrodes are made instead from alternative materials such as lithium cobaltate (for the cathode) and graphite (for the anode).

Lithium-ion batteries have a higher energy density than other battery technologies such as nickel-cadmium (Ni-Cd) and have a small package size and weight. Unlike Ni-Cd, lithium-ion batteries do not suffer from 'memory effect' and they also have a low self-discharge rate which means they can be left unused for longer.

While they offer many advantages when compared to other battery technologies, lithium-ion requires more extensive testing than other forms of battery technology to demonstrate stability in the final product as it is sensitive to temperature and susceptible to capacity deterioration over time.

To ensure that potentially dangerous batteries remain safe throughout their normal life, they are routinely tested for functional performance, susceptibility to various environmental conditions (including transportation) and safety. For lithium-ion batteries that rely on electrical protection circuits to remain safe in normal use typical tests include electrostatic discharge (ESD) to simulate static from touching or being carried around by the user, as well as surge testing.

Looking to the future, sodium-ion batteries are currently under development as an alternative to lithium-ion batteries. These offer a potential advantage as sodium salts are more widely available than lithium salts, which should reduce production costs.

This new chemistry also has better performance during deep discharge. Lithium-ion batteries have cathode electrodes made of copper, which under deep discharge conditions react, causing the copper electrodes to dissolve. However, with sodium-ion batteries both the anode and the cathode are made of aluminium, so this condition does not occur.

Lithium-ion batteries are also more sensitive to heat, whereas sodium-ion batteries are more robust. Higher recommended charging ambient temperatures are also possible with this new chemistry, and it is predicted to have potential for static urban energy storage. For example, it has already been used in e-bikes to demonstrate its potential.

Other 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 they 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.

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