Rechargeable batteries are an essential component
'With certain types of rechargeable batteries, letting them run completely flat and then recharging them back to full capacity each time is the best way to prolong their life,' says Professor Maria Skyllas-Kazacos from the School of Chemical Engineering at the University of New South Wales. 'That's because some batteries, mainly nickel-cadmium and nickel-metal-hydride suffer from a drop in voltage that is often referred to as the 'memory effect'. This happens when you cycle them through only short amounts of discharge before recharging.' 'It then becomes extremely hard to recharge these batteries higher than a certain point meaning they won't last as long between recharges, and need to be recharged more often. Most of the memory effect problems experienced by nickel-cadmium batteries, were caused by overcharging and poorly designed chargers,' says Skyllas-Kazacos. Many consumer electronics such as laptops and cameras, however, now contain lithium-ion batteries, which don't suffer from the 'memory effect'. Skyllas-Kazacos says it is often recommended that these type of batteries receive regular 'top-off' charges during the day when possible, and fully discharging and recharging the battery every 30 charges or so. This reduces the likelihood that the cells get out of balance, and helps maintain the accuracy of the 'fuel gauge' on the device. How do they work? Nickel-cadmium (Ni-Cd) batteries, which are the original type of rechargeable battery used in consumer electronics, use nickel hydroxide and cadmium electrodes with potassium hydroxide as the electrolyte. They have been largely superseded by nickel-metal-hydride (Ni-MH) batteries. Ni-MH batteries, which are used in consumer electronics and electric cars, offer twice the energy density of nickel-cadmium batteries and are less environmentally damaging as they don't contain heavy metals such as cadmium. They also differ by using a metal alloy negative electrode capable of storing a large amount of electrons. The metal-hydride is produced as the charging product. Both types of battery have a long cycle life and a long shelf life when looked after properly, says Skyllas-Kazacos. Depending on their application, these types of rechargeable batteries should last between 500 and 800 charge cycles or about three years, so the more you recharge, the shorter the batteries life span. 'The main cause of capacity loss is that rechargeable batteries are usually battery packs containing more than one cell, and as they're recharged over time these cells tend to get out of balance as one cell gets slightly more charge than another.' 'And so the whole battery pack capacity is limited by the one that charges first and the one that discharges first,' says Skyllas-Kazacos. Lithium-ion batteries are made from layered sheets of aluminium foil coated with cobalt oxide acting as the cathode, with the anode being a carbon mineral coated copper sheet. The electrodes are separated by a plastic separator and immersed in a lithium electrolyte medium. Lithium-ion batteries produce even higher energy levels than Ni-MH batteries, don't contain toxic materials such as cadmium or mercury, and are about 35 per cent lighter than the other types of batteries. The future While lithium-ion batteries will remain as the principle power source for portable electronic items such as laptop computers, they're likely to be replaced in electric cars by lithium-oxygen batteries in the future, predicts Skyllas-Kazacos. 'And that means you'll only really have lithium in the negative half-cell, while the positive half-cell will run off oxygen from the surrounding atmosphere.' This will produce even greater energy densities and longer driving ranges, she says.