A boost to battery power

A boost to battery powerSteve Bush Improvements in primary (disposable) cells are making them a viable option where once only rechargeable cells could cope. Primary cells, particularly alkaline and zinc-chloride types, are well suited for use in low consumption portable equipment – test meters are a prime example – where the load matches the cell characteristics well. However, these chemistries fall down in high-drain applications where high internal impedance limits output current and dramatically reduces capacity. For example, a D-size alkaline cell has a capacity of 18Ah at 50mA load, which drops below 5Ah at 2A load. Where high current is needed in portable equipment, rechargeable chemistries – nickel cadmium (NiCd), nickel metal hydride and sometimes lithium ion – are employed. These have lower absolute capacities, but they are far less affected by drain current. A 5Ah D-size NiCd has almost the same capacity at 50mA and 5A drain. Motivated by emerging consumer applications including GSM phones, digital cameras and Mini-Disk players, Energizer and Duracell have produced a new generation of primary cells with much-improved high current performance. Power cell … AA and AAA-size disposable batteries are getting much better at supplying higher power loads. The curves are of Duracell’s standard and new high-performance ‘Ultra’ AA cells. Energizer has a similar new product. “Battery operated devices used to have drain rates between 30 and 300mA, new high-tech products like phones and personal organisers are drawing higher currents including 400mA to 2A pulses,”said John Grady, senior technologist at Energizer. “We have improved our cells to match the developing demands of these new uses.” Both Energizer and Duracell have concentrated on their AA and AAA-size cells, these being the most popular in their native US market. The improvements have been two-fold. The amount of active material in the cells has been increased and attention has been paid to the sources of internal resistance. As a result, Energizer’s latest AA cell is claimed to have twice the capacity of its counterpart of two years ago, and much improved voltage hold-up at high current. Power hungry devices like portable signal generators can now reasonably be supplied from primary cells, or at least have an option to use them when their internal rechargeable runs flat at an awkward moment. Duracell has chosen to market its high performance alkaline cells separately from its standard alkalines under its ‘Ultra’ banner. Energizer’s tack is to turn over all its production capacity to its new design which will carry an ‘Advanced Formula’ logo. Replacing secondary cells with primary cells still carries with it the additional environmental impact of throwing cells away and the increased cost of continual replacement, but it allows field users to get their equipment back in service with a simple trip to the local supermarket.

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