4 October 2014

Primary Batteries – What Are The Choices Part 2

The Li-FeS2 cell comes with PTC thermal switch, a safety device. The switch limits the current delivered at high temperatures. It is not possible to recharge this cell like NiMH batteries. Recharging, mixing them with used batteries or other types of batteries or placing them backwards can lead to a leak or an explosion.

The most common formats for primary batteries are AA and AAA. The AA batteries, referred to as penlight batteries, became available as a consumer product in 1915. They were used in pocket lights which served as spy tools during the World War I. In 1947, American National Standards Institute standardized the AA format. The AAA battery was developed in 1954 with a view to bring down the size of the Polaroid and Kodak cameras and other portable devices. AAAA batteries, an offshoot of 9V batteries, were produced in the 1990s for use in LED penlights, laser pointers, headphone amplifiers and computer styli.

The AAA cell’s capacity is half of that is an AA cell, but the price is almost the same. In other words, the cost of energy of AAA batteries is two times that of AA batteries. Energy cost is often neglected when the focus is on downsizing batteries. Further, device manufacturers prefer to use the smaller AAA batteries. For example, AA batteries would only marginally increase the performance of bicycle lights, but they would deliver two times the energy at the same cost. Therefore, proper design consideration is essential to protect the environment by reducing the waste generated.

The retail price and performance alkaline AA cells vary a great deal. A study conducted by Exponent, an engineering firm in the US, on eight different brands of alkaline AA batteries showed that the discrepancy in terms of performance between best and lowest brands was 800 percent. A practical way of testing the batteries is by counting the shots that a digital camera can provide with one set of cells. Actually, the current pulses required for operating a digital camera is much higher than that needed for operating a kitchen clock or remote control. A regular alkaline battery that cannot be used in a digital camera any longer could still be used to power a kitchen clock or remote control for up to 2 years.

If two each of NiMK, alkaline and lithium Li-FeS2 cells are used in series to provide 3V, 433mA and 1.3W power to a digital camera and pulses or shots counted, the highest would be for Li-FeS2 (690 shots) and followed by NiMH (520 shots) and alkaline (85 shots). Internal resistance, more than the capacity, determines the shot count.

The rated capacity can be used for measuring performance usefully when the discharge currents are low. This is because the power factor starts to play a key role at higher loads. A Ragone Chart is made use of for the purpose of illustrating the relationship between the ability of the battery to drive the current and capacity. The Ragone Chart is named after David V. Ragone. It evaluates the energy and power of batteries.

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