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The hard cell

A bit of design thought can make batteries last longer. By Len Sherman.

In spite of accomplishing amazing feats, many consumer products still manage to frustrate and disappoint for the simple reason that they run out of power. Sometimes, it is unavoidable; more often than not, it happens because the consequences were not considered.
Battery choice is important. AAA and 9V alkaline ‘transistor’ batteries have significant disadvantages compared to AA cells. Both have poor output impedance, so their limited capacity is made worse as losses increase with load peaks. Whilst AAA cells should be avoided, except where mechanical design mandates their use, 9V ‘transistor’ batteries have some justification; they allow a 5V system to be powered with a cheap linear regulator. For low current loads, 9V capacity is comparable to AA. However 9V batteries have high output resistance and capacity is very dependent on load current. This is fine for a smoke alarm, but frequent battery replacement is expensive and the cell’s poor output resistance compounds this problem.
Modern low cost dc/dc converters make it easy to generate an efficient 5V supply from two AA batteries (see Figure 1). The circuit shown typically doubles the time between battery changes and is four times cheaper for the user than 9V batteries.

Graham Pitcher

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