Posted on Feb 4, 2014

[The following circuit is adapted from battery monitors and testers published in The Smith-Kettlewell Technical File (SKTF). The first appearance of this system was `The Smith-Kettlewell Battery Tester, Where Silence is Golden`, SKTF, Volume 11, No. 1, Winter 1990. ] The application here was inspired by a request from a visually impaired ham-radio

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operator whose "rig" (transceiver) specifically requires that its supply voltage be above 12 volts under load. When powered by a storage battery (as in emergency operation), his unit would malfunction on transmit, which could not be detected by the operator. Thus, he suggested that a sensitive warning system be devised (other than a multi-digit meter) by which the operator would be alerted to an impending low battery condition. Since the time of that request, we have become interested in providing an alarm for riders of motorized wheelchairs - an audible indicator that would warn the user that he should put a charge on the battery before "deep discharge" damages it. Thus, the values and adjustments shown here are for voltages below 11. 66 volts. Smith-Kettlewell`s "Rehabilitation Engineering Research Center", funded by the National Institute on Disability and Rehabilitation Research, devised a battery-checking scheme which has found its way into several of our designs - a vibratory hearing-aid button-cell tester for the deaf-blind, an auditory adaptation of the Radio Shack "Handy Checker" (see the above byline), and a test instrument for servicing "talking book" players. The principle of our readout is to generate bursts (audio "beeps", or vibratory "buzzes" for those who cannot hear audio signals); their duration indicates battery voltage. (In this implementation, the output is auditory. ) Specifically, an exhausted...

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