Folks who depend on battery power for their daily needs obviously have more than a passing interest in the state of charge of their storage banks. And although that level can be checked with a hydrometer, doing so is a time-consuming, messy chore. Fortunately, there’s an alternative that will make the burdensome task less of a bother.
As battery charge drops, so does battery voltage. In fact, voltage can provide an accurate gauge for evaluating battery condition. Fig. A shows a curve of voltage, plotted against the percentage of charge, for a 12-volt battery. But rather than asking you to consult the graph and a voltmeter whenever you want to check your system, we’ve wrapped the whole procedure ( plus some fringe benefits) into a neat little electronic package.
MOTHER’S Battery Monitor is essentially nothing more than a very easy-to-read voltmeter. The voltage of the battery being tested is indicated by one of ten LED’s (light-emitting diodes). The scale of the readout is arranged so that each of the consecutive lamps represents 1/2 volt. The first indicates a charge of 10.5 volts, the second 11.0, the third 11.5 … and so on up to 15.0 volts.
The lights are switched on and off by an integrated circuit (designated LM3914), which is actually a large collection of transistors in one package. In addition, the lights are color coded to indicate the urgency of the situation. Lamps between 12.0 and 14.5 are green, while the 11.0 and 11.5 items are yellow, and the 10.5 light is red . . . indicating that the battery is almost entirely discharged.
At the other end of the scale, the 15.0 is also red . . . since excessive charge voltage can cause a battery to boil. (This situation might occur if the temperature control in your voltage regulator goes haywire.)
Finally, to be sure you’ll be warned of a potential disaster, both the lowest and the highest readings are hooked to a piercing alarm. Whenever voltage drops too low or soars too high, integrated circuit 2 (known as 4011 CMOS) will call for help.
The monitor has been designed for easy assembly, and all the parts are readily available (for under $20) at Radio Shack outlets or similar electronics stores. You can either prepare your own circuit board, or purchase one, ready made, from the source included in the List of Materials.