Posted on Feb 5, 2014

Figure 1 is a schematic of such an arrangement with four batteries. The diagram in Figure 2 uses one voltmeter and a two-pole multi-position rotary switch to select the battery to monitor. While accomplishing the goal of bringing the metering of batteries out of the battery compartment, this design has some safety problems. A direct connection to any point

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in the battery pack is dangerous because of the shock hazard and also because a short-circuit could result in injury or fire. The risks can be reduced by revising the design as shown in Figure 3. By placing current limiting resistors right on the battery terminals, only micro amperes are allowed to flow on the wires of the instrument, even in the case of a short circuit. Note that the meter itself is not a voltmeter but is a micro ammeter with a scale marked in Volts. If the switch is eliminated by providing separate meters for each battery, then the batteries can be compared under identical load conditions. In 1989, I installed just such a meter system in my Sebring-Vanguard Citicar. The dashboard sported eight vertical panel meters, one for each 6-Volt battery. I included trimmer potentiometers behind the dash to provide calibration of the meters. Figure 4 is the circuit diagram for one meter. These meters did an excellent job in the Citicar. It was easy to spot a weak battery and individual cell failures were immediately evident by the 2-Volt drop in the meter. I wanted to have similar metering in the Solectria Force. The dashboard appearance of the Force would suffer if I installed thirteen meters of any sort. But there is room in the instrument cluster for one small instrument. I decided to make a display which could show all 13 batteries at once in bar graph format. So the challenge was to design something that could...

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