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Circuits designed by David Johnson, P.E.

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Last Updated on: 11/01/2009 03:06:29 PM

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Lead-acid Battery Low Voltage Indicator
Lead-acid batteries should not be discharged below a certain voltage. Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity. Large battery banks are critical in off-grid energy storage systems. One bad cell in a series wired string will weaken the whole bank. The circuits below are designed to be connected across either a single 2 volt cell or a 6v three cell battery bank. It will alert the user of a weak cell by turning on a red LED when the cell voltages drop below 1.8v. The circuit takes advantage of a low voltage LMP2231 op amp from Nation Semiconductor and a LM385 voltage reference in the first circuit. In that circuit the red LED must have a forward voltage of about 1.6v at 5ma, in order to operate properly. In the second three cell bank circuit, a TLV3701 voltage comparator is used. The circuits are simple enough that they could be placed on a postage stamp size circuit board.