## Vdc

I used an IRFZ40 MOSFET to rest the adder by shorting out the capacitor C1. Other MOSFETs such as the IRF511 (Radio Shack #276-2072) will probably also work. Note that the MOSFET has an integral diode to keep the tantalum capacitor from going more than a few tenths volt reverse polarity.

### Comparator Section

Figure 4 shows the COMPARATOR section in detail. It uses an LM339 comparator. As there are 4 comparators in a single LM339 package, the other 3 comparators are available for a 2nd Amp-hour meter or other circuitry (I used 2 comparators to implement a high battery voltage/low battery voltage detection circuit). The zener diode in the schematic is a precision reference diode LM385Z. When the adder output reaches -2.5 Volts, the comparator output will rise to approximately 3.7 Volts, which will both increment the counter and reset the adder. The comparator output will remain high until the adder output approaches 0 Volts. The comparator output will then go back to approximately -5 Volts.

values specified. R1 and C1 are selected to obtain the correct counting rate. Their values are also dependent on the resistance of the shunt (Rs). The values can be approximately determined from the equation R1 = 1.44/ (Rs x C1), with R1 in kiloOhms, C1 in ^F (microFarads), and Rs in Ohms (W). I would recommend choosing C1 from available values, then calculating R1. R1 should be between 1k and 200k Ohms.

For example, my input shunt is 0.001 Ohm. I chose a 15 ^F capacitor. R1 is then calculated to be 1.44/(.001 x 15) = 96 kiloOhms. My output shunt is 0.0001 Ohm. I chose a 4.7 ^F capacitor. R1 is then calculated to be 1.44/(.0001 x 4.7) = 31 kiloOhms. To compensate for component tolerances, I would recommend using a potentiometer for R1 and adjusting it after the circuit is built for proper calibration (see CALIBRATION). Also, R2 should be chosen approximately equal to R1 (within 10%). Capacitor C1 should be tantalum for best accuracy. Make sure the capacitor is installed to match the indicated polarity.

Resistor R3 is selected to optimize offset adjustment using the 100k Ohm potentiometer. Offset adjustment is necessary to assure that the Amp-hour meter is accurate for low or zero input currents. If R3 is too low a value, it is very difficult to set the pot to the optimum value, since a slight change in the pot will cause a large output variation. If R3 is too high a value, no pot setting will compensate for the offset. I used two 10 megaOhm resistors in parallel (5 megaOhm effective) on my two meters.

Figure 4

Comparator Section

Figure 4

Comparator Section

LCD Counter Section

Figure 5 shows the LCD COUNTER section in detail. The LCD counter is Radio Shack #277-302. As 1.5 Volts is generated by the power converter circuitry, no 1.5V AA battery is needed. The zener diode in the schematic is a precision reference diode LM385Z-1.2. It is used along with the 22 kW resistor to provide the correct logic level to the counter. The reset switch is used to reset the counter display to 00000. The display will read up to 999.99 Amp-hours. There is no decimal point in the display. Perhaps one could be "taped" onto the display if desired.

Figure 5

LCD Counter Section reset

## DIY Battery Repair

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.

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