Battery Circuit DiagramsLCD-LED Display

Car battery monitor

THE charge-discharge-idle (C-D-I) monitor described here is suitable for all vehicles with a 12- V or 24-V battery of which the negative terminal is connected to the chassis (point B in the drawing).

The current drawn from the battery is measured by monitoring the voltage drop across the heavy cable between the negative terminal of the battery, and the chassis. Usually, this cable is extended to the engine (point C). The positive terminal is usually connected to two cables, a solid one to the starter motor (point A) and a thinner one to the ignition switch.

Two LEDs fitted in the car interior indicate whether the battery is charged or discharged with a significant current, providing a reassuring check on the generator function. A third LED is provided to indicate a kind of neutral area in which the battery is only lightly charged or discharged.

The circuit is essentially a window comparator based on opamps. The voltage drop that exists across the chassis cable of the battery is fed to resistor R1, which forms part of a measuring bridge consisting of Ry-R2-R3-R4-Pi. The small voltage unbalances the bridge and is amplified 100 times by opamp IC2, which s wired as a noninverting amplifier. In practice, input voltages as small 2 -+2.5 mV or -2.5 mV are detected reliably by the balanced bridge.

The output voltage of IC2a controls a window comparator built around IC21 and IC9c. The LEDs at the opamp outputs indicate whether the battery is charged (D4 lights), discharged (D6 lights), or is it a ‘neutral’ state (D5 lights).

The two positive feedback networks associated with IC2b and IC2, are decoupled at the output of IC2a by R6 and R7 to ensure that the hysteresis of the window comparator does not affect the reference voltages supplied by R8-R9-R1o. If necessary, R9 may be made smaller to make the ‘idle’ (neutral) range smaller.

The fixed resistors in the bridge must be close tolerance types mounted such that RR they are in thermal contact with each other. The circuit has an internal power supply – based on the ubiquitous 7805 three •. terminal voltage regulator. The regu-la tit requires a heat-sink only when the 6111 is installed in a vehicle with a battery voltage higher than 12 V.

Car battery monitor Schematic diagram

Adjusting the circuit is straightforward. Start the engine and let it idle. Set preset P2 to mid-travel. Next, adjust P1 until the idle LED, D5, lights. Carefully readjust P2 until IC2a supplies an output voltage of 2.5 V. Rew up and check that the charge LED D4, lights.

With the component values shown here, the circuit will indicate a charge or discharge current greater than about 1.5 A, which corresponds to 18 W at a battery voltage of 12 V. Suggested colors for the LEDs are green for D4 (charged), red for D6 (discharged) and yellow for D5 (idle). Alternatively, the charge and discharge LEDs may be triangular types which can be fitted to point up and down respectively. The idle LED is then a rectangular type fitted in between. Finally, the circuit draws about 30 mA when connected to a 12-V system.
Parts list
R1-R4, R6, R7 = 1.2 kΩ
R5 = 270 kΩ
R8, R10 = 2.2 kΩ
R9 = 680 Ω
R11, R12 = 120 kΩ
R13 = 120  Ω
R14 = 10  Ω
R15=68 kΩ
R16 = 470  kΩ
P1 = 250 kΩ preset H
P2 = 47 kΩ preset H
C1 = 100 μF, 40 V
C2 = 10 μF, 10 V
C3 = 1 μF, 10 V
C4 = 100 nF
IC1= 7805
IC2 = LM324
D1 = 1N4002
D2, D3 = 1N4148
D4 = LED, green
D5 = LED, yellow
D6 = LED, red
Car battery monitor Schematic diagram

Car battery monitor Schematic diagram

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