Battery Circuit DiagramsLCD-LED Display

Car battery monitor Schematic Circuit Diagram

Charge-Discharge-Idle (C-D-I) Monitor for Vehicles

This C-D-I monitor is designed to be compatible with vehicles equipped with 12-V or 24-V batteries, where the negative terminal is linked to the chassis (refer to point B in the diagram).

Current Measurement Setup

To measure the current drawn from the battery, the system monitors the voltage drop along the hefty cable connecting the battery’s negative terminal to the chassis. Typically, this cable extends to the engine (point C). The positive terminal usually connects to two cables: a sturdy one leading to the starter motor (point A) and a thinner one going to the ignition switch.

LED Indicators for Battery Status

Inside the car, two LEDs serve as indicators, providing visual feedback on the battery’s charging or discharging status when a substantial current is involved. This setup offers a reliable way to confirm the generator’s functionality. Additionally, a third LED signals a neutral zone where the battery experiences only slight charging or discharging.

Car battery monitor Schematic diagram

Measuring Voltage Drop

This circuit functions primarily as a window comparator, utilizing operational amplifiers (opamps). The voltage drop across the battery’s chassis cable is directed to resistor R1, forming a segment of a measuring bridge comprised of R2, R3, R4, and P1. This slight voltage imbalance in the bridge is magnified by a factor of 100 through opamp IC2, configured as a noninverting amplifier. The balanced bridge reliably detects input voltages as low as 2 +/- 2.5 mV or -2.5 mV in practical scenarios.

Window Comparator Operation

The output voltage from IC2a controls a window comparator integrated around IC21 and IC9c. LEDs connected to the opamp outputs serve as indicators, illuminating to denote whether the battery is charging (D4 lights up), discharging (D6 activates), or in a ‘neutral’ state (D5 is lit).

Hysteresis Control and Neutral Range Adjustment

To prevent hysteresis from affecting the reference voltages provided by R8-R9-R10, the two positive feedback networks linked to IC2b and IC2 are isolated at IC2a’s output by R6 and R7. If necessary, adjusting the value of R9 can alter the ‘idle’ (neutral) range, making it smaller as needed.

Precision Component Requirements

For accuracy, the fixed resistors within the bridge must be high-precision types placed in thermal contact with each other. The circuit incorporates an internal power supply based on the common 7805 three-terminal voltage regulator. A heat sink for the regulator is necessary only in vehicles with a battery voltage exceeding 12 V.

Car battery monitor Schematic diagram


Adjusting the Circuit for Optimal Performance

Making adjustments to the circuit is a simple process. First, start the engine and let it idle. Set preset P2 to the mid-travel position. Following this, tweak P1 until the idle LED, D5, illuminates. Carefully fine-tune P2 until IC2a generates an output voltage of 2.5 V. Rev up the engine and confirm that the charge LED D4 lights up.

Circuit Performance and Current Indication

With the specified component values, the circuit is designed to indicate a charge or discharge current exceeding approximately 1.5 A, corresponding to 18 W at a battery voltage of 12 V. Recommended colors for the LEDs include green for D4 (indicating a charged state), red for D6 (indicating discharge), and yellow for D5 (indicating idle). Alternatively, triangular LEDs can be used for charge and discharge indicators, positioned upwards and downwards respectively, with the idle LED being a rectangular type placed in between. Notably, the circuit consumes 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

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