Battery Level Meter

With this circuit, you can assess the voltage level in Lead Acid Car battery or Inverter battery. It gives LED indications in Dot Mode display to show the level of voltage in the battery. A Lead Acid battery attains 13.8 volts and tubular battery 14.8 volts in the fully charged state. If the battery voltage level drops below 10 volts, it needs immediate charging, and if it is not attaining full charge, it cannot be used to drive the load.

The circuit uses the popular display driver LM 3914 to give the LED meter reading. Input voltage is applied to the IC from the battery terminals through R1.The lowest reference voltage is set as 10.5 volts and highest level as 13.8 volts. Variable resistors VR2 sets the lower voltage level and VR1 sets the upper voltage level.

How to set

To set the meter, a variable power supply is required. LM 317 based 1.5-30 volts variable power supply is a good choice. Adjust the output of the variable power supply to 10.5 volts and check it with a digital multimeter. If it is ok, then connect points A and B to the variable power supply and adjust VR2 till D2 lights. This is the lower level. Then adjust the variable power supply to 13.8 volts. Slowly adjust VR1 till D9 lights. This is the upper level.  You can also calibrate the steps of voltages between 10.5 and 13.8 in the similar way and mark the voltage level at which each LED lights. Now the meter is referenced between 10.5 and 13.8 volts. Connect it with the battery using Crocodile clips.

Some facts about LM3914

LM 3914 IC is used in display circuits to drive either individual LED or Matrix LED. These are mainly used in circuits where precision output display is needed. Its each output becomes low one by one with the increment of 125 milli volts in the input.

1.      LM 3914 Internal resistors have equal value. Produce linear response. Used as volt meter.

2.      These ICs have 10 outputs each capable of sinking current to light LEDs brightly.

3.      Up to 4 LEDs can be connected to each output serially if the supply voltage is more than 9 volts.

4.       LED do not require a series resistor since the IC can regulate output current according to the value of the Programme resistor in the pin 7.

5.      To reduce signal to the input, a variable resistor is needed. Otherwise all LEDs will light (in bar mode) or turn off (in dot mode).

6.      If the input is not connected to the 0 rail through a preset or resistor, Last LED at pin10 will lock on.

7.      Supply voltage It can be as low as 3 volts. It must be at least 1.5 volts more than the reference voltage applied to the “high end” (pin6) of the resistor chain. Standby current is around 3 mA with 5V supply when all LEDs are off.

8.      Reference Voltage Pin 7 and 8

Reference voltage of 125 milli volts is brought out from the IC at pin 7 and 8. The reference voltage at pin7 can be increased up to 12 volts by connecting the reference adjust pin 8 to 0 rail through a resistor .This gives the advantage of setting the sensitivity of IC over wider limits.

9.      Fixing LED current –LED current is programmed by the resistor connected to pin7 and pin8. In LM 3914, total resistance of the internal resistor chain is 10K. So the LED current can be calculated as

LED current =10(1.25V / R + 1.25v / 10K) Where R is the value of programming resistor at pin7, 8 and ground. With 1.2K programming resistor, individual LED current will be10 (1.25/1200+1.25/10,000) = 0.0116 Amps or 11.6 milli Amps.

10. To get more brightness to LED, value of the programming resistor should be below 1K. If the value of the programming resistor is 560 ohms, the current through the LED is 23.5 milli ampere which gives sufficient brightness. Current should be between 20 to 30 milli ampere.

11. DOT Mode and BAR Mode

If the pin 9 of IC is left free, it shows DOT mode display ( if one LED lights, the previous one turns off ) and if pin 9 is connected to positive supply, it shows BAR mode ( All LEDs remain lit) display.


Why you use resistor with variable resistor?

Often we want to easily change a resistor value, so we use a variable resistor. 
For example, we may want to change the resistor that controls the power sent to a LED, so we can easily make it brighter or dimmer. 
Often if we use a variable resistor, there is only a very narrow range that is useful. 
Continuing our example, sometimes we use several LEDs, and we use the variable resistor to set them all to the same brightness. 
In this case, the resistance range that sets the LED to be twice as bright as the the other LEDs, and the resistance range that sends so much power to the LED that it is permanently destroyed is even less useful. 

So we add a fixed resistor in series with the variable resistor — the fixed resistor sets the minimum net resistance, no matter how we turn the knob on the variable resistor. 
In our example, the addition of the fixed resistor allows us to turn the variable resistor throughout its whole range, and the LED gets brighter and dimmer; without that resistor, a certain range of the knob on the variable resistor would allow so much power to go to the LED that it would be destroyed.

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