Battery
Level Indicator
Normally in mobile
phones, the battery level is shown in dot or bar form.This lets
Easily recognize the
battery level. Here we present a circuit that lets you know the battery level
of a device from the number of LEDs that are glowing. It uses ten LEDs in all.
So if three LEDs glow, it indicates battery capacity of 30 per cent. Unlike in
mobile phones where the battery-level indicator function is integrated with
other functions, here only one comparator IC (LM3914) does it all. The LM3914
uses ten comparators, which are internally assembled in the voltage divider
network based on the current-division rule. So it divides the battery level
into ten parts.
The circuit derives the
power supply for its operation from the battery of the device itself. It uses
ten LEDs wired in a 10-dot mode. The use of different colored LEDs makes it
easier to recognize the voltage level on the basis of the calibration made. Red
LEDs (LED1 through LED3) indicate battery capacity of less than 40 per cent.
Orange LEDs (LED4 through LED6) indicate battery capacity of 40 to less than 70
per cent and green LEDs (LED7 through LED10) indicate battery capacity of 70 to
fewer than 100 per cent. The brightness of the LEDs can be adjusted by varying
the value of preset VR2 between pins 6 and 7. Diode D1 prevents the circuit
from reverse polarity battery connection. The tenth LED glows only when the
battery capacity is full, i.e., the battery is fully charged. When the battery
is fully charged, relay driver transistor T1 conducts to energies relay RL1.
This stops the charging
through normally-open (N/O) contacts of relay RL1.
For calibration,
connect 15V variable, regulated power supply and initially set it at 3V. Slowly
adjust VR1 until LED1 glows. Now, increase the input voltage to 15V in steps of
1.2V until the corresponding LED (LED2 through LED10) lights up. Now the
circuit is ready to show any voltage value with respect to the maximum voltage.
As the number of LEDs is ten, we can easily consider one LED for 10 per cent of
the maximum voltage. Connect the voltage from any battery to be tested at the
input probes of the circuit. By examining the number of LEDs glowing, you can
easily know the status of the battery. Suppose five LEDs are glowing. In this
case, the battery capacity is 50 to 59 per cent of its maximum value.
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