Abhijeet Deshpande, People’s Education Society Institute of Technology, Bangalore, India; Edited by Martin Rowe and Fran Granville -August 12, 2010
Properly maintained rechargeable
batteries can provide good service
and long life. Maintenance involves regular
monitoring of battery voltage. The
circuit in Figure 1
works in most rechargeable
batteries. It comprises a reference
, which operates
at a constant current of 1 mA and
provides reference light of constant
intensity regardless of battery voltage.
It accomplishes this task by connecting
in series with the
diode. Therefore, even if the battery
voltage changes from a charged state
to a discharged state, the change in
current is only 10%. Thus, the intensity
remains constant for a
battery state from a fully charged
state to a fully discharged state.
This circuit works in most rechargeable batteries. It comprises a reference LED, LEDREF
, which operates at a constant current of 1 mA and provides reference light of constant intensity regardless of battery voltage.
The light output of the variable
LED changes with respect to changes
in battery voltage. The side-by-side-mounted
LEDs let you easily compare
light intensities and, thus, battery
status. Using diffused LEDs as
crystal-clear LEDs can damage your
eyes. Instead, mount the LEDs with
sufficient optical isolation so that the
light from one LED does not affect
the intensity of the other LEDs.
The variable LED operates from 10
mA to less than 1 mA as the battery
voltage changes from fully charged to
fully discharged. Zener diode DZ
series with resistor R2
causes the current
to change with battery voltage.
The sum of the zener voltage and
the drop across the LED should be
slightly less than the lowest battery
voltage. This voltage appears across
. As the battery voltage varies, it
produces a large variation of current
. If the voltage is approximately
1V, then 10 mA will flow through
, which is much brighter than
. If the voltage is less than 0.1V,
then the light intensity of LEDVAR
be less than LEDREF
, indicating that the
battery has discharged.
Immediately after the battery has
charged, the battery voltage is more
than 13V. The circuit can withstand this
voltage because it has a 10-mA margin.
If the LEDs are bright, quickly release
pushbutton switch S1
to avoid damage
to the LEDs (Figure 2
This circuit can withstand 13V because it has a 10-mA margin. If the LEDs are bright, quickly release pushbutton switch S1
uses a 12V lead-acid battery
indicator as an example, but you can extend the design to accommodate
other types of chargeable batteries. You
can also use it for voltage monitoring. It
uses two green LEDs to indicate whether
the battery has charged above 60%.
A set of red LEDs indicates whether
the battery charge drops below 20%.
10-kΩ resistors R1
. For the
variable-intensity LEDs, a zener diode
works in series with 100Ω resistors R3
. Diodes D1
, and D3
the required clamping voltages. Table 1
shows how LED intensity
the variable intensity
for the green LED:
a green-LED current
of 1 mA, VBATT=10−3
×100+0.6+0.6+1.85+9.1=12.25V. The selected LEDs have a
drop of 1.85V at 1 mA.
If the LED has different characteristics,
then you must recalculate the resistor
values. At this voltage, the LEDs
have the same intensity, and the battery
is 60% charged. See Reference 1
for lead-acid-battery voltages.
The following equation
the variable intensity for the red LED:
a green-LED current of 1 mA, VBATT
At this voltage, both red LEDs have
equal intensities, and the battery is 20%
is off. Figure 3
that both variable-intensity LEDs are
brighter than the reference LEDs, indicating
that the battery is 100% charged.
Both variable-intensity LEDs are brighter than the reference LEDs, indicating that the battery is 100% charged.