Mr. Lung wrote to me, asking for some help with a battery
circuit. After some discussion, we arrived at the the low parts
high performance "Battery Low" circuit. Mr. Lung prototype it
it to work to his satisfaction. The "Battery Low" checker
the red LED if the battery is too low. A second circuit was
the "Battery Good" circuit, which lights it green LED when the
voltage is high enough.
Why two circuits? Because different applications have different
According to Mr. Lung, the lowest safe voltage across a lead
in long term storage is 2.0 volts per cell, or 6.0 volts for a 6
battery. Gates Energy Products, the
load must be
disconnected from a lead acid cell when the voltage reads 1.8
less, in order to avoid damage. For a 6 volt
battery with three cells, this critical voltage is 3 x 1.8 volts
volts. Choose your threshold depending upon your
there is a good chance that the battery will not be recharged
after running down, be safe and go with the 2.0 volt per cell
These circuits can also be adjusted for other battery voltages
other types of battery. This circuit should work well with
Nickel Cadmium (ni-cd or nicad), Nickel Metal Hydride (NIMH) or
rechargeable as well as primary cells types such as Carbon-Zinc
Alkaline. Just make sure that you don't exceed the ratings of
the parts for your particular application.
The practical minimum threshold voltage for the Battery Low
about 3.5 volts while the practical minimum voltage for the
Good circuit is about 4.5 volts.
Battery Checker Circuit
This low parts count
be adjusted to operate over a
wide range of trigger voltages. Be careful to choose
the correct value and wattage rating for R1.
As specified by Mr. Lung, this circuit indicates when the
voltage is too low. His purpose for the checker is to
a rechargeable lead acid cell has run down to its safest
To calibrate this circuit for a
lead acid cell:
1. Connect the circuit to a power supply set to just above 6.0
2. Hold down the button.
3. Adjust R2 to the point that the LED just comes on.
The TL431 has an internal threshold of 2.5 volts. Pot R2 is
that when the battery voltage equals the desired threshold, the
of R2 will be 1.25 volts. When the voltage across the battery is
the preset threshold, the voltage at the wiper of R2 is
volts, the TL431 conducts, shunting current away from the LED,
the LED off.
When voltage drops below the preset threshold, the voltage on
of R2 drops below 2.5 volts, and the TL431 conducts
current, only up to about 1 milliamp, and the anode voltage
the LED conducts.
I tested this with a 6 volt threshold rather than a 5.4 volt
When adjusted for a 6 volt threshold, the LED turns on at 6.0
then slowly fades out as the battery voltage decreases, until it
reaches about 2.5 volts, when the LED is too dim to see. The LED
above 6.0 volts, but (for reasons I have not investigated) the
comes back on at voltages above 8.35 volts. The "come back on
is proportional to the threshold setting, thus, if the circuit
adjusted to turn on the LED when the voltage dropped below 12
the LED would also come on at voltages above
(8.35/6) x 12 = 16.7
The minimum voltage across the TL431 is 2.5 volts, which would
leave enough to illuminate D2 (which requires about 1.8 volts),
was added to provide an additional 0.7 volt drop in series with
that D2 will not illuminate when the TL431 is sinking the
The luminance of the LED can be adjusted by changing the value
The push-button is in series with the circuit so that it will
to the drain on the battery when the battery voltage is not
checked. Unless actually checking the voltage, this circuit
Mr. Lung, who identified the need for this circuit, built and
the prototypes, and has experienced using the circuit in
actual application, can be reached at
email address is a image).
Battery Checker Circuit