Introduction
Mr. Lung wrote to me, asking for some help with a battery
checker
circuit. After some discussion, we arrived at the the low parts
count,
high performance "Battery Low" circuit. Mr. Lung prototype it
and
found
it to work to his satisfaction. The "Battery Low" checker
lights
the red LED if the battery is too low. A second circuit was
devised,
the "Battery Good" circuit, which lights it green LED when the
battery
voltage is high enough.
Why two circuits? Because different applications have different
needs.
According to Mr. Lung, the lowest safe voltage across a lead
acid
battery while
in long term storage is 2.0 volts per cell, or 6.0 volts for a 6
volt
battery. Gates Energy Products, the
load must be
disconnected from a lead acid cell when the voltage reads 1.8
volts or
less, in order to avoid damage. For a 6 volt
battery with three cells, this critical voltage is 3 x 1.8 volts
= 5.4
volts. Choose your threshold depending upon your
application. If
there is a good chance that the battery will not be recharged
soon
after running down, be safe and go with the 2.0 volt per cell
threshold.
These circuits can also be adjusted for other battery voltages
and
other types of battery. This circuit should work well with
Nickel Cadmium (ni-cd or nicad), Nickel Metal Hydride (NIMH) or
other
rechargeable as well as primary cells types such as Carbon-Zinc
and
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
circuit is
about 3.5 volts while the practical minimum voltage for the
Battery
Good circuit is about 4.5 volts.
Battery
Low
Battery Checker Circuit
This low parts count
circuit can
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
battery
voltage is too low. His purpose for the checker is to
detect when
a rechargeable lead acid cell has run down to its safest
discharge
voltage.
To calibrate this circuit for a
6 volt
lead acid cell:
1. Connect the circuit to a power supply set to just above 6.0
volts
2. Hold down the button.
3. Adjust R2 to the point that the LED just comes on.
Circuit operation:
The TL431 has an internal threshold of 2.5 volts. Pot R2 is
adjusted so
that when the battery voltage equals the desired threshold, the
wiper
of R2 will be 1.25 volts. When the voltage across the battery is
above
the preset threshold, the voltage at the wiper of R2 is
above 2.5
volts, the TL431 conducts, shunting current away from the LED,
keeping
the LED off.
When voltage drops below the preset threshold, the voltage on
the wiper
of R2 drops below 2.5 volts, and the TL431 conducts
less
current, only up to about 1 milliamp, and the anode voltage
rises until
the LED conducts.
I tested this with a 6 volt threshold rather than a 5.4 volt
threshold.
When adjusted for a 6 volt threshold, the LED turns on at 6.0
volts and
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
is off
above 6.0 volts, but (for reasons I have not investigated) the
LED
comes back on at voltages above 8.35 volts. The "come back on
voltage"
is proportional to the threshold setting, thus, if the circuit
were
adjusted to turn on the LED when the voltage dropped below 12
volts,
the LED would also come on at voltages above
(8.35/6) x 12 = 16.7
volts.
The minimum voltage across the TL431 is 2.5 volts, which would
still
leave enough to illuminate D2 (which requires about 1.8 volts),
so D1
was added to provide an additional 0.7 volt drop in series with
D2 so
that D2 will not illuminate when the TL431 is sinking the
maximum
current.
The luminance of the LED can be adjusted by changing the value
of R1.
The push-button is in series with the circuit so that it will
not add
to the drain on the battery when the battery voltage is not
being
checked. Unless actually checking the voltage, this circuit
draws no
power.
Mr. Lung, who identified the need for this circuit, built and
tested
the prototypes, and has experienced using the circuit in
an
actual application, can be reached at
(this
email address is a image).
Battery
Good
Battery Checker Circuit