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Charge Monitor for 12V lead acid battery.
Click here for circuit diagram.
A battery is a vital element of any battery-backed system. In many cases the battery is
more expensive than the system it is backing up. Hence we need to adopt all
practical measures to conserve battery life.
As per manufacturer's data
sheets, a 12V rechargeable lead-acid battery should be operated within 10. IV
and 13.8V. When the battery charges higher than 13.8V it is said to be overcharged,
and when it discharges below 10.IV it can be deeply discharged. A single event
of overcharge or deep discharge can bring down the charge-holding capacity of
a battery by 15 to 20 per cent.
It is therefore necessary for all concerned
to monitor the charge level of their batteries continuously. But, in practice,
many of the battery users are unable to do so because of non-avail?ability
of reasonably-priced monitoring equipment. The circuit idea presented here will
fill this void by providing a circuit for monitoring the charge level of lead-acid
batteries continuously. The circuit possesses two vital features:
First, it reduces the requirement
of human attention by about 85 per cent.
Second, it is a highly accurate and sophisticated method.
Input from the battery under test is applied to LM3914 1C. This applied voltage
is ranked anywhere between 0 and 10, depending upon its magnitude. The lower
reference voltage of 10.IV is ranked '0' and the upper voltage of 13.8V is ranked
as '10.' (Outputs 9 and 10 are logically ORed in this circuit.) This calibration
of reference voltages is explained later.
1C 74LS147 is a decimal-to-BCD
priority encoder which converts the output of LM3914 into its BCD complement.
The true BCD is obtained by using the hex inverter 74LS04. This BCD output is
displayed as a decimal digit after con?version using IC5 (74LS247), which
is a BCD-to-seven-segment decoder/driver. The seven-segment LED display (LTS-542)
is used because it is easy to read compared to a bar graph or, for that matter,
an analogue meter. The charge status of the battery can be quickly calculated
from the display. For instance, if the display shows 4, it means that the battery
is charged to 40 per cent of its maximum value of 13.8V.
The use of digital
principles enables us to employ a buzzer that sounds whenever there is an overcharge
or deep discharge, or there is a need to conserve battery charge. A buzzer is
wired in the circuit such that it sounds whenever battery-charge falls to ten
per cent. At this point it is recommended that unnecessary load be switched
off and the remaining charge be conserved for more important purposes.
simple combinational logic circuit can also be designed that will sound the
buzzer when the display shows 9. Further charging should be stopped at this
point in order to pre?vent overcharge.
The circuit is powered by the
battery under test, via a voltage regulator 1C. The circuit takes about 100
mA for its operation.
For calibrating the upper and lower reference levels,
a digital multimeter and a variable regulated power supply source are required.
For calibrating the lower reference voltage, follow the steps given below:
Set the output
of power supply source to 10. IV.
Connect the power supply source
in place of the battery.
Now the display will show some reading. At this
point vary preset VR2 until the reading on the display just changes from 1 to
The higher reference voltage is
calibrated similarly by setting the power supply to 13.8V and varying preset
VR1 until reading on the display just changes from 8 to 9.
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