Sunday, June 5, 2011

Charging Monitor for 12V lead acid battery

A battery is a vital element of any battery-backed method. In plenty of cases the battery is more pricey than the method it is backing up. Hence they must adopt all practical measures to preserve battery life.
As per manufacturer's information sheets, a 12V rechargeable lead-acid battery ought to be operated within ten. IV and 13.8V. When the battery charges higher than 13.8V it is said to be overcharged, and when it discharges below ten.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 twenty per-cent.
It is therefore necessary for all concerned to monitor the charge level of their batteries continuously. But, in practice, plenty of of the battery users are unable to do so because of non-availĂ‚­ability of reasonably-priced monitoring equipment. The circuit idea introduced here will fill this void by providing a circuit for monitoring the charge level of lead-acid batteries continuously. The circuit possesses vital features:
First, it reduces the requirement of human attention by about 85 per-cent.
Second, it is a highly correct and sophisticated system.
Input from the battery under check is applied to LM3914 1C. This applied voltage is ranked anywhere between 0 and ten, depending on its magnitude. The lower reference voltage of ten.IV is ranked '0' and the upper voltage of 13.8V is ranked as '10.' (Outputs 9 and ten 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 in to its BCD complement. The true BCD is obtained by using the hex inverter 74LS04. This BCD output is displayed as a decimal digit after conĂ‚­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 simple 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 example, if the display shows two, it means that the battery is charged to 40 per-cent of its maximum value of 13.8V.
The use of digital principles lets us employ a buzzer that sounds whenever there is an overcharge or deep discharge, or there is a necessity to preserve battery charge. A buzzer is wired in the circuit such that it sounds whenever battery-charge falls to0 per-cent. At this point it is recommended that unnecessary load be switched off and the remaining charge be conserved for more important purposes.
Another simple combination logic circuit may even be designed that will sound the buzzer when the display shows 9. Further charging ought to be stopped at this point in order to preventive overcharge.
The circuit is powered by the battery under check, by a voltage regulator 1C. The circuit takes about 100 MA for its operation.
For calibrating the upper and lower reference levels, a digital multimeter as well as a variable regulated power supply source are necessary. For calibrating the lower reference voltage, follow the steps given below:
Set the output of power supply source to ten. IV.
Connect the power supply source than the battery.
Now the display will show some reading. At this point vary preset VR2 until the reading on the display changes from one to 0.
The higher reference voltage is calibrated similarly by setting the power supply to 13.8V and ranging preset VR1 until reading on the display changes from 8 to 9.

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