12V Battery LED Level Indicator

Discussion in 'The Projects Forum' started by hazim, Mar 12, 2011.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Hi.

    Here is an easy circuit I designed today for a UPS 200Ah battery at home. I'd like to share it to benefit others. Undoubtedly, this circuit measures any 12V battery types and sizes.

    The circuit consists of two quad op-amp ICs, the LM324, a voltage regulator, LM7808, several trimmer potentiometers (pots) and resistors and the LEDs. I also added a buzzer (I put a lamp instead in the circuit diagram) to warn for low battery, 'Warning'.

    When the battery's voltage is 15V and above, the (top) red LED (LED1) lights indicating for overcharging. As the voltage goes down, the LEDs turns off gradually from the top till LED7. When the voltage decreases under 11V, LED7 turns off and the lower red LED (LED8) turns on indicating that the battery if empty. The buzzer (or warning lamp) starts when the voltage is between 11V and 10.5V, under 10.5V it stops and the "Empty LED" turns on.

    [​IMG]
     
  2. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Hi.
    This circuit should work fine as I have tested on Multisim. But now after I build it I have something wrong. On potentiometer R1, the voltage on the middle pin that is connected to the op-amps inputs changes suddenly from around 12V to around 1V when I try to set it to 8.25V. This potentiometer is a voltage divider I should be able to adjust the voltage I need but there is something wrong. I thought the pot. is bad so I tried another one.. same problem.

    I checked the wiring, they are right. This problem happens if there is a short circuit or if the input of the op-amp drains a real current where it's not like that. None of the two ICs become hot assuming there is a bad IC.

    Any idea? Later I will try to measure the current at the inputs of the op-amps connected to the potentiometer.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You have pin 3 connected to pin 4 so the pin 3 input transistor is completely turned off.
    You are trying to adjust the input voltages to a voltage that is way too high.

    The input common mode range for the inputs of an LM324 is 1.5V less than the pin 4 voltage so the inputs probably will not work properly in your circuit where their voltages are the same or higher than the pin 4 voltage.

    Instead of using millions of parts, why don't you use an LM3914 dot/bar LED voltmeter IC? It has a resistor ladder, 10 comparators, an adjustable voltage reference and 10 LED drivers with a regulated output current without needing current-limiting resistors.
     
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  4. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
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    Thank you Audioguru. You are right concerning LM3914, I have LM324 and the other parts so I did it using parts I have..
    Now I have already built it, do you think connecting Vcc of the ICs directly to the battery (before the regulator) will solve the problem?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Simply reduce all the input voltages of the opamps to below the regulated 8V supply to the opamps minus 1.5V.
     
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  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your circuit is going to add extra load to your battery, draining it faster - not much compared to your inverter of course - however, you should consider having only one LED on at a time instead of all of them.

    An LM3914 will make this a lot easier to do using dot mode. Otherwise, you'll need to make window comparators for each voltage range.

    Don't use an incandescent lamp; they are very power-hungry.

    Draining your battery to 10.4v will be very hard on it; that's 100% discharged, and your battery service life will be very short.

    Look in the Electronic Tips & Tricks thread, reply #38. Link:
    http://forum.allaboutcircuits.com/showpost.php?p=262143&postcount=38
    I attached an image of a spreadsheet giving voltages vs charge vs temp and battery life hints.
     
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  7. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Use 2 or 3 series diodes to drop the voltage?

    SgtWookie, I'll use a buzzer not a lamp, I said that before. Also the buzzer will start at around 11V which will indicates that one should turn off the computer.. I also replied earlier on using LM3914. The circuit will not turn off the inverter.. When the voltage is less than 10.4V the last LED will be ON indicating that the battery is completely empty. I may increase it to 10.6V or so just by adjusting the pot. Thank you for the link.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    Your circuit has many timpots. All their voltages are set too high especially the first one. Simply turn them all down so that an opamp input voltage never gets above +6.5V.
     
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