How to improve standby time of 12V Lead Acid Battery

Discussion in 'General Electronics Chat' started by tvibakar, Jul 22, 2011.

  1. tvibakar

    Thread Starter New Member

    Jul 22, 2011
    2
    0
    Hi,

    I am new to this website. This is my first post to this site. I am using emergency lamp with Lead Acid battery. The lead acid battery is of 12V 7.2AH 12AL007 SMF-VRLA and its manufacturer is Amaron-Quanta. I am using this emergency lamp for more than six months. When I started using, the battery was fully charged and the standby time was more. But after six months, even after the battery was fully charged, the voltage was dropped. I don't know the reason. I inquired about the lamp manufacturer. He assured that there is no defect in the emergency lamp. I think there is defect in the battery. I saw also posts pasted in other websites by the customers that the lead acid battery wasn't withstanding for longer times.

    I want to know how to increase the withstanding time of battery and to reduce the voltage dropping. Please help me in this problem. I enclosed the charging circuit as an attachment with this mail.

    Thanks in advance for your help.

    Thanks,
    Thomas.
     
  2. pistnbroke

    Member

    May 9, 2011
    32
    1
    The lead acid battery has a self discharge of something like 1/3 % per day ..you will not stop this unless you freeze it ( no dont try ) you need a top up charge say once a month or leave it connected to a float charger ..the voltage is on the battery ..probably 13.8v
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I received your E-mail. As my signature line states, I do not help people via E-mail. These forums are not interactive, and it may take more than a day to receive a reply.

    Images posted inside of Word documents are not convenient, as one has to download the file, and have Word installed on their computer. It takes much longer to do that than if one simply uploads a .png-format image file; as the image can be viewed directly with a Web browser.

    I have extracted the schematic from the Word document:

    [​IMG]

    The charger will work OK as long as the float voltage level is set properly for your ambient temperature, and as long as it has input power. R1/Q1 controls the maximum charge current; 0.5 Ohms will give a max current of around 800mA.

    The proper battery float voltage decreases as the battery internal temperature increases. The charger design does not include compensation for temperature variations. The basic float voltage should be around ~13.6v at 25°C/77°F, and decrease at a rate of 18mV per 1°C increase of battery core temperature.

    Also, if mains power is lost, the charging circuit will tend to discharge the battery via R3/R2/R5 (nominal 10mA-11mA current), and through the regulator itself. While that is not a lot of current, it will drain the battery if the charger is not powered for extended periods of time.

    Overcharging a battery (too high of a voltage) is worse than undercharging it, but either will reduce the life of the battery.

    Deeply discharging the battery will kill it faster than anything else. For the longest service life, you should not discharge the battery below 80% of full charge, but people routinely exceed that because they don't know.

    If you routinely discharge the battery below around 50% of full charge, you will reduce the battery life by 2/3 from the expected life.

    If you routinely discharge the battery completely, it will be dead with only a few charge/discharge cycles on it.

    The worst thing you can do to a battery is to discharge it completely and leave it discharged for long periods of time in a hot climate.

    [eta]
    Here is a link to the datasheet for your battery: http://www.quanta.in/images/pdf/smf-battery-12V-7.2Ah.pdf
    Same manufacturer, same part number that you specified with your post.
    Datasheet was found on this page: http://www.quanta.in/products.asp
    Look at the "Cycle life in relation to depth of discharge" chart on the 2nd page.

    On the 1st page, under "Characteristics", look at the "charge method (constant voltage)" area.
    For trickle use (which is what your charger circuit is), they only give 13.5v to 13.8v @0.72A maximum. This is for maintaining a charged battery.
    For cyclic use, where the battery is drained and then re-charged, they suggest a maximum current of 1.8A and charge up to 14.2v to 14.4v. When that terminal voltage is reached, the trickle use parameters should be used to maintain the battery.
     
    Last edited: Jul 22, 2011
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  4. tvibakar

    Thread Starter New Member

    Jul 22, 2011
    2
    0
    Hi SgtWookie,

    Thank you very much for your reply and valuable advice. As per your mail, the charging circuit will tend the battery to discharge through R3/R2/R5. To avoid this we are using a diode 1N5408 before the end of battery. This can help the discharging of battery. And also, I forget ti tell a thing yesterday. I am sorry for that. As per the manufacturer of emergency lamp we are using, the battery will not discharge completely. At 12V power supply. the lamps glow. When lamps continously glowing, the battery discharges. At a point of 9V, the lamps are switched off. Then the voltage of battery increases and it gets charged. And also, the emergency lamp setup with the battery is always connected with the AC supply. Once the battery reaches 9V, it gets charged. It is indicated by the LED in the unit. Once the battery gets fully charged, the LED is switched off.

    This is the setup we are using as per the manufacturer's unit. I use this setup for the past six months. There is no problem in the standby time of battery. But nowadays, the discharging happens frequently. From the data sheet you sent, I checked the charging current while charging the battery. It was 0.72A when it was started charging. When fully charged it was 0.65A. Please help me in this situation if I have to change my charging circuit or lead acid battery. If I have to change the charging circuit, please tell the component which has to be changed.

    I enclosed the charging circuit with the diode as an attachment with this message. Kindly pardon me if you have any problem in opening the attachment. Please help me.

    Thanks,
    Thomas.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    You're discharging your battery too deeply.
    Those sealed lead-acid batteries are considered 100% discharged when the voltage per cell is 1.75v @ 25°C; which makes the total battery voltage 10.5v @ 25°C.

    If you discharge the battery 100%, or down to 10.5v @ 25°C, it will last only around 200-250 charge/discharge cycles. You are discharging it far more than what is considered 100%, which is why your battery life is so short.
    If you discharge the battery 50%, 11.65v @ 25°C, it will last 400-500 cycles.
    If you discharge the battery 30%, 12.11v @ 25°C, it will last 1100-1200 cycles.
    A completely charged battery will measure 12.8v @ 25°C.

    The 100% voltage reading @ 25°C will need to be adjusted for differences in temperature, at a rate of -3mV per cell per 1°C difference from 25°C; since your battery contains 6 cells, that's -18mV per 1°C difference from 25°C.

    If your light is not lasting long enough with discharging it 30% to 12.11v @ 25°C, then you need a battery with greater AH capacity.
    To determine what size battery you need:
    1) Time how long it takes to discharge the battery from 12.8v down to 12.11v with your load.
    2) Then divide the time you need the light to be on by the time that a good battery powered the light.
    3) Multiply that result by the AH rating of the battery used in the test.
    The result is the AH rating you need.
     
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  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I've attached a schematic and simulation of a circuit very similar to what you are using now, except this one has compensation for temperature variations.

    Q2 must be a 2N3904 or PN3904. For the circuit to work properly, Q2 must be thermally coupled to the battery positive terminal. If you cannot thermally couple the transistor to the battery positive terminal, the float voltage will not be correct.

    I used a modern Schottky diode instead of the ancient 1N5408 silicon diode.
     
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