How to measure car battery capacity

Discussion in 'General Electronics Chat' started by Sonoma_Dog, Sep 9, 2008.

  1. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
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    0
    The only way i know how to do this is to measure the battery output voltage[from 12.7v(100% capacity) to 11.8v(almost dead)]. Is it the only way to measure battery capacity? And does the voltage drop vary from different batteries?

    Thanks in advance
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, 12.7v is considered fully charged, and 11.4v is considered fully discharged, for lead-acid batteries.

    Sulphation begins occuring at about 12.4v. Battery life is shortened considerably if sulphation is allowed to occur. If you keep your battery voltage above sulphation level, you will get maximum life out of it.

    The voltage drop will vary if the chemistry of the battery varies from lead-acid. It is not good to subject a standard automotive battery to deep-cycle use. They are designed for maximum output for very short durations, and then need to be charged immediately.

    Deep-cycle batteries, such as marine batteries (for trolling motors, boat lighting, etc) are designed for slow discharge over long periods. They have a different internal structure from auto batteries. Their plates are thicker. They do not have the surge capacity that auto batteries do. However, they are still vulnerable to plate sulphation, and should be immediately recharged after they are exhausted.

    Testing an automotive battery generally requires a special tester, which would be expensive to purchase. In the States, you can go to an "Auto Zone" store, who will loan you equipment free of charge to perform the tests. You will, of course, have to make a deposit or provide a credit card as security; you're essentially buying the unit for a short time, and returning it for a full refund.

    The tester puts a heavy load on the battery, perhaps 100A- 200A. It is up to you to know beforehand what the amp-hour rating of your battery is, and the "cold-start" rating of the battery. Voltage at the start of the test and the end of the test will give you a good idea of the condition of the battery.

    The test can easily "kill" a weak battery.

    Arm yourself with knowledge beforehand. Unscrupulous mechanics may attempt to bilk you with arbitrarily severe tests that may leave you stranded.

    I had a service technician at a major chain store (which will remain unnamed) test my alternator years ago, that I had just rebuilt. I'd gone there since I was having trouble starting my car, and suspected the battery, which I'd also purchased at that chain store. The technician told me that my alternator was "a little weak". However, I'd watched the technician deliberately overload the electrical system on my vehicle, and I knew from the instrument that he was abusing that my alternator was putting out it's full rating of 65 Amperes.

    Had I not known how to read the instrument, I might have fallen for the technican's story, and been bilked out of a good deal of money for a new alternator. Instead, I said, "A little weak? How do you figure it's a little weak, when it's putting out the 65A it's rated for?"

    I received a replacement battery with no charge.
     
    Last edited: Sep 9, 2008
  3. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
    99
    0
    Thank you for the detailed reply, it makes a little clear now.
    ok, here is what i am trying to build, can you please give me some more advices.

    I am trying to light up several LED lights (LED signs for advertising purposes) when the car is turned off and the LEDs is going to cut off itself after the battery capacity is lower then maybe 70%. I am not sure how much current the LEDs is going to draw, but probably going to be .2 to 5A. I am thinking to use a small microcontroller to control the cutoff threshold and PWM to control the LEDs

    So do you think discharging directly from the automobile battery will do me good in a long run? or should i use a rechargeable battery to operate the LEDs when the car is turned off, and charge up the rechargeable when the car is turned on.

    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Offhand, that seems to be a reasonable approach.

    I would use a separate battery to power the LED display. After all, you want it to be lit for as long as possible, right?

    Use a marine-type battery for the LED display. They tolerate long, slow discharges far better than automotive-type batteries. If the battery has to be inside the car/trunk, you must use a SLA type (Sealed Lead-Acid) battery, or there will be a potentially explosive buildup of hydrogen/oxygen.

    Don't use the primary car battery unless you can tolerate a very short lifetime from it.
    [eta]
    You must separate the primary car battery from the LED display battery using a MOSFET or high-power Schottkey LED.

    This separation must occur when the current is no longer available from the charging system; the primary auto battery must be kept charged near peak at all times.

    Your LED sign battery will require charging also. This is not a trivial item. You must not overcharge it, nor let it fall below a reasonable charge level.
     
    Last edited: Sep 9, 2008
  5. Sonoma_Dog

    Thread Starter Active Member

    Jul 24, 2008
    99
    0
    Thanks for the advices.! looks like i need to use separate power source for this design and I am sure i will need some more help later on.

    Thank you, really appreciate your help !!!
     
  6. eb5agv

    New Member

    Sep 16, 2008
    1
    0
    Hello!

    I am also dealing with a discharged car battery (due to Summer holidays it was left unused for some weeks...) and can tell you that, if you can access the battery elements, best way to know exactly charge condition is to measure electrolyte density. Reason for this is that, as in my case, if one element is bad (or different than the rest), you can't tell from total voltage. Yes, you would find voltage is lower than expected but wouldn't know the exact cause.

    It seems that the battery in my car discharged so one of the elements reached a deeper discharge than the others. After trying to charge ir, all densities were fine (100% charge) except one. Aplying an slow charge (3 Amperes), after 24h, density was about 25% this morning... I hope it will continue to raise up to 90% or even 100%.

    Hope this helps!

    Best regards,

    JOSE
    http://jvgavila.com
     
  7. vinoddips

    New Member

    Sep 26, 2008
    1
    0
    I am planing to desigen one capicty test pannel

    The tester puts a heavy load on the battery, perhaps 100A- 200A. It is up to you to know beforehand what the amp-hour rating of your battery is, and the "cold-start" rating of the battery. Voltage at the start of the test and the end of the test will give you a good idea of the condition of the battery.

    The test can easily "kill" a weak battery.

    Arm yourself with knowledge beforehand. Unscrupulous mechanics may attempt to bilk you with arbitrarily severe tests that may leave you stranded.

    I had a service technician at a major chain store (which will remain unnamed) test my alternator years ago, that I had just rebuilt. I'd gone there since I was having trouble starting my car, and suspected the battery, which I'd also purchased at that chain store. The technician told me that my alternator was "a little weak". However, I'd watched the technician deliberately overload the electrical system on my vehicle, and I knew from the instrument that he was abusing that my alternator was putting out it's full rating of 65 Amperes.

    Had I not known how to read the instrument, I might have fallen for the technican's story, and been bilked out of a good deal of money for a new alternator. Instead, I said, "A little weak? How do you figure it's a little weak, when it's putting out the 65A it's rated for?"

    I received a replacement battery with no charge.[/quote]
     
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