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Circuit to indicate charge remaining in any battery bank

Discussion in 'Power Electronics' started by Migo, Oct 25, 2014.

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  1. Migo

    Migo

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    Oct 25, 2014
    Hi Guys,

    i'm new to this thing.

    i need a circuit that can indicate the amount of charge left in any type power bank. does it exist? where can i find it?

    thanks
     
    Last edited: Oct 26, 2014
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    No.
    You have a circuit that can tell you the current Voltage across the terminals, and can tell you the current amount of amperage it is putting out.
    You can also build a circuit to measure the amperage capability of a power pack...
    In order to tell how much charge is left, you need to determine the charge characteristics of the power pack.
    I suggest you take a look at various discharge graphs for different battery chemistries.

    If you build it for a specific type of pack, you can determine the remaining charge by looking at the voltage when the pack is under load. This same circuit will not reliably work across different battery chemistries though, and will be even worse for detecting charge level left in a supercapacitor power bank.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Estimating the amount of energy in a battery is difficult because of factors such as temperature and the age and condition of the battery.

    You can try to estimate it by measuring the battery's terminal voltage, with no load and/or with a known load, but both of those factors affect the result you will get.

    You can also make the battery "smart" by adding circuitry to it that monitors the current in and out (as well as the terminal voltage and the temperature), so it knows how much energy has been taken from it, and keeps track of the age and number of charge/discharge cycles the battery has undergone. This will give a more accurate answer than simply measuring the terminal voltage but is much more complicated to implement.

    For both of these methods, you need to know the battery chemistry - different types behave quite differently - and the pack capacity.
     
    hevans1944 likes this.
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Linear Technology offers integrated circuit battery management devices capable of operating with any common battery chemistry, the LTC-4020 for example. The user selects the chemistry and charging method during the application design phase. These devices are complicated and should be used with a thermistor attached to the battery to prevent overcharging and to prevent attempting to charge a defective battery. Application specific information is provided in the datasheet.

    These are not devices for the faint of heart or inexperienced, although they are not as sophisticated as the systems @KrisBlueNZ mentions that track battery usage by monitoring the charge/discharge cycles. Such battery management systems are generally a part of an overall system design that requires reliability and expense is not a factor. Typically, portable equipment that spends a lot of time idle, but must be instantly available when needed, is a candidate for such designs. Think heart defibrillators in ambulances. There are also specialized battery packs with monitoring electrodes for use with specific chargers designed to operate with, and only with, these batteries. If you have a particular battery in mind, the manufacturer can recommend a safe charging algorithm.

    Supercapacitors are a whole different ball game.
     
    Harald Kapp and KrisBlueNZ like this.
  5. Migo

    Migo

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    Oct 25, 2014
    thanks guys for the insightful input. they are really helpful
     
  6. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    "You can also build a circuit to measure the amperage capability of a power pack."

    How can you do this ????? I will buy 1,000 of these !!!!!!.
     
  7. Gryd3

    Gryd3

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    Jun 25, 2014
    The idea is simple, but implementing it may not be.
    You need to put a large load on the battery and watch the voltage on the battery.
    If the terminal voltage drops too low, you have a pretty good indication of the amount of current the battery / pack can put out.

    This is not an exhaustive test, and will not give you precise numbers. The 'too low' point of the voltage drop will depend on your operation.
    This would also be an invasive test, as the battery would discharge at a fast rate during the test.
     
  8. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    That's what the carbon-pile battery testers do but they are big, expensive and dissipate an enormous amount of heat.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    The "full load" battery test is available free at Auto Zone to allow you to determine whether your lead-acid car battery is capable of cranking the engine. Monitoring the battery terminal voltage while heavily loading it does provide some indication of its capability, but IMO it is only a go/no-go test useful for deciding whether or not to replace the battery. The manufacturer determines the capability of a battery (or a power pack) during their design phase, and then they label it so the consumer knows which one is appropriate for their needs. After that it is all down hill as the chemistry eventually fails to maintain charge and deliver rated maximum current after recharging.

    NiCd cells are prone to "memory" effects, refusing to accept a full charge after several charge/discharge cycles that do not "empty" the cell. It's almost as if there was a little guy in there monitoring the usage, and when he sees you are only using, say, ten percent of the rated capacity, then he makes sure that is all you get: ten percent of rated capacity. Some of the "smarter" chargers will discharge NiCd cells before attempting to recharge them, an effort to defeat the "memory" effect. Better to use a different battery chemistry and avoid NiCd altogether if you need a reliable power source. I like Li-ion technology, but there are some "gotchas" associated with that too. Caveat emptor.
     
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