Connect with us

How do chargers determine what battery pack is installed?

Discussion in 'Electronic Repair' started by DaveC, Jul 18, 2004.

  1. Ian Stirling

    Ian Stirling Guest

    With all modern semiconductor based power sources, the compliance
    voltage is a few tenths of a volt below the maximum output voltage.

    The number of powertools whose multi-battery constant current charger uses
    a resistor, and a power supply of many times the voltage (apart from the very
    cheapest untimed low power ones) can be probably counted on the fingers of
    one ear.
  2. Ted Edwards

    Ted Edwards Guest

    Don't know what sort of constant current sources you use but the ones
    I've designed and used have a fixed voltage supply, Vs, driving an
    electronic current limiter. The output voltage remains approximately
    constant at Vs as long the load draws less current at that voltage than
    the rated current, Io. Any attempt to draw more than Io results in the
    voltage dropping. You thus get a nice square V/I curve. For a charger,
    you simply need to choose Vs greater than the max battery voltage you
    wish to charge.

  3. I read in that Ian Stirling>) about 'How do chargers determine what battery pack
    is installed?', on Mon, 19 Jul 2004:
    Sure, but I was responding to a purely technical point, not a practical
  4. I read in that Ted Edwards
    'How do chargers determine what battery pack is installed?', on Mon, 19
    Jul 2004:
    That isn't a constant-current source. It's a constant-voltage, current-
    limited source.
    Read all the thread. Your comment is true but irrelevant.
  5. Bob Wilson

    Bob Wilson Guest

    Naturally. That was the assumption.

  6. Bob Wilson

    Bob Wilson Guest


    Another way to state John's point is that a theoretical current source
    generates whatever voltage it needs to (no matter how high) in order to push
    the specified (constant) current through the load. If the load's resistance (or
    in this case its terminal voltage) is very high, the current source
    automatically adjusts for this.

    A high "Compliance voltage' is just another way to say that the current source
    is capable of a very large range in output voltage, should this be necessary to
    ensure meeting the required output current.

  7. DaveC

    DaveC Guest

    Could this possibly be one way a charger designer determines that a battery
    pack is faulty? If the internal battery resistance rises with age (true?),
    and the compliance voltage is high as the charge current is applied, the
    charger's controller could determine that this pack is out-of-spec, and
    decide to not charge it?
  8. Rich Grise

    Rich Grise Guest

    John Woodgate, I'm shocked! Please check your maths here[0]. I'll let
    this slide for now, because it's very late. ;-)


    For those who don't get it:
    On my constant-current charger with sufficient voltage compliance,
    V0 adjusts itself to Vbatt + (Ibatt * Rcontact), so unless your
    contact resistance is something like a light bulb, Vo doesn't
    need to be very much greater than Vbatt at all.
  9. I read in that DaveC <dave-
    > wrote (in <[email protected]>) about 'How do chargers determine what battery pack
    is installed?', on Mon, 19 Jul 2004:
    Yes, it could, but it won't charge it anyway, because the battery
    voltage is presumably too high to allow the normal charging current to
    flow, unless the charger accepts batteries with a large range of nominal

    I have an old charger for wet lead-acid batteries that works from a
    single 2 V cell up to 24 V. It has a 'constant-current' transformer
    (high leakage inductance) and the compliance voltage is 32 V.
  10. I believe this method will either leave both under-charged or destroy a
    NiMH cell. The generally accepted (rapid) charge method for NiMH is a
    constant current with a temperature cut-off. During charge perhaps 10%
    of the energy goes into heating the cell. After charge 100% goes into
    heat so it's rather easy to sense this difference. This method works
    well for both technologies. V or delta-V sense works for NiCd, but is
    highly "not-recommended" for NiMH.
    Yes the "Rechargeable Batteries Applications Handbook", originally
    published (and given away[*] to customers) by Gates Energy is/was a
    very good reference manual. I believe the rights were sold (Stoneham?)
    and Gates itself bought (?). The book is available on Amazon:

    [*] who swiped my copy?!
  11. Ted Edwards

    Ted Edwards Guest

    Any constant current source that does not have infinite compliance is
    exactly that except your scheme results in a much sloppier arrangement.
    I bet you say that to everybody who disagrees with you.

  12. I read in that Ted Edwards
    'How do chargers determine what battery pack is installed?', on Wed, 21
    Jul 2004:
    I don't just disagree with your opinions; your definition of 'constant-
    current source' is simply and completely wrong. For load where the
    voltage is constant, the current is ipso facto not constant.
  13. Ted Edwards

    Ted Edwards Guest

    Quoting from your post:
    "No. A theoretical current source develops an infinite voltage on no-
    load. This is inconvenient, so practicable current sources have an
    internal impedance Z in parallel with the current generator, so that the
    no-load voltage Vo = IZ, I being the constant current. Vo is the
    compliance voltage.

    The current I can be achieved (approximately) only if Vo is very much
    larger than the battery voltage."

    So your description has an open circuit (as seen externally) voltage
    Vo. As you apply load, the voltage out drops in rather sloppy voltage
    as the external load current increases. Eventually, when the load
    current forces the output voltage down so that V<<Vo, the current will
    approach a limiting value.

    There is no real difference between a current limited voltage source and
    a voltage limited current source other than the portion of the V/I curve
    over which it is designed to operate.

  14. I read in that Ted Edwards
    'How do chargers determine what battery pack is installed?', on Wed, 21
    Jul 2004:
    I expect you know what that means, but since I *defined* Vo as the no-
    load voltage, your assertions that Vo changes when a load is connected
    is not sensible.
  15. Ted Edwards

    Ted Edwards Guest

    Are you incapable of discerning the difference between "Vo" and "output

    This is getting rather silly and I'm rapidly becoming convinced that it
    has been many years, if ever, since you designed and _built_ a practical
    current source.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day