current limiting batteries

Discussion in 'General Electronics Chat' started by suyash, Oct 30, 2015.

  1. suyash

    Thread Starter New Member

    Jan 11, 2015
    8
    0
    hey guys,

    have a quick question here. there are many commercially available power banks with rated amperages as high as 20000 mah. what i wonder is that they usually have 2 ports one being 1 amp and the other being 2 amp.how is 20000mah limited to 1/2 amp.i know these things have multiple batteries in them but still each has a individual amperage of about 2000 mah or higher.

    are such limiters available to be used with your lipo batteries that can be just attached to the batteries if so what are they marketed as ? and can i buy them online.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    mAh is not a current rating, it is the energy capacity rating of the battery (amperes times hours) and a battery is usually rated at a 10 or 20 hour discharge rate.
    That rating is only loosely related to the maximum current the battery can deliver which also greatly depends upon the battery type and construction.
     
    Last edited: Oct 30, 2015
    recklessrog and shiva007nand like this.
  3. recklessrog

    Member

    May 23, 2013
    338
    102
    Beware though, that the current limit advised on the outlet is stated at a maximum level that will not cause damage. Batteries are able to provide current at sometimes many times their mah (see the post above) rating for short periods if shorted out which can result in wiring/ contacts/ and the batteries over heating and catching fire! lIPO'S especially seem to have a propensity to catching fire!
    Your post seems to imply that you think that that the current is "limited" so that no more than the 1 or 2 amps can be drawn. In most cases this is not so, although there may be a fuse included that will blow in case a short circuit is encountered.
    There are many hundreds of devices that can regulate current, look on this site or google "current regulators, and voltage regulators"
     
    Last edited: Oct 30, 2015
  4. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    As I previously stated, maximum battery current is not directly related the the mAh rating, which is a energy rating for the battery.

    "Many times their mah rating" has no meaning. It's like trying to determine the maximum RPM of an engine from its horsepower rating.
     
    Last edited: Oct 30, 2015
    recklessrog likes this.
  5. recklessrog

    Member

    May 23, 2013
    338
    102
    Yes i could have phrased that differently, but I think the term mah does signify the potential capacity of the battery, I.e in simple practical terms, a 17ah lead acid gel battery in a car booster starter can withstand a very short period of high current discharge to start the vehicle, current, time. My intention was to make the thread starter aware of the possibility to draw excessive current from the battery which could result in damage, fire etc. I just didn't want him to assume that he could connect any load he liked and the current would not exceed the stated maximum permissable.
     
    Last edited: Oct 30, 2015
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Fair enough.
    I just don't want to promote the faulty idea, which seems to be somewhat common, that the battery mAh rating somehow tells you what the maximum battery current is.
     
    Last edited: Oct 30, 2015
  7. recklessrog

    Member

    May 23, 2013
    338
    102
    I absolutely agree, and that was not my intention :)
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    Traditionally; fuses were used - but you have to replace those when they blow.

    The latest thing is the Polyfuse self resetting fuse, its a PTC thermistor with a sharp knee curve. Up to a certain current they have low resistance, but if you increase the current beyond a certain point they start self heating and the resistance rises sharply.
     
Loading...