Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does V or I remain constant in a discharging battery?

  1. Jul 8, 2015 #1
    In a discharging battery, do current and voltage decrease over time? if so how does the battery accomplish the same amount of work over the course of its discharge with a lesser voltage or current?
     
  2. jcsd
  3. Jul 8, 2015 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    Voltage stays more or less the same until the point where the battery is no longer able to supply the current required by the load, at which time the voltage starts to decrease.

    I suspect you can find full expositions about all that using Google.
     
  4. Jul 8, 2015 #3

    berkeman

    User Avatar

    Staff: Mentor

    Note that at higher output currents, the internal battery resistance is more of a factor. You can find discharge curves like the one below by looking at the battery's datasheet...

    http://www.steveduncan.net/assets/images/mfg_curves.jpg
    mfg_curves.jpg
     
  5. Jul 8, 2015 #4
    The discharge characteristic of batteries varies according to the type of battery, its capacity (Amp-hr rating) , the age of the battery. temperature. The graph in @berkeman post looks like one for a Lead Acid car storage battery. Their voltage typically drops to 90% of initial voltage at full discharge. In fact the terminal voltage is used as a measure of the state of charge of the battery. Mercury and silver batteries will maintain the same voltage until they reach their capacity. The voltage of alkaline batteries loose drop about 15% after using 50 % of their capacity.and about 35% at full discharge. And the normal dry cell Zn-C battery voltage drops 50% at 80% discharge.

    So the energy delivered with time drops with discharge for many batteries.
     
  6. Jul 8, 2015 #5

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    Just to make it clear, the actual current from the battery depends on the circuit it is connected to more than the battery characteristics.

    Constant power load circuits are possible, but unusual. In most cases, the battery will not supply constant power during discharge.

    On second thought, I use a nearly constant power load every day. It is a 12v refrigeration system. The lower the voltage, the longer the compressor runs each cycle. Averaged over a day, the power is nearly constant.
     
    Last edited: Jul 8, 2015
  7. Jul 8, 2015 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    It helps to get things the right way round in this sort of problem. Assuming the load is the same value all the time (a constant resistance), it will be the Volts that determine the Current that will flow (cause and effect) and Power delivered will be V2/Rload.
    As a battery discharges, the (unloaded) Volts will drop towards the end and the internal resistance could also be effected. If it increases, the terminal volts of the battery under load can fall faster than the unloaded volts would. So a battery cannot be expected to deliver ("accomplish") the same amount of power over its discharge period without some help. If you want to achieve this, you need to have some sort of regulation circuit which will supply a lower voltage than the raw volts from the battery (as per a normal voltage regulator).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook