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

Power consumed during Battery charging

  1. Jan 24, 2012 #1

    I have a basic query, regarding the charging and usage of battery.
    When I charge a battery it needs some energy, say 100 watt. Does the charged battery now will provide me back 100 watt (or more or less)?

    My confusion is specially due to, we use invertors in most of the places to overcome load-shedding (power cut to save energy).

  2. jcsd
  3. Jan 24, 2012 #2


    User Avatar
    Science Advisor

    Battery charging is not very efficient.

    Even power that actually reaches the battery is not all recovered.

    Depending on the battery charger, there may be large losses in the charging process as well.

    Normally, there will be some type of current limiting and this can consume a lot of power.

    More modern chargers using switch mode techniques are better but there is still some loss.

    Usually we accept these losses for the convenience and portability of batteries.
  4. Jan 24, 2012 #3


    User Avatar

    Staff: Mentor

    Hmm - I was under the impression that battery charging was pretty efficient. Like on the order of 90%.
  5. Jan 24, 2012 #4


    User Avatar
    Science Advisor
    Gold Member

    Every battery has an internal resistance. You can think of an actual battery as a perfect battery in series with a resistor. As you charge the resistance converts some of your charging energy to heat. (note that watts = power = energy/time = Joules/sec).

    Say you're charging a 10V battery at a rate of 100 watts (stored energy per time). For a perfect battery that would require you apply 10V at 10 Amps. But if the battery has a 1 ohm resistance you'd actually need to apply 11V meaning you'll need to supply 110watts of power to get 100watts to the battery. The remaining 10 watts goes as waste heat.

    But that's not all! You again loose energy discharging the battery. But that's is a variable amount. If you drain the battery at low power for a long time you get better efficiency than if you drain it quickly at high power. That's why EV drivers with a lead foot dramatically shorten their vehicle range.

    Finally the chemical process releasing energy in the battery also occurs slowly when not in use and you loose energy over time with the battery just sitting. In effect there's also a very high short circuit resistance.
  6. Jan 24, 2012 #5


    User Avatar
    Science Advisor
    Gold Member

    That depends on the battery design. Also note over charging (which is bad for some batteries) will also waste energy. Once the battery is fully charged you're getting 0% efficiency in the energy you add.
  7. Jan 24, 2012 #6


    User Avatar
    Science Advisor

    Most of what I have seen is anecdotal, but I imagine that efficiency figures for different battery types is available.

    I know that with Lead Acid and also NiMH, the amp-hours in always has to exceed the amp-hours out by about 30 %.

    I did some detailed tests on this for NiMH batteries where I charged up batteries for varying times and then discharged them while they were operating a mechanical clock. When the battery was flat, the clock stopped.
    I could reach a charging time when more charge was entirely wasted.

    As Jambaugh mentioned, the routine practice of overcharging batteries just to make sure they are charged is very inefficient as well as destructive to the battery.

    This doesn't even include the losses in the usual battery charger where current is controlled by a simple resistor in some cases.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook