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Calculation of power for battery charging

  1. Sep 12, 2008 #1


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    Hi - I am trying to calculate the power required to charge a battery in order to compare the energy requirements of an electric scooter versus an internal combustion scooter. The electrical scooter specifications state:

    battery: 12V/50AH x 4
    time to 90% charge: 4.5 - 6 hours
    charger: 400W / 7A

    Am I correct in assuming that to recharge a discharged battery to 90%, it would take 2.4 kWh (i.e. 400 W x 6 h)?

    Thanks for any advice you can offer.

  2. jcsd
  3. Sep 13, 2008 #2


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    I think when you recharge a battery it is only about 50% efficient. I think your calculation is right.
  4. Sep 13, 2008 #3


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    Staff: Mentor

    I always thought battery chargers were more efficient than that. In any case, your numbers give 135W for the battery and 400 for the charger, an efficiency of 34%.

    When someone asks me a question that involves charging efficiency (such as evaluating an electric vehicle), I typically use something like 90%. But I don't really have any basis for that.
  5. Sep 13, 2008 #4


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    I guess we have to assume you are charging a lead acid battery.

    Given that assumption, your assumption is not correct.

    A good charger will have 2 stages:
    1. Maximum current with a rising voltage
    2. Maximum voltage with a descending current

    So the charger will not be operating at the rated power and current over the full charge.

    Without having a http://www.thinkgeek.com/gadgets/travelpower/7657/", you can estimate the power transferred to the battery by graphing the voltage and current about every 30 minutes.
    Last edited by a moderator: Apr 23, 2017
  6. Sep 13, 2008 #5


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  7. Sep 15, 2008 #6


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    Thanks for all of the replies. The problem I have is that I do not have the batteries and charger to actually measure the power demand during a recharge cycle. I am trying to estimate this so that I can calculate (again, an estimation) the equivalent CO2 emission of electricity generation versus the same for a gasoline engine. I already know the CO2 emissions per kWh for the local electricity supply and so need to estimate the kWh required to charge the e-scooter battery.

    Best regards,

  8. Sep 22, 2008 #7
    Chargers might reach 86% efficient but the charging process is a similar figure and then there are losses in the vehicle particular if 3 phase AC motors are used.

    You must not discharge a battery too much otherwise it wont last long. They has been a debate what the Ah means?? Real useful power without demaging the battery or a 'theoretical' figure if the battery was 100% discharged.

    Then there are transmission losses from the power station of 7 - 10%..

    Long chains involving energy conversions add up to large overall losses.
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