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Simple question regarding the Ah rating of a battery

  1. May 12, 2013 #1
    Hi all!

    Excuse the simple nature of this question, but i'm a mechanical engineer working on building his own PAS bicycle and need to understand this to go ahead with my work.

    I understand that a 12V 7.2 Ah battery ideally discharges 7.2 ampere of current at a nominal voltage of 12V for 1 hour. But in reality, it is only capable of doing this for 45 minutes.

    So am I correct in assuming that as the battery is an SLA (sealed lead-acid), if the application demands 20A, it will deliver the current at nominal voltage for 16.2 minutes? I mean, is this relationship linear?

    Thanks in advance!
  2. jcsd
  3. May 12, 2013 #2


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    Staff: Mentor

    The concept of ampere-hours does lead you to expect that. But in practice, the heavier the current, the less realistic that is. From a 7.2AH battery you may get only 6 to 10 minutes at 20A before the voltage sags too low to be useful.
  4. May 12, 2013 #3
    Thank you so much!
  5. May 12, 2013 #4
    In theory its linear yes, again just like NascentOxygen said it will drop too much after 10 minutes which makes it useless. If the manufacturer says it discharges that in 1 hour in practice you need to consider less than that, probably even less than 45 mins. It also depends on the battery's age.
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