Simple question regarding the Ah rating of a battery

1. May 12, 2013

motivehunter

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?

2. May 12, 2013

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.

3. May 12, 2013

motivehunter

Thank you so much!

4. May 12, 2013

Dundeephysics

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.