I was wondering if it is at all possible to work out just how long a 9V battery would last if connected to a circuit that is constantly drawing 6.5mA?

Can this be calculated and how?

Thanks Folks -JDR04

 

Depends on the battery and the current. If you look at the 100ma curve posted in this link yours will last longer with the low discharge current. So picking one in the middle the time would be greater than 350ma hours / 6.5 ma or around 53 hours.

http://www.powerstream.com/9V-Alkaline-tests.htm

 

About 10 h. Est. 9 V battery @ 700 mAh/ 6.5mA= 10.7h

I think you skipped a decimal, it would be more like 100 hours. I thought 9V batteries were 500 mA/hr ballpark? That would be around 80 hour ballpark at 6.5 mA.