If you want to figure out how long a particular battery will last with a particular load, use the Peukert calculator on Uve's battery page:

http://www.geocities.com/CapeCanaveral/Lab/8679/battery.html

The calculator (towards the bottom) uses Peukert's equation to calculate the capacity of any lead-acid battery for any load. It works quite well, actually.

You need the capacity under two different loading conditions, like the 20 hr rate capacity (generally the a-hr capacity given on the label) and the "reserve capacity" which is the length of time the battery will hold a 25 amp load.

The page lists a lot of batteries, but not many that you would use for an airplane.

If you want to know the details about Peukert's equation, you can find them here:

http://www.thermoanalytics.com/support/publications/batterymodelsdoc.html

Once you find the Peukert exponent using the calculator on Uve's battery page, you can use it in Peukert's equation in your calculator to find the capacity at any discharge rate.

I should note that temperature makes a huge difference in the capacity of a lead-acid battery. Thus, if you have a very cold battery, you will have only fraction of the capacity that you would have at room temperature (or above).
Bill Dube <LED@Killacycle.com>
http://www.killacycle.com/Lights.htm