What consideriations do you take in choosing a battery for a project?

[Copernicus]

Ive been looking at battery options for my project, does it really matter what you choose? 4 AAA's or 2 AA's are basically the same capacity. 9V batterys seem on the low end for mAh capacity. For my project I need a 9V rail and a 40V rail .  I could use a 9V battery for the 9V rail and a boost converter to get the 40V rail, or I could use 2 AAs  and boost 1.5V -> 40V and use a voltage divider to get the 9V rail. Does the nominal voltage of the battery really matter? Any tips?

Thanks!

[Ian.M]

First ask how many watts you need and how long you need it for, then list your supply rails  their current consumption, and whether or not they need to be regulated.

Its more difficult to keep the losses low in a boost converter than in a buck converter due to the higher current in the switching transistor, so a higher battery voltage than the most loaded supply rail is often desirable.

[Brumby]

First ask how many watts you need and how long you need it for, then list your supply rails  their current consumption...

This is the first thing you need to do.

I always get anxious about battery operated circuits and whether battery life will be adequate until those calculations are done.  Wanting to run a 40V rail as part of that makes me wary - unless the current draw is going to be low.

and whether or not they need to be regulated.

This is the second step - and one key purpose is to address the issue of drooping voltages as the battery drains.