I want to create a low energy project with arduino mini pro
I read about low energy project with arduino pro mini and I understand that I should remove the power led and the regulator so the project will be more power efficient (most of the time it will be in sleep mode and once on an hour it wakes up for a few seconds).
The problem is that I want to support both rechargeable and alkaline batteries
If I take two AA batteries, In alkaline batteries case the board will get a 3 volt and it should work but in rechargeable case the board will get only 2.4 v and that shouldn't enough.
If I take 3 AA rechargeable batteries the board will get 3.6v and with alkaline it will get 4.5 v
I don't know which configuration to chose for getting the most working time (I planned for over a year) with supporting a1.5v or 1.2v AA batteries
If you review the Speed Grade chart for the 328P, see attached, you will see that you can run with a 8 MHz system clock over quite a wide range of voltage. 2.4V would be the lowest voltage to run fully in spec at 8 MHz..
Vout = 1.8V + 0.9 * ((8 - 4) / (10-4) ) = 2.4V
(8 MHz being 4/6 of the way between 4 and 10 MHz, and 0.9V between 1.8V and 2.7V)
Be sure to set the Brown Out detection fuses for a lower voltage to support running at 2.4V if you decide to go that route.
You could also look at a LiPo type battery, with voltage from 4.2V down to 3.7V.
Be sure to review the capacity of the batteries. For example, AA alkaline is about 2500mAH.
I don't know which configuration to chose for getting the most working time (I planned for over a year)
In addition to sleep mode, there are lots of other tricks for reducing power usage to an absolute minimum. Here is an excellent reference for just about all of them.
Id use a step down converter, they are very efficient compared to linear regulators and youd get 3v3 stable.
mart256:
Id use a step down converter, they are very efficient compared to linear regulators and youd get 3v3 stable.
Are they?
Dropping from what voltage?
What quiescent current?
I tried 3 alkaline AA connected to a 3.3 step-down converter, for a Pro Mini 3.3. It sleeps 10 minutes, wakes up, reads the environment temperature, sends it via HC-12 and keeps in standby for 5 seconds waiting for possible messages from the 'MASTER' station. Both the Pro Mini and converter power leds are crushed.
LOL, the test lasted exactly 3 days cause the step down converter needs at least 4 (maybe more) volts in input.
So, meanwhile that I am waiting for some stepdown stepup components ordered online, I removed the step-down converter and the test continues 
I read somewhere that the Pro Mini 3.3 can tolerate up to 4.5-5.0 volts to the VCC pin, the only downside is that the voltage at its I/O pins will vary. Is it right or am I f****** up my Pro Mini?
mart256:
Id use a step down converter, they are very efficient compared to linear regulators and youd get 3v3 stable.
There are linear regulators that have extremely low drop outs and standby currents.
Eladh:
I want to create a low energy project with arduino mini pro
Unfortunately you cannot just consider your project from the standpoint of what the processor on a Pro Mini needs to run.
Its the other stuff, sensors etc, that you needs to consider, they might well have very specific power requirements.
There is little to be gained in knowing that the processor on a Pro Mini can run at 2.4V, if you have sensors etc that need 3.3V minimum.