Hi!
I have a project running with a ProMini 3.3v/8MHz, and it runs from a single Li-Ion cell (NCR 18650).
It seems to run ok, but the voltage might get as high as 4.2v when charging the battery, is that ok?
What is the voltage range for a Pro Mini, what is e.g. different between the 3.3v and the 5v versions (except for clock frequency)?
I cannot see (by the datasheet) that the ATmega328 has different voltages (1.8-5.5v)?
BR
-Alf
It's on the Pro Mini product page, under specifications. 3.35-12 V (8MHz/3.3V model) and 5-12 V (16MHz/5V model). The on-board regulator supplies the appropriate constant voltage for processor stability. ATmega328 datasheet won't know about how the Pro Mini development board was designed.
The only differences are the clock frequency and the onboard regulator. If you take the regulator off you can use any voltage between 24V - 5.5V on the 3.3V pin.
And when connected with the battery directly to Vcc it may be anything between 2,4V and 5,5V. It's still the same chip. It only needs a minimum of 2,4V to run stable at 8MHz and a minimum of 3,8V to run stable at 16MHz. The /3.3V only indicates that the onboard regulator is of the 3,3V type.
@Jiggy-Ninja , lost a decimal 
Thanks for quick and "spot on" replies, then I seem to be ok with my battery hookup.
I've removed the regulator (and the power led) to save power, thus the battery is connected directly to the VCC-pin, not the raw power input.
This together with sleep mode in the code gives really low power consumption (~4uA) and long battery life!