Hello Forum,
I am using a 3.7V Li-Po battery for a project using an Arduino Pro Mini 3.3V module.
The spec on the Pro MIni 3.3V for Vraw is 3.4V - 12V.
Once the battery life depletes BELOW 3.4V, let's say 3.1V, will the 3.3V voltage regulator have an issue with this or will the output just be 3.1V?
Or should I connect the 3.7V Li-Po battery directly to Vcc? The spec says Vcc max is 3.6V.
TIA,
--Neal
The regulator cannot put out more voltage then it is given. It also cannot put out the same voltage it is given. There if a forward voltage drop sometimes referred to as head room or drop out voltage. It sounds like you may want to get an inexpensive SEPIC or Boost converter then you can use the last of the energy in the batteries and not worry about over voltage.
Well, connecting the battery directly to Vcc will work just fine. The Atmega328P in that module is the same as the one in the 5V Pro Mini, and it will work fine up to 5.5V. So to the first approximation, that setup will give you longer battery life because you avoid the voltage drop across the regulator as the battery becomes discharged.
But it's also the case that the 328P will draw less current running at 3.3V than it does running at, say, 4.0V. So that that would give you shorter battery life. Which one of these prevails will depend on how the Pro Mini is used, but it probably isn't going to make a significant difference either way. I usually remove the regulator and the power-on LED, and connect the battery to Vcc.
But if you're going to connect the battery to Vcc, there's a good argument for removing the regulator because otherwise some current will flow back through the regulator to ground from Vcc.
geometrix:
Or should I connect the 3.7V Li-Po battery directly to Vcc? The spec says Vcc max is 3.6V.
What "spec" says that?
Not this one - which is misleading in any case. 
Hey Paul,
Thank you for your reply. I looked all over for the range for Vcc and I saw this "spec" on one website. I cannot find it at this time.
So what is the range for Vcc? Can I connect a 3.7V battery to Vcc? And then what is the minimum Vcc that will work with the Arduino Pro Mini?
I have 2 other 3V components in my project, so I figured that the Pro Mini 3.3V module was the best fit for the project.
If I use a Pro Mini 5V and then use a voltage divider to supply voltage to the 3.3V Bluetooth module and 3V vibrator motor, I will need a booster for my 3.7V Li-Po battery to power the 5V microcontroller. I think that this is what Gilshultz was suggesting.
Thank you.
"Vin" is useless - and is so for most projects. If operating from a 3.7 V battery remove the useless regulator - and the "pilot" LED as it wastes power.
Connect your battery to the "5V" pin and ground. If the Pro Mini is the 3.3 V version, it should have a clock of 8 MHz and it should be loaded with the 8 MHZ bootloader which also sets the "brownout" threshold to a lower value which is - I think - somewhere around 2.5 V. Someone else may know exactly. 
For the 5 V option - you cannot use a voltage divider to power a Bluetooth module, you would need a LDO regulator. As to a vibrator motor, the ones I bought recently apparently will tolerate higher voltages (than 3.7 V) for short periods, or you could use a (single, series) resistor or a couple of diodes to drop the voltage from 5.
Just for the record, a 3.7V lipo is rarely 3.7V when you're using it because it's 4.3 V when it's charged.
So when it is fully charged, it is barely enough input voltage for the 3.3V regulator.
If you have a Vin pin that can take 9V in , use a 2S Lipo (7.4V 2 cell lipo that charges to 8.4V.
That should work fine. Just don't let the Lipo get below 7V or you'll damage it.