Powering Pico with a 3V lithium battery

Hello
I want to power my Espruino Pico with a Lithium battery CR 2450 (3V and probably down to 2.5 V). I read the doc on powering, and I see that Vbat should be > 3.6 V.
Is it possible to power Pico with a 3V Lithium battery ?
On which Espruino pin shall I connect the + of the battery ?
Thanks a lot
GeekBot

Pico chip chip STM32F401CDU6 6.3.1 General operating conditions - p. 60 - and Table 15.
Features depending on the operating power supply range - p. 61 - specify what you can expect... The current schematics gives you protection great operation stability. Feeding of 3V would have to happen very directly - 'behind (regulator) enemy lines' - with no poly fuse and voltage regulator and... I]m not sure if you can connect the battery+ right 'behind' the voltage regulator without permanently removing it...

Thank you allObjects, looking at the schematics, as you say, we see that the 3.3V regulator output is connected on the 3.3V pin. I will have a look at the regulator datasheet to see whether it accepts 3.3V external feeding on the output or not. May be somebody else knows...

Looking at the datasheet, it seems possible. However, a resistor bridge at the output will probably consume some current. No idea how much.
I hesitate to do a try...

It won't hurt to try at all (as long as you don't put the battery in backwards :) ).

I've literally just got some Lithium batteries (1/2 AA cell) this week. Having tried it out, you can connect to the 'Bat' pin quite happily. At low power draws (~20mA or so?) the voltage drop across the regulator is very low (0.02v when I measured at 10mA). Since the Pico will operate down to 1.7v you'd be able to flatten a CR2450 without much trouble.

Powering via the 3.3v rail would also be fine and would avoid the regulator's voltage drop, but you then couldn't use USB or you'd end up putting 3.3v on the battery and charging it

Thank you Gordon for these good news, that is very clear.

@Gordon, eventually I have a doubt; the 'Bat' pin you talk about is the 'VBat' pin (PIN2 of J2 on Pico, called 5V on the schematic) ?
Because there is also the VBAT on the schematic which is the PIN 1 on J1, and called BAT_IN on the Pico coloured diagram on the web site.
According to our discussion, I guess the 'Bat' is the 5V on the schematics, but I need to be absolutely sure.
Thanks a lot
GeekBot

It's the Pin 1 - BAT_IN on this diagram (your second choice): http://www.espruino.com/Pico#pinout

Unfortunately the pin naming is a bit confusing. The other pin will have 5v on it when USB is connected, so you definitely don't want to put the lithium battery on that :)

Thank you Gordon. So I connected on BAT_IN as you suggest, it's clearly much safer and easier to manage with USB connection. The voltage drop is about 0.3 V. That's 10% of the battery voltage, but acceptable for my application. I guess this drop comes from the MOS transistor?
Best regards
GeekBot

Where are you measuring the voltage drop? The FET shouldn't really drop any noticeable voltage. It could be from the voltage regulator, but when I measured there was very little voltage dropped. Are you powering something else as well that might be drawing a bit of current?

@Gordon, thank you, I will do accurate measurements and reporting tomorrow. I come back to you.
Best regards
GeekBot

I did some more measurements.
Battery voltage 2.78V
3.3 V output : 2.5V
LED1 turned on, Espruino running.
I confirm that the transistor has just some mV of voltage drop.
Since, I estimate the current over 35 mA, I have to look at the regulator datasheet to see if it is an expected value.
Anyway, I manage with these figures!
GeekBot