Powering the LoPy with 2 AA batteries?

[mahe]

Hi,

I wan't to power the LoPy with 2 AA (or AAA) batteries.. I was wondering if it might be possible to power it directly on the 3V3 pin ? (Pycom note: DO NOT POWER THE LoPY from the 3.3V pin, that pin is only output) The ESP32 should work down to 2.3V as far as I understand, how about the rest of the hardware? Disclaimer; I'm not an electronics expert, so please bear with me if this is a completely stupid question. :)

Thanks!

[livius]

@mahe
First 3V3 pin is output only, do not use it for powering board.
Instead of use 5V pin, it have feature to baypass regulator if voltage is small (if less then 3.4 V and have voltage drop <60mV)
and i wonder if 3V will be enought then.

[robert-hh]

@mahe You can run the LOPY from the Vin pin reliably down to 3V. But AA batteries are not good. Two AA would supply between 3.2 down to 2 V, and at the current the LoPy daws the voltage would almost all the time below 3V. If you want to do that, you should place an buck converter in between, like the Pololu S7v8F3 Step-Up/Step Down converter, which provides 3.3V output for an input range of 2.7-11.8V, which give you a lot of freedom for supplying. So you could also use 3 or 4 AA's, 3 or 4 NiMH batteries, or a single Li-Battery or LiIon rechargaeble battery.

[mahe]

@robert-hh Thanks! Just to be clear: when you say the Vin pin, it's the 5V on the pin out? https://docs.pycom.io/chapter/datasheets/downloads/lopy-pinout.pdf . How about just connecting 3 AA batteries directly to this, avoiding the converter?

[robert-hh]

@mahe Yes, that's the 5V Pin. It has an suitable range of <3V - 5.5 V. And yes. you can use 3 AA's there (or a single LiIon cell, or 4 NiMh batteries), as long as the input voltage does not exceed 5.5V. I would add a large capacitor (like 4700µF) close to the input to ensure a high current availability for short power drain peaks, especially when the batteries are low.

[JasonTimmins]

@mahe @robert-hh Hi Guys, I want to run a LoPy off AA batteries too so I followed your advice Robert and implemented a Pololu S7v8F3 converter. I got it working this morning and it seems to work very nicely. My voltmeter says it's 6.6v on the output of my 4 cell AA battery holder but it's 3.3v on the output from the converter. I wired that output into GND and 3V3 on the LoPy and it started working straight away. Very nice, thanks for the info.
Cheers
Jason.

[JasonTimmins]

Hmm, that said, things are getting pretty hot! The batteries are warm and the converter is too hot to touch. Is that normal? My device would cook if I closed the lid... and who knows how long the batteries will last.

[robert-hh]

@jasontimmins NEVER connect teh power supply to the 3v3 V Pin of LoPy.- That is an OUTPUT, not an input. Always supply through Vin. Even if it tells 3.4 .. 5.5V, it works well at 3.3V.

[JasonTimmins]

Oh, well, my four AA batteries were flattened in ~30 minutes... that's hardly my glittering dream of ultra low power remote sensing! <mutter>

[robert-hh]

@jasontimmins There is simething wrong. The capacity of a AA battery is about 2500 mAh. The supply current of LoPy is like 200 mA, so it should last about 10 hours. BUT, primary batteries have a lousy high current behavior. Better use NiMh rechargable batteries.
And again: The high current could have been caused by the wrong wiring of the LoPy. The Vin to 3.3V regulator on the LoPy could have been killed, at which point it consumes a lot of current.

[JasonTimmins]

@robert-hh OK, great advice. I'll try it again. [Can you smell burning? ;-)]

[JasonTimmins]

@robert-hh Things are much better now it's wired correctly! :-) The batteries and the converter have been at room temperature for the last few hours and my LoPy and sensor are working fine. Thanks again for your help Robert.