Powering ZERO with Li-Ion cell

[EYYE]

Hello everybody,
although I'm quite new to Rpi I have some knowledge with electronics in general. I need to run my Rpi ZERO by one 3.7 V lithium cell. Since voltages needed for Rpi ZERO are generated from 5V USB input by integrated circuit PAM2306 on board itself, and PAM2306 datasheet says that input can be from 2.5 to 5.5 V, can I just desolder USB micro connector and replace it with direct CABLE way to Li-Ion cell? As I see it from schematics (https://www.raspberrypi.org/documentati ... matics.pdf) it would not be a problem, right? All I lost during this are GPIO pins 2 and 4 because they use the 5 V input directly. That is totally not a problem for me since I'm not about to use them. Also HDMI output use 5V but again, I have no use for it (will use TV output anyway).
Anyway, there can be same reason blocking this approach I don't yet realized. What do you think?

[drgeoff]

Hmm! That might work. (Though there is no need to remove the micro-USB sockets.)

But:

1. How will you know when the cell is too far discharged? AIUI, 2.5 volts (at which point the PAM2306 will still be able to power the Zero according to your thinking) is past the point at which the cell becomes damaged in some way.

2. Irrespective of the above, at some point you'll need to recharge that cell. You'll need a charging circuit specific to a Lithium cell. Why not use a USB powerbank which is a lithium cell plus a charging circuit plus ......................, oh yes, a dc-dc converter which produces 5 volts (which the Zero will certainly work from) plus ......................,oh yes, a protection circuit which will stop the cell being discharged too far?

[Douglas6]

Adafruit's Powerboost 1000C includes a charging circuit, hot-swapping, and under/over voltage protection. It's how I roll.

[EYYE]

Well, cell protection I want to solve independently from Rpi. I dont want to stress the cell, so I will limit its output to the point of 3.0 V. Under this condition the device will cut the power. The point of not using powerbank is to save space and have whole device soldered together to archieve high durability (it will be one purpose machine - forever). And also, I use 18650 cells in many applications so I want to have uniform power supplies in all of my gadgets...

[shadetree01010100]

Ignoring the mentioned battery management issues, your run of the mill Li-ion cell runs between 3.3 and 4.2V and shouldn't be any problem for the on-board DC-DC converter. As you suspected, your 5V GPIO pins will be at (slightly less than) battery voltage rather than 5V.

I wouldn't hesitate to try it.

[davidcoton]

You will also not get 5V on the (other) USB socket, so some peripherals will not work reliably. AIUI the Pi's supply parameters are aligned with the USB spec.

[EYYE]

Sure. Have no use for USB port either Only TV, SPI and I2C interfaces.
Thank you all for four answers and comments.

[steveb4pi]

You can get basic 1200mAHr PowerBank from Poundland == this even comes with micro-USB-full-USB cable !
.. or go on eBay for a multi-cell PowerBank for longer run-time

NOTE few PowerBanks allow recharge AT THE SAME TIME as power out i.e. when you plug in a USB power-block (into the PowerBankl microUSB socket to recharge it), the PowerBank cuts off the 5v to it's the full-sized USB socket output (althought it's easy enough to wire up a couple of schottky diodes as a DIY switch-over cct).