Looking at the RPi Zero schematics, shows a PAM2306AYPKE which takes 5 V
from a USB connector and generates 3.3 V and 1.8 V supplies:
The PAM2306 datasheet shows:
The Recommended Operating Conditions shows the Supply Voltage range is 2.5 to 5.5 V
Is a dual step-down current-mode, DC-DC converter. I.e. requires an input voltage greater than the output voltage.
The description shows the input is designed accept USB or a single single-cell LiPo battery:
The PAM2306 supports a range of input voltages from 2.5V to 5.5V,
allowing the use of a single Li+/Li-polymer cell, multiple Alkaline/NiMH
cell, USB, and other standard power sources.
The Efficiency VS Input Voltage graph shows that for a 3.3 V output, the input voltage can be as low as 3.5 V:
So, in theory it should work with a single-cell LiPo battery (3.7V) connected to the input of the PAM2306.
I tried connecting the positive power output from the Li-po charger directly to either a 5 V or 3.3 V input on the RPi Zero.
Connecting the Li-po to a 5 V pin on the header of the RPi Zero should avoid damage, since that connects to the supply input on the PAM2306.
However, connecting the Li-po to the 3.3 V pin on the header of the RPi Zero possibly caused some damage since:
- Applied a higher than normal voltage
VDDIO2
pins on the BCM2835
chip used on the RPi Zero, which expects nominally 3.3 V. The question text says the Li-po voltage is ~3.8 V, but a comment from @AndrewMorton said the fully-charged voltage of a Li-Po cell is typically 4.2 V
. The only Broadcom document I can find for the BCM2835
chip is a description of the peripherals. Since can't find a full datasheet don't know what the absolute maximum supply voltage is before permanent damage occurs.
- Will only power the
BCM2835
via the VDIO2
supply, and PLL_1V8
supply won't be powered.
It might be worth checking if the RPi Zero still works if you power it over the USB connector. However, if the RPi Zero has been damaged due to supplying an over-voltage due to connecting the Li-po directly to a 3.3 V pin, perhaps try powering with a USB charger rather than connecting the RPi Zero to a PC USB port, in case the RPi Zero damages the PC.
Update in response to a comment:
My RPi is luckily not damaged and works fine when connected to USB power. My battery currently outputs ~3.8V. I tried connecting it to either of the 5 V pins on my RPi Zero. It turns on the green power LED on the RPi, but it occasionally blinks and I can't seem to SSH to it at any point. AFAICT by using a multimeter, the voltage is pretty constant at 3.8V, so I'm not sure what's going wrong. Any ideas? – Amir Rachum
From a search have found other articles which suggest the RPi Zero can directly operate with a LiPo cell. The Running the Pi Zero directly off the battery voltage section of Running a Raspberry Pi off a LiPo battery contains:
With that proviso, I was surprised to read this treatise on Hackaday that says the PiZero will work fine straight off a LiPo cell, the onboard regulator will still work down to 3.3V. The one provison is DO NOT CONNECT A POWERED DEVICE TO USB. You will be stuffing 5V bus power straight into your LiIon battery, result likely misery.
As to why are unable to use SSH to connect to the RPi Zero powered by a Li Po, perhaps trying to use networking increases the power consumption which causes the power supply voltages to dip below the minimum. In which case using an oscilloscope to monitor the 1.8 V and 3.3 V rails on the RPi Zero would help to check if that is happening.