Exploring alternate methods for providing DC power to Raspberry Pi

Question

I'm in need of a battery power solution for a Raspberry Pi? 
 My plan was to use a 5VDC lithium-ion battery connected via the USB micro connector to power the Pi. I have had to rethink this because of extreme space limitations. The power connector on the Pi makes it a difficult fit in the space that I am using. 
 My next thought was to connect the battery directly to the 5V and GND pins. I seem to recall that this could be used on Pi's. I then stumbled across an article that suggested, if this method is used, the circuit would require a Universal Battery Eliminator Circuit (UBEC). I have no voltage level issues. The battery supply is 5VDC. Unless the UBED offers some protection I'm not aware of, I don't need its capabilities of managing different input voltages. This shook my confidence of using the pins. 
 What are your thoughts on providing alternate power to a Pi? Can I do it through the pin header? This is my preferred solution. Are there any requirements I need to be aware of to make this an alternate solution?

robogary · Answer

Hi gpolder - seeing is believing :-) reminds me of the Journey song - Dont Stop Belivevn' 
 To properly reply to your question on RPI 4, I just did an experiment on a Raspberry Pi 4 , I used an adjustable power supply to simulate a battery. Current limit was set to 3.1A in case I screwed up. From the + terminal of the power supply, I wired in a series 3Amp diode, Diode Anode to the + terminal. 
 I always use series diodes from a battery or external power supplies since I forget and will plug in a PC, or vicea versa, or both, and end up backfeeding somebody and unintentionally powering sensors & stuff. 
 From the out side of the diode (cathode) I used an alligator jumper, to a cheap chinese Dupont jumper , with a socket end that plugged into GPIO Pin 2. 
 I put a voltmeter on GPIO Pin 4 to read the voltage directly at the RPI4. I also used a single cheapy run of the mill jumper to GPIO 6 GROUND. 
 The RPi 4 has a keyboard dongle installed on one USB port. The RPI 4 also has a NOIR camera installed. 
 On initial bootup, the RPI 4 only drew 100-150mA, and increased as it went . The Power Supply was set for 5.2V ( 5V + the measured no load Vf drop of the diode was .2V). 
 As the RPi 4 continued in bootup, l had to turn up the power supply voltage to compensate for the diode IR drop. I dont think the load was so much the 28 AWG wires were dropping significant volts. 
 The power supply ammeter does dance around a bit, and I'm sure I'd see some higher peak excursions if using a scope on CFB, BUT the largest amps I witnessed was .6A. in the power supply's screen. 
 This measurement isnt the absolute chart all for everyone's applications, but maybe instills some confidence in colporter that its not that bad. 
 DISCOVERY ! 5V at the 5V pin was not enough to boot up reliably. I missed this on my first pass because I didnt have a monitor plugged in. 
 My "battery" ended up being 6.6V for 5.3V to 5.5V at the RPI4 GPIO pin, I'd feel better having a better regulated voltage there as well. Its worth a test without the HDMI plug again tho. 
 For robots & roving vehicles, I will tend to use a 7.4 V or 11.1 LIPO batteries, and a buck power supply to feed the 5.x volts, and often several for peripherals power, at 6V (RC receiver), 3V (sensors) 5V (sensors) , 12V (fans if any, some LED drivers,) etc... 
 Where volts were measured 
 . 
 the test setup. I'd like to check to see if the HD camera load makes any difference from the NOIR camera. 
 
 The power supply volts and amps, and the volts read at GPIO4.

robogary · Answer

The RPI4 uses the USB-C connector, the link is for that adapter. The RPi 3 uses a micro USB. https://www.amazon.com/gp/product/B07PJYVH12/ref=ox_sc_act_title_2?smid=A3MDC9KUHU6B27&psc=1 
 There are a variety of these adapters depending on the USB type. 
 I'm starting to really like the idea of powering a project this way. It makes it easy to unplug the upstream battery, interlocking & buck power supply, and plug in the AC power supply when doing the project development & debugging.