The secret to connecting the 3 pin of the RPi with a single charger

[minhchau0211]

I'm working on a robot project that uses three separate raspberry pi's (two Zero W's and one Pi 3 B), and I'm trying to figure out the best way to have them be battery powered without needing 3 separate batteries and 3 separate chargers. Essentially, I want to adapt this idea to three separate boards:

I imagine these are the options and I'm just not sure which is most feasible or would work best:
3 boards, 3 shims, 3 batteries (easy to implement, but pain to charge them and will go dead at different rates)
3 boards, 3 shims, 1 large battery (not sure how to wire this)
3 boards, 1 external charging module, 1 large battery (not sure what module to use or how to wire it)
Any thoughts or suggestions?

EDIT: Just to clarify, the Zero W's will each have a camera module and operate as "eyes" with some minimal pre-processing using OpenCV, and the Pi 3 B will be the proper "brain" of the robot receiving that input (and other sensor input) and controlling motion.

[pootle]

I have run 2 pl zeros from 1 usb powerbank using simple splitter cable. No need for shims. You would need a beefy one to run a pi 3 as well though.

[Z80 Refugee]

There is no particular problem with running muplitple RPis from one battery pack. All you need to make sure of is that the regulator between the battery and the RPis is capable of supplying the current needed. What you could do is have separate regulators - one for the Pi3 and another for the two Pi Zeros. The inputs to the regulators would then both come from the one battery pack. It's unlikely you would be able to find a powerbank (which has a built-in charger and output regulator circuit) with enough output current to run all three RPis.

Don't skimp on the batteries. You need to choose batteries which can supply the current without getting hot. The lithium polymer batteries used for RC modelling would be a good choice. Using switching regulators, the higher the voltage you supply to them the lower will be the current required from the battery (1A @ 6V = 0.5A @ 12V).

Regulator: something like this https://www.ebay.co.uk/itm/3A-1-5MHz-28 ... 2817919658

Battery charging: don't keep the batteries in series for charging, that way leads to a short life. Lithium polymer battery packs have multiple connections for balanced charging. If using standard NiMH AA/C/D batteries (don't use AAA), take them out of the holder for recharging in a proper charger like this: https://www.amazon.co.uk/dp/B07239TM65 or this https://www.amazon.co.uk/dp/B01AXZDGCU

[Imperf3kt]

I agree with what Z80 Refugee said, but I'd also add if you decide to use Lithium batteries, be careful of fake 18650's and similar - Ebay and Amazon are rife with them.
Buy brand name batteries, from a reputable supplier. For 18650's the recommended brand is Panasonic with a capacity of ~3000mAh, IIRC.

For Lithium Polymer technology, pretty much any supplier is okay. Note that the wires presoldered onto these things, are typically 1A limited, so you'll have to put your own on instead. DO NOT let the battery get too hot, be sure you solder quickly and efficiently or you could have a fire and some nasty burns. If you mess up, let the battery cool for 10 or 15 minutes before trying again.

You may want to check the C rating of the battery to see if it is capable of supplying multiple regulators.
Also be aware that a regulator alone, is probably not enough. When the battery runs out, you won't know, because the regulator will not drop in voltage as the battery drains.
For this, you'll either need to know how long your Pi runs off the batteries (and preferable not let it drain below 2.9v - that'll shorten the battery's lifespan and letting it run to 0v will kill the battery permanantly), and shut it down at least a few minutes before this time (which will get shorter with age) or have some kind of battery voltage monitoring circuitry that automatically shuts off the Pi once the batteries dip below ~2.9v

[Z80 Refugee]

All good points. You could source LiPo cells with built-in power management, but if not you must use a regulator/converter with LiPo management functionality.

Of course, a single LiPo cell can be stepped up to 5V, but I envisaged a gang of three or four providing current to two step-down converters. This reduces the output load on each converter, and reduces the overall current demand from the battery (same wattage output, but higher supply voltage therefore lower supply current).

To avoid any risk with LiPo, stick with a gang of 6 or 8 NiMH.