Post date: Jul 20, 2016 5:9:31 AM
Since the Pi Zero power supply circuitry is so simple, I've wondered for a while if it could be powered directly from a rechargeable Lithium battery. Just the battery, no boost converter.
I finally got around to testing it as part of a recent project. It works! The Zero regulator dropout voltage is near zero volts, so the Zero stops working around 3.3 volts. Coincidentally, a rechargeable Lithium battery has used up about 95% of it's capacity at 3.3 volts. So, all you have to do is connect the battery to the ground and 5V terminals on the Zero's GPIO header, and you'll be able to use most of the battery's capacity!
BE SURE TO ONLY USE A BATTERY THAT HAS A BUILT-IN PROTECTION CIRCUIT. You don't want any fires!
ALSO, DON'T PLUG A POWER SUPPLY INTO THE ZERO WHILE THE BATTERY IS CONNECTED - THIS INCLUDES POWERING IN GADGET MODE. You'll overcharge the battery and the protection circuit will probably permanently disable it.
Of course you need to be VERY careful with charging. Use a charging module: they're available for under a dollar. You'll have best results if you rig things so that the battery isn't powering anything when charging. You also need to be careful about over-discharging: you could use my analog2pi technique to monitor the battery voltage, and sound an alarm or disconnect the battery when it's too low. You could also rely on the battery protection circuit, but permanent capacity reduction will probably result if you wait until it triggers. Or you could calculate battery run-time in software.
There is, of course, one little problem with all this: USB devices will be powered at the battery voltage, not 4.75-5.25 volts that the USB spec requires. But this isn't a total show stopper: some USB devices operate internally on 3.3 volts or less, and as such have a regulator on-board. These should work correctly at battery voltage, as long as this regulator is a low-dropout type. Specifically, they'll function until the battery reaches the sum of the device's regulator dropout voltage and internal operating voltage.
So which ones work correctly? My keyboard doesn't work on low voltage: it likes 5 volts. So I popped in a Bluetooth dongle and used a Bluetooth keyboard. It seems to work just fine, at least with a mostly charged battery running the Zero. I haven't had a chance to test a flash drive, wireless mouse dongle, or WiFi adapter.
Let me know if you have a chance to test any USB devices, or even reverse-engineer them to find internal operating voltage and regulator dropout. I'll add the results here.