Reusing a phone battery to power a Pi

Question

I know this question might be too broad or too generally worded, but I'll ask it anyway, because I can't find it anywhere else.

Is there any simple way how to convert any old phone (fully functional) to a home-made power-bank/UPS, so that I could use it to power a Pi? For example:

• phone as a charger, battery used separately
• phone + battery used together to power

I don't require whole instructions as I'm not giving a specific model of a phone. ( I'm asking in general). I'm just curious if it is actually possible and if the power supply would be sufficient enough, to at least boot a Pi.

Let's say I have at least Li-Po 900 mAh battery, judging from these calculations a Pi Zero could be powered with this for a while. Then again, it'd be a raw battery, not a power-bank, so... Any ideas?

Answer 1

I think this looks like a slightly problematic question. The unspeakably bad things I do to phone batteries while I'm using them don't leave them good for much once the phone's lived out its natural life - overcharging, undercharging, temperature extremes, moisture, impacts, constantly swapped crappy chargers, etc. Mileage is going to vary substantially between specific batteries. In the grand scheme of things you're likely going to have a better time just buying a suitably rated power bank. That said, let's take a quick look:

• If your battery still holds a reasonable amount of charge there's nothing to stop you from using it as a battery and the phone as a charger, the same way you would have throughout the useful lifespan of the phone. Provided the battery still provides at least 5V (preferably slightly above that, around 5.25V), and still holds enough charge to provide the 200mA-ish the Zero needs it'll work exactly as any other battery would. Identify the correct terminals, connect them to the Pi's USB power input through something like a UBEC (important for regulation), start the Pi as normal.

• Using the phone and battery together like a UPS may be problematic. There isn't a maximum specified current draw from a USB OTG device (I'm presuming USB OTG availability on the phone), leaving that up to individual manufacturers. You're going to get very different results from different handsets. I'd be reeeeeeally surprised if there were many that provided more current than would be useful for, say, a flash stick or portable keyboard. To test it you just need to run a cable from the USB-OTG out on the phone to the Pi's USB power input.

In either of these cases, if it were me, I wouldn't test any of it without having the battery in a reasonably solid flameproof container with a nice big bucket of sand nearby. Screwing around with batteries can get somewhat explosive when you get near the edge values, particularly if they're old.