Is it safe to use old tool batteries for a DIY power bank?

The USB part is what usually gets people in trouble, not the battery itself. Those drill packs can put out way more current than a phone expects, and if you just run wires to a USB socket the phone will try to pull whatever it wants. I’ve seen a buddy’s setup get warm enough that he unplugged it and never bothered again. If you’re going to do it, at least use one of those little car USB adapters that already handles the voltage step down and has its own fuse inside. That’s the lazy version and it’s still safer than raw wiring. The cells themselves are usually fine if they still hold a charge, it’s the charging and output side that people hand wave. For camping I’d honestly just grab a cheap power bank and save the drill packs for a flashlight project or something.

I used an old 12v pack for a little camping fan setup last summer and it worked fine for a few trips, but it died way sooner than I expected. Honestly the cells in those things are usually more tired than people think.

I’ve messed with a couple of those packs for a little LED light setup and the thing that surprised me was how much the cells had drifted apart from each other. One pack had two cells that were still fine and one that was basically dead weight, so the whole thing acted weird even though the pack as a whole still showed a charge. That part doesn’t really come up when people just talk about wiring a USB port on. If you’re pulling from a pack that’s been sitting a while, the cells inside aren’t necessarily all on the same page anymore. Not saying it can’t work, just that a pack reading good voltage doesn’t mean every cell in there is healthy. I ended up using the better cells separately and ditching the rest.

Frying your phone is overblown, that part isn’t the real risk.

I went down this road with a couple of old Ryobi packs and ended up using one for a little speaker project. What I noticed was the age of the pack matters way more than the brand or anything. One pack that was maybe four years old still held a charge fine and ran the speaker for a whole weekend. The other one, which was older, dropped off a cliff after about twenty minutes. So it’s a bit of a gamble depending on how the cells have held up. You won’t really know until you try it. As for frying your phone, that wasn’t what I worried about. I just used a little USB adapter that plugs into the pack and it worked. The main thing for me was whether the pack itself was still in decent shape. If it charges up normally and doesn’t get hot, that’s a good sign. Mine didn’t, so I just used it for the speaker where a sudden cutoff wasn’t a big deal.

Frying your phone is actually the part I’d be least worried about. The real question is whether the pack itself is worth the hassle, since old drill batteries are usually half dead anyway. I tried one for a camping fan and it barely lasted an evening.

Depends how old. Mine were dead weight.