Aaaagh. Use spade terminals, not razor blades! At least they’re… mostly taped.
And don’t do this. I really, really wish he’d have shown the voltage on the “dead” battery before doing that, because depending on the voltage, the “good ideaness” of this behavior ranges from “It’s fine” to “You really, really shouldn’t be doing this.” Given the “15 to 20 minutes” timeframe mentioned, I’d wager it’s in the “You really, really shouldn’t be doing this” end of the spectrum.
Many tool packs (and, apparently these…) are “always live” to the terminals. I’ve torn down some that have a set of transistors of some variety that can turn the output off, but almost all the newer ones are “dumb as doornail batteries” - which I think is the right answer. The battery is hooked straight through to the pins, always live, and all the “smarts” related to balancing, low voltage cutoff, etc, live in the tools and chargers. This means the battery pack is as cheap as possible, which beats the really expensive, somewhat exotic early lithium tool packs with really fancy BMSes (that were, of course discarded when the battery was worn out - I have plenty hanging on the outside of my office now for flair).
Either internal self discharge or some parasitic drain of the onboard PCB has drained the cells down below the low voltage safety point that the charger will tolerate, so it (rightly) doesn’t charge dead lithium cells. Lithium is physically stressed at “fully charged” and “fully empty,” and as you go past “fully empty” (about 2.5V/cell per most datasheets), the cell is increasingly physically stressed. Let it go to zero volts, or even negative (I’ve seen some quite horrifying negative voltages on series cells in a fully discharged pack - several volts negative), and the physical stresses internally are significant, and can do very real damage to the internals.
So, he goes about hooking them up to a live battery, and charging them back up, in an entirely uncontrolled manner, not having checked pack voltage, balance, etc. That’s dumb. I’m wary of anything under about 2.5V/cell, and I won’t recharge anything below 2.0V/cell unless I know exactly how it got there, and it’s not been there very long. Some random pack of unknown origins and history below 2.0V/cell? Nope. Not going to risk that.
The problem is that you’ve no idea how the cells are damaged internally - and generally, whatever damage they’ve taken won’t show up immediately. It will show up sometime later in their life, when the additional stresses of cycling combine with the over-discharge damage and you get an internal short, runaway cell, and a lithium ion blowtorch. It doesn’t always happen, and a lot of people get away with it, but it can and does happen, often enough that it’s a pretty well stupid idea to play that game.
But, forced charging like that will bring the voltage up to whatever point the charger considers the safety cutoff, and it will charge the (probably damaged) pack happily, because at that point, it can’t tell the difference.
Now, there’s another behavior you’ll occasionally see on higher end packs that’s quite different, and that’s a “storage mode” or “low power mode.” A BMS draws current when operating, even if the pack isn’t being used, and will discharge the pack over time as it monitors things. It’s not a problem in a daily use pack, but it is an issue with a pack that sits for 6 months or a year. So, some better BMS designs will, after a period of inactivity, go into a storage mode that puts them into a super low current (typically uA, on the order or less than the self discharge of the cells) mode. But they won’t come out of this mode automatically - something has to wake them up. With a tiny bit of cleverness, you can wire things so that plugging it the charger will provide this wakeup signal, bring the BMS board back online, and let you get on with life. But this won’t be (or, at least, shouldn’t be) a “nearly dead” pack activity - you’d want to trigger it around 65-70% SoC, if the pack hasn’t seen activity. Or, lower, if the pack has sat for a while.
And I guarantee that’s not the case with this tool pack.
So, this is yet one more person on YouTube, who clearly doesn’t know a damned thing about lithium, abusing them (and encouraging other people to abuse them) in ways that look fine at first, and have a way of biting down the road when you least expect it. Business as usual, sadly.
Most of the non-Tesla EV owners I know carry a wide range of adapters from 120V to 240V and back, since a lot of the portable EV adapters will run fine on 120V and 240V. I can, for instance, take a 240V 14-50 outlet, convert that down to run over a 120V extension cord, and convert back to the 20A 240 plug one of my chargers uses. The insulation is rated for 600V, so it’s fine… I just carry L1/L2 instead of L1/N over the extension cord. But don’t plug other stuff into the end.