I'm starting to think the videos might be bunk.
Sometimes you can bring a dead multi-cell pack back to life like this, but you have to consider that the pack itself may not have any kind of charging controller in it - it may be in the tool itself. You can run quite a risk hooking an essentially unlimited current to what may be a dead short. If there is an onboard controller, it's probably smart enough no to turn on until some handshake is given, so your efforts are futile in this case.
I'd only connect two packs together like that as long as you realize there's a very small but measurable chance that you could wind up with a big hot fire.
No fires. But it didn't work. Something was definitely going on because the good battery lost voltage and the bad battery seem to gain a little bit of voltage but when I put it on the charger is still instantly flashes bad battery. I tried it with two of my dead batteries and it didn't work with either one of them.
If Lithium batteries get too low, they may not charge properly again. That may be why your charger is doing that - it knows the voltage is lower than the "this is bad" threshold. It could also be looking at current drawn and making a decision based on that.
It sounds like the packs are dead. You may be able to get someone to rebuild them, but lots of companies won't touch name-brand packs.
I have a couple of ruined kobalt batteries just as you do, and must have watched the same video because I also attempted to restore them with no luck.
I have an older battery powered leaf blower which I discovered has some faulty circuitry which causes batteries to excessively discharge over time even when its not being used, to the point that it will 'kill' the pack.
Now I just remove batteries from all tools whenever I'm not using them. Its safer that way anyhow.
(post is archived)