I wouldn’t mix cells of varying voltages if you paid me. I’m from the RC drone world, a lot of us use balance boards to charge several 3-6s lipo packs at a time in parallel. These boards connect the main discharge leads in parallel, and the balance leads in parallel.
Rule of thumb is never connect packs with cells that are larger than 0.1v difference to each other, or balance current can melt wires/damage cells. I push that rule to 0.2v sometimes, but I make sure to connect the higher voltage pack to a bank of lower voltage packs, never the other way around. Why? Discharge current. These batteries don’t like being charge rapidly. They’ll take it, sure. But they don’t like it.
The packs I use are rated for 5c charging current. That’s basically mAh capacity converted to Ah capacity, and then charge at an amperage equal to the Ah rate. Have a 3,000mah pack? That’s 3Ah, so a 1c charge rate is 3 amps. 2c is 6amps, 3c is 9amps, etc.
Discharge current between two packs at 0.2v difference per cell can be 5 or 10 amps, sometimes more, at least briefly.
To connect two batteries, one full and one dead, together, is to essentially cause a dead short. You’re gonna pull a crap ton of current out of the full battery, and dump it into the low one.
This would be bad enough with my lipo’s, with their high charge rate capacity of 5c. But the cells we use usually have a max charge rate of 0.5 or maybe 1c. The 30q has a max charge rate of 1.3c, or about 4amps. But in my experience they get warm charging at 0.3c, or 1amp.
So to mix cells of large voltage difference, and generate that massive balance current? Melting springs? Yeah, no thanks. Maybe it won’t blow up in your face, in fact it probably won’t. But dang it if it isn’t really bad for the battery, and potentially your face. No thanks, I’ll pass.
Sorry for the rant, of course you’re welcome to do as you please. But I agree with TK, it’s not an urban legend. Even if it’s not as bad as I’ve made it out to be, it’s still not good. Please don’t do it.