I never clicked the link, but johnny Bs mention of 18650s made me realize what was atttempted.
18650 lithium cells are indeed used in tesla battery packs, as well as many laptop battery packs.
Their quality ranges from horrid and dangerous, to quite good, the best cells being SAmsung, Sanyo, Panasonic and LG. It seems their capacity and ability to deliver hige currents are still not available in the same cell, ie a high capacity cell is not able to maintain high currents, a High current cell cannot have a high capacity.
I have about 2 dozen 18650 cells I have extracted from failed laptop batteries.
Their remaining capacity is almost a joke, in comparison to the panasonic NCR 18650 cells I outright bought.
I have a USB power source which uses a single 18650 cell, and a USB power meter. The Panasonic NCR 18650s will wind up producing 3 to 8 times as much usable capacity as the laptop extractions when powering my bluetooth speaker or recharging my phone.
There is so much absolute crap on Youtube in terms of DIY stuff. Far too many halfwitted self important people giving dangerous advice and acting as authorities. I would never ever in a million years trust a conglomeration of laptop 18650 batteries in series parallel to form a high capacity ~12 volt battery. This is a fire waiting to happen the same as a open bucket of gasoline in a hot shed, next to a fireworks convention.
I have a hard time trusting my laptop extracted 18650s. Anytime one of them heats to 110f when recharging, It gets put into a fireproof bucket for recycling. These cells still have usable capacity, but are simply not worth the risk. I mainly use them in a powerful LED headlamp, and they cannot maintain the higher light levels of 'high' or 'turbo' whereas the panasonic cells, which are not high current cells, easily can maintain these rather absurd light levels for a while before the light gets too hot and throttles back output.
The Panasonic NCR18650b cells have now dropped in price as they are no longer the highest capacity cells available at 3400mah, the newest cells are 3600mah.
Do not believe ANY claim of 18650 capacity over 3600mah at this point in time. Many cells on amazon and Ebay will claim higher capacities, but all they really are are used rewrapped laptop cells, already compromised, and dangerous.
These batteries should not be treated like Alkaline/Nicad or NiMH batteries, but their cylindrical shapes will have many people doing exactly that, and the ignorance could easily cost them, greatly.
Beware of DIY stuff on Youtube posted by self important yet highly ignorant nimrods, and especially where lithium batteries are concerned.
Making a huge DIY batterypack of used 18650 cells is criminally negligent