Noticed this study from Project Farm (YouTube), a site that tests tools and, IMO, performs salient, objective evaluations. The batteries were from manufacturers in China, Japan and Korea. General findings: the Chinese batteries listed about three times the amp-hours of those from Korea and Japan, while actually being about 20% lighter, having 10% or so of their projected capacity (while the others were at 100% or so), more internal resistance, and half or less of the capacity in a use test. Interestingly, some of the batteries didn't lose much when tested at freezing temps. Purchasing batteries is a mine field IMO unless from a major OEM (obviously for their models), ebikesca or em3ev.

One has to avoid the scams, There are chinese cells that aren't fake.THis one is actually sold on aliexpress, and it is funny to read reviews. One buyer said that he knew 30,000mah is impossible for 18650, but he expected it to do at least 9000mah, but he only got 1000mah.

I prefer batteries that have proven themselves in flashlights and e-cigarettes or RC-models.
In my experience (and I have been using these batteries since 2003), there are hardly any more robust batteries than the Sony 18650, even if the capacity is supposedly lower than other manufacturers I would currently go with the Murata / Sony Konion US18650VTC6 - 3120mAh.

When ordering multiple items from ChiCom sources I usually expect that there may be defective items in the batch. I don't feel to bad about it cause its expected. Surprisingly, lately I am doing the same thing for made in USA items too.

So you found a good battery... Bravo, I am going to look for it now and cross my fingers...

If you have to solder 50 batteries to a single pack, that would need a lot of fingers to cross

I have a coworker who does this for bikes, and also battery backup systems for travel and homes. It's actually not as complicated as I'd thought. Takes a bit of knowledge, and safety is paramount. Once you understand the correct safety procedures, the possibilities are only limited by what you're will to attempt. Me, I'd just pay him for a build.

Oh I do solder batteries myself. But I would never solder such a big pack with a cheap unknown type of battery. ;-)

Who said to use cheap, crap cells? My coworker only uses Panasonic, Sony, or LG cells. Cheap cells are known to fail, and oftentimes cause fires/explode.

Sorry, my answer referred to zandovals reply.

No worries!!!

I'd also not solder expensive know type of batteries. Spot welding is what's required.

And a plus 2 to spinnanz advice to NOT solder on batteries. The heat required to flow the solder usually damages the battery too.