I assume you are talking about Li-ion type batteries. I think in some ways a lot of the concerns are overblown. Li-ion technology is in so many things now, and the incidents of fire are pretty low. In many cases it's people pushing the batteries too far and putting them in things that are modded, etc. This is just anecdotal, but it is what I have observed when you get to the details of the mishaps.
However, there's no doubt in my mind that the cheaper you go, the higher your chance of problems. Again, if you treat them well, you will probably be fine. I choose not to take that chance, and go with reputable brands. I'm probably "overpaying" in some ways, but it's a small price to pay to keep things safer. I still don't take it for granted, and I still follow reasonable safety practices when charging, etc.
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The danger is actually more related to the ultrafires, suretocatchfires, trustmetocatchfires, uranusfires, etc, being populated with recycled laptop cells...that are rewrapped and sold as new cells.
So their internal resistance is typically higher than for a new cell, and, dangerously, they are inconsistent. That means loading more than one in series for example is very very dangerous.
Even the protected versions may have fake protection, and not really be protected, and so forth.
That's all in addition to them also not being anywhere near their rated capacities....so they are old worn out cells sold as new cells, but you have no way of knowing what's inside the wrapper.
So, sure, it MIGHT have a "genuine Panasonic cell" inside, from a used laptop...or, it might have a coupla AAA inside, etc.
It might be LABELED as a protected 10,000 mAh high capacity 3.7 v 18650, but inside is ANYTHING...no rules. The one's labeled 10,000 mAh might be worse than the one's labeled 2600 mAh, or better, there's no baseline.
:shrug:
So you can "treat them well" and carefully charge them etc...but, they're still garbage, and, potentially dangerous.
If there was a way to tell from an ad how bad they really were...that would be great...but there is not. Even the seller's ratings are almost entirely manufactured by networks of paid posters (That's an actual job description in China).
So, if a maker is honest, and really taking brand new cells and adding protection, or not, and reselling them with their own private label, labeled correctly...you can't tell who they are, they look exactly like the scam artist's cells.
That leads to the DEALERS being the ones who GET the cells, hopefully do some batch tests at least, and then reselling those private labeled cells...and if the dealer's wares prove trust worthy, eventually, with any luck, buyers give them their business even if the real cells cost more.
Even the dealers can miss a bad batch from a previously trusted supplier...so, its just a big crap shoot.
