Hi,
It's possible that
if the married pair of batteries were used in a Series Mod like the Sigelei 150W, one may have gone bad
if they were not rotated every other time. I have mine marked as 1a and 2a for pair #1, with 1b and 2b on pair #2. Since I have two sets, and charge one while the other is in use, I'll put 1a & 2a in with 1a first and 2a upside down for the Series Stack. I'll do the same with pair #2. Then when I use pair #1 again, I put in 2a first with 1a upside down, and do the same with pair #2. After that, I start over with pair #1 and 1a in first.
Batteries in double-stack and triple-stack devices need to be rotated, because each one can be discharged differently while in Series. This is why a Balanced Charging System is so important for Series Lithium Polymer batteries. They cannot be easily taken out and charged separately, so the Balancer monitors each battery to make sure that all of them are charged properly and evenly.
This is not a problem when batteries are used in Parallel.
Stepping onto the soapbox now.
. . (not targeting you, Cool. Just giving general info while on the subject)
It is also possible that one battery was less perfect than the other. Especially if they were anything other than Sony, LG, Samsung, Sanyo, or Panasonic. Companies like Efest (and others) do not actually manufacture their own batteries. They buy less perfect (more inferior) batteries from a battery manufacturer, take the manufacturer's wrapper off, put their own wrapper on, and usually exaggerate the Amp Rating and mAh Rating. There is no such thing as a 35 Amp 18650 battery, and certainly not 40 Amps and above. Any 18650 that is labeled higher than 30 Amps is using a Pulse rating. But a pulse rating is meant for uses of less than 1 second, and almost no one takes a puff that quickly. It is best to only use batteries with a known Continuous Amp rating and suggested Pulse rating.
Using a battery that is mislabeled, like Efest, can be dangerous when vaping at high Amps, especially when you start reaching the exaggerated Amp rating on the wrapper. For instance, the Purple Efest 3100mAh 20A 18650s are actually only 10 Amp Continuous Discharge batteries. They
might actually be capable of 3100mAh when new, but battery testers with the correct equipment have tested them under various Amp loads and shown that they are 10 Amp batteries with somewhere around 2800mAh. Using these
supposedly 20 Amp batteries in something like the Sigelei 150W at higher Wattages would heat them up quickly and damage them... possibly to the point of a catastrophic failure with continued use. It has been shown that most if not all Efest batteries have exaggerated ratings, as do many other brands that do not manufacture their own batteries.
It is important to know what you are buying, and buying an Efest battery from many eBay sites will get you an even
more inferior battery that is labeled to
look like an Efest, but isn't. While there are companies like AW that use only very high quality batteries and rewrap them, the 5 companies listed above actually manufacture their own batteries and rate them properly, so these are the main battery brands to trust. They will save you having to do the research to find out what is inside the wrapper,
as long as you get them from a reputable source.
Please be Safe!!! Stepping off the soapbox now.

Back to your regularly scheduled vaping.
.