What is a battery marriage?
The term "married" is used quite often in reference to unprotected 18650 lithium ion batteries. A battery marriage will consist of 2 or more batteries that will be used together at all times, with the same exact specification ratings for millamp hour, maximum discharge rating, and size. Essentially a perfect marriage would be purchasing 2 or more batteries (of the same brand and model) that will only be used together simultaneously, without ever substituting a battery, no matter what other batteries you may have available for backup.
Why is battery marriage important?
When using batteries that are the same exact age, specification and brand; your likelihood of damaging your vape mod or batteries are rather slim. When using one 18650 from brand A, and one from brand B, you're likely going to notice that one will discharge at a slightly difference rate than the other. Once you drain both batteries down far enough, your vape mod will not detect that one battery is below nominal discharge voltage and will continue to demand a certain power draw, which jeopardizes the batteries safety and will create a bit of internal stress on the electrical regulatory system of your mod, which can cause damage to the vape mod.
Our best advice is to always buy your batteries in pairs and label them to indicate the date of purchase or some other form of labeling so you won't accidentally mix up your old and new batteries.
We must remember that the vast majority of high power output regulated box mods will rely on unprotected flat-top 18650 batteries. If you stress them hard enough, they will risk explosion.
For more information regarding battery safety and marriage please check out this blog post: http://blog.mtbakervapor.com/marrying-batteries-vape/