What's not to understand?
Batteries and e-liquids (specifically nicotine) do not like heat. You don't want either of them in a hot car where the nicotine breaksdown to non-effectiveness, or the batteries decide to play 4th of July fireworks.
Not sure what extreme cold would do to anything, but there's probably something not good about that, too.
Batteries dies unexpectedly. Juice delivery devices stop working at the most inopportune times. Unless you are confident you can go several hours without your vape, you need backups.
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Ego cases are available which can house a couple of batteries, a few clearomizers or cartomizers, and a couple of bottles of juice.
Whatever your setup and supplies that you need to get you through your day (including backups), find a carrying case or bag to haul it all wherever you go. I like to bring a few mods with me to work, including some extra cartotanks and spare batteries for "just in case" situations. I use a mini-laptop zippered bag that has three pockets and a shoulder strap which is large enough to carry all that gear and can still fit easily in my clothes locker at work.
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If I'm just going out for a couple of hours, I don't bring all of that. But I do bring a spare battery and a filled second tank with me.
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Little cases like the above are useful to carry a spare battery or a tank. Always carry batteries in such a case or a plastic battery case. Never carry in a pocket of loose change or car keys, or in a purse. Any metal can create an electrical circuit with the battery and result in something like this:
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