IMO, the charger and batteries should never be placed in a sealed container. Doing so will only increase heat buildup.
Most of the problem with charging is due to cheap/poor quality charging circuits. It's not the fact that you're using any type of USB power source, which should be 5 volts, but may vary, and it shouldn't matter if it does. The charging circuit should take the applied voltage, then regulate the output voltage and current to charge your battery at the proper rate. When charged, it should cut things down to a very safe maintenance charge. Shouldn't matter if the USB output is 250 ma. or 2.5 amps, 4 or 7 volts, etc. It's the charging circuit's job to properly do these things. Cheap charging circuits have little or no protection built into them that prevents dangerous conditions should something go wrong.
Chargers are built into many boxes and even in every ego battery. And when these charging circuits are as cheap as they can be, such as in these sub 10 dollar batteries, things go wrong quite often. Even in the more expensive boxes the charging circuit uses nothing more than these cheap methods.
Battery charging is relatively easy and safe. Use a quality charger; the latest and greatest frills and displays aren't usually necessary and the money spent on frills only takes away from the needed quality. Don't charge them at the fastest rate you possibly can just to save a little time; if adjustable, set your charger's current to a lower level. It's safest to charge them at the slowest rate possible. Easier on the charger, safer on the battery, less heat, and more peace of mind.
Been vaping 5 years, using the same charger, some of my batteries are over 2 years old and still going strong. Never had a venting or failure situation and I charge all night sometimes. It's when you push your batteries or chargers to the limit of their capability that things go wrong.
