This has always been a controversial subject.
Statistically, 80% of battery incidents occur while the battery is being charged.
E-Cigarette Explosions: Comprehensive List
There is evidence that say's that you shouldn't leave batteries in a charger overnight. Doing so allows the charger's "trickle" charging stage to kick in to keep the battery's charge at 4.2 volts. This leaves a battery at its most excited and vulnerable state, and if you put your battery into your mod directly from the charger, you could be at an additional safety risk.
Once a battery receives a full charge (4.2 volts), it will naturally start to slowly lose some of that charge, even when not in use. Manufacturers want their customers to be happy with fully charged batteries when they need them, so they design algorhythms which keeps the battery's charge "topped off" while sitting in the charger waiting to be used. Sounds like a great idea, but its not so great for the battery's health.
Trickle charging leaves a battery in a highly excitable and vulnerable state. This is why we tell people to remove their batteries from the charger as soon as they are fully charged, to not leave batteries in a charger overnight while sleeping, and to "rest" batteries before putting them in a mod.
"Rest batteries after charging--
One commonly-reported factor in almost all the incidents we hear of where batteries failed violently while in use is that they were taken directly off the charger and then used immediately, at which point they failed.
Because of this, we think it may be a good idea to rest batteries after charging them. This advice will not be found in the usual 'reference bibles' on batteries, but we see more and different reports than others. Therefore we now advise:
Do not use batteries directly after charging them. Use a battery or batteries you previously charged, and that have rested for several hours. This is especially important if using a stacked pair for higher voltage, as statistically the risk is far higher." --Rolygate, ECF Forum administrator
The benefit to resting is increased battery lifespan & minimize the chance of venting."
Additional info from resident battery expert Mooch:
- Make sure the charger you use turns off once the charge is complete. Check the instructions for the charger you want to use.
- Never use a trickle charger with Li-Ion batteries! The continuous holding of the battery at the trickle charge voltage damages it.
Guide to Choosing a Li-ion Battery Charger