I have left my cells in overnight many times but not on purpose, to tired of just forgetting. I use the Miboxer C4 12 and have noticed a slight discharge up to 4% in the mornings.
My opinion: It is not safe to leave any cells or battery charging without actively monitoring them—lead acid, NiCad, NiMH, or Lithium.
i have experienced chargers fail to stop in all four chemistry with overheating and acid spewing, so i learned the hard way not to trust unwatched chargers anymore, especially on the first use of a new device.
i have dismantled chargers and found incorrect resistors in the circuit that controls the cutoff voltage, tiny surface mount resistors hard to read the value, e.g. 354 vs 454, easy to grab or load the wrong one. They don’t get proof tested by the vendor, think the manufacturer has time to do it? Not in today’s throw-away mentality.
Safe to leave batteries in charger overnight? … probably not.
Do I do it? … every single night!! 
Using a Nitecore intelicharger i4 and only good quality cells.
Its like playing russian roulette with a 10,000 round revolver.
Odds are high nothng will ever happen, but if that one round roles around to the right place. 
Li-ion batteries are an easy target because big companies aren’t actually harmed. You don’t know the cause of an accident, just blame the Li-ion batteries and call it a day.
How many people charge their mobile phone on the bedside table overnight while they are asleep ??
Just some trivia to think about 
Cheers David
I’m a diver and I’ve been on boats and land based situations like this. In general, battery knowledge and equipment has improved. But, the resorts and situations I have frequented have been pretty avid photographers, people that spend money on equipment and time getting to know it.
There absolutely is a lot of pretty sketchy equipment available, and a lot of people are using it, possibly unaware of the danger they are causing. They may well believe their 9000mA li-on cell is great, and the charger that came with it is reliable. I had a number of very experienced divers approach me on my last Philippine outing about their strobes not working well. In every case it was old NiMh (usually Energizer), that had years under their belt, and had run downhill badly. Their chargers were OK but they had no IR or capacity testing ability.
In land based situations everyone generally gets a station. If a single station ‘went up’ it would not be good, and could lead to a disaster, but the probability is lower. On live aboards the charging station is usually centralized with a hoard of chargers all going in the same small area, and at the same time, after a dive to get ready for the next. People seldom watch them as they are eating, napping, socializing, or relaxing. At night the attention would likely be almost non-existent.
I try to avoid charging going on all night, while I’m not around.
Sometime it can’t be avoided, like when I refresh larger cell’s.
But my mancave above the garage has a kitchenette so I put the charger in a (dry) sink.
Do not over charge the battery. Leaving overnight does not matter. Let say you left the battery on charging for whole night. When you woke up in the morning you found that it was not fully charged. In that case, it will not effect the battery life. If the situation is other way round. And it was fully charged, then it is going to decrease the battery life. I hope you understood my point.
Tough to grade a high capacity cell and not leave it charging over night, those take forever. I’m not about to watch a battery cycle for 18 hours.
Friend had one light up. While he was in the room sleeping and had the lithium batteries from his vape pen in a charger (he didn’t recall the brand). He woke up and the bed he was on and his bedroom was on fire. Used up the 2 fire extinguishers in the room, crawled out and went for another one. He said there was an orange glow in the thick smoke, he didn’t know which way was out of his bedroom or which direction was which it was so thick. But he crawled around and found the exit, ran down and grabbed the last extinguisher, when that was used up he scrunched up the mattress and forced it out of his window. It smoldered all night in the yard.
If I am in the house but not in the room, I’ve been sliding the charger as it charges lion cells, into an Efest “Safety Bag” and sealing the flap.
First off, glad your friend is alright, that is a very scary situation. Maybe it’s time to get a metal box for charging. On another note, it’s very dangerous to try to put out a fire by yourself, if you have to go down and get a third extinguisher, you should probably evacuate the building and call the fire department.
Lit-Ion batteries are everywhere but fortunately I have not heard of many incidents. I leave my laptops, cellphone, power tools and flashlight batteries overnight all the time. Of those the tool I use least is the flashlight. So far so good.
I wouldn’t. I have read HKJ’s charger reviews where some continue to trickle charge after the batteries are completely charged which could overcharge a cell.
What I’ve read: (or at least my impressions from reading this forum)
- The only safe way to recharge li-ion batteries is to do it outside.
- Well away from any structures.
- With the charger sitting on brick or concrete platform at least 10 meters from anything flammable.
What I do:
- I leave them in the charger in my room all the time, including overnight.
I think I’ll leave this on my Nightstand while I sleep tonight — See you’ll in the Next Life
I NEVER leave 18650s charging overnight —- After you’ve had one vent in your house you get a lot more respect for what can happen ( I vented one while taking apart a tool pack—Totally my fault)
Anything we leave plugged in (especially overnight ) I verify it trickles down and ultimately to 0 amps — pretty easy to check usb devices with some sort of in line tester
I also verify that a charger shuts off when it’s suppose to — you never know when you’re gonna get a charger that cooks your cells to 4. whatever volts
I’ve noticed some of the cheapest type of li-ion battery charger may not have a proper termination, however, while they do not terminate charging completely (a good li-ion battery charger is supposed to terminate charging completely), the charging current becomes very very small (like 2-3mA charging current, but which continues almost indefinitely, as long as the charger is still plugged to the power source). Examples of these chargers would be the Eizfan NC1 (Yonii-C1), Yonii Q1 and some others. (note: some Yonii do terminate charging properly (several Astrolux chargers are rebranded versions of some Yonii charger models).
I understand this is not good for the battery, but I wonder is a continuous 2-3mA “trickle” charge indefinitely to a li-ion battery may cause a fire or not (battery life will get damaged, but could it cause something like a fire?)
does all this advice apply to internal charging flashlights as well?