What is safe temp for 18650?

Just wanted to know when to get worried. Charging in my Nightcore I2 get them to around 35-40c at the hottest. After a few min in my Xsearchervn can get my Sony VCT5's up to around 50c.

According to the datasheets this is in the safe zone just wanted to hear others opinions on what temp is to hot for 18650's.

Somewhere above 80°C there is risk for venting, but I have not seen problems at lower temperatures.

HKJ said:

Somewhere above 80°C there is risk for venting, but I have not seen problems at lower temperatures.

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Thanks, good to know ive got some headroom.

Do you happen to know if running them at high temps(say 50+) has any negative effects on either cell life or capacity?

Rifter said:

Thanks, good to know ive got some headroom.

Do you happen to know if running them at high temps(say 50+) has any negative effects on either cell life or capacity?

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Generally cells has shorter lifetime and much faster self discharge at higher temperature, but I do not know if a minute or two once in a while has any significant effect.

My experience with high temperature is from my test stations where I frequently has high temperature, due to high discharge current. This also means when I measure 80°C on the surface of the cell, it might be 120°C inside (I have no idea about the real temperature inside).

HKJ said:

Generally cells has shorter lifetime and much faster self discharge at higher temperature, but I do not know if a minute or two once in a while has any significant effect.

My experience with high temperature is from my test stations where I frequently has high temperature, due to high discharge current. This also means when I measure 80°C on the surface of the cell, it might be 120°C inside (I have no idea about the real temperature inside).

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You are a brave man to run them at 80C temps on purpose!

Rifter said:

You are a brave man to run them at 80C temps on purpose!

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Not really, they are usual rated for that, but as long as they can vent without pressure buildup and sparks it is fairly safe.

Depends on what aspects of "safe" you consider. Below I'll address two aspects: safe from thermal runaway / venting and safe from accelerated degradation.

First, 50°C is well below the lowest reported thermal runaway temperatures. However, we need to consider the affect of the temperature on the entire system - including the charger. Higher temperatures will cause greater stress on the components of the charger (esp. caps) which may lead to premature failure of the charger. For consumer-level chargers (unlike laptops etc) there are no design standards that help to ensure that the charger will likely fail in a safe manner. Nor does anyone test their failure modes. It is possible that the charger may fail in a way that causes the cell to be overcharged or shorted, which could possibly lead to thermal runaway.

Second, higher temperatures will accelerate internal degradation processes in the cell (esp. at higher SOC). For example, the graphs here show that at your 50°C, Sanyo UR18650E cells have 107 days lifetime stored at 100% SOC, but 2.7x that (288 days) stored at 95% vs. 100% SOC, and 14x that (4 years) stored at 0% SOC.

Generally, the rule of thumb is that the more time the cells spend close to 50% charge and non-extreme temperatures the healthier and safer they will be.

Most manufacturers state that the max operating temperature is 60C.
But that not means it's good to use them this hot.
Li-ion is damaged by heat. The more you heat the cells up, the faster they will die. You can see this effect on mobile phones very clearly.
60C is also the temperature that humans start to feel the 'too hot effect'. When charging cells, if they get over this temperature it usually means the cells are bad. Of course, if you're charging them at 45C environment temperature, that won't be a valid observation anymore.

I also noticed that many of the even high end chargers get pretty hot under normal conditions, which causes cells to overhear. Therefore I use 12V PC coolers to help out when mass-testing 18650 cells.

When using such cells in normal temperature conditions there is no reason for them to overheat. If they do, you're either using bad cells or you're using the wrong type (like low drain / laptop battery cells in high drain things like power tools).

Overmind said:

Most manufacturers state that the max operating temperature is 60C.

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But the cells themselves are safe at higher temperatures. I tend to use 30Q's and VTC6's in high-drain lights, and IIRC they are safe up to 75-80C. They can get that hot when used at very high currents, i.e., above their 15A continuous current rating. However, only my FET lights push them that hard, and FETs tend to drop back current pretty fast because the cell voltage drops. Plus, the light head gets hotter much faster than the battery. So, I doubt I've ever gotten cells anywhere close to 80C, probably more like 60C max.

If you're using a flashlight in 60C operating conditions... yuck... wait for night. I would guess a flashlight could get a battery hotter than 80C if used in 60C weather.

Rifter, you didn't state the ambient temperature. I'll suppose it's less than 30°C. In this case it's normal that batteries get quite hot in high current flashlights. They get hot due to the current and, mainly, due to the heat that comes from the head of the flashlight. It certainly does no good to the battery life but that's the price you pay for high output flashlights. Also, discharging the battery at high currents also reduces their life. If you want maximum durability don't go to extreme conditions... I cannot put numbers in the degradation, and it differes a lot between batteries.

Now, if during charging they get 10°C above room temperature either the cell is bad and you'd better throw it away because its performace will be much lower than it should, or you're evidently charging with a waaaay too large current (which you didn't mention either). When I charge my batteries they do not get even warm; I cannot notice a temperature difference between the cells and around with the hand (I always supervise charging). This excessive charge current will also kill the battery life. If you need the battery fast get spares.