Regarding the max temperature limit of lithium battery charging and discharging

[turenabo]

I tested the temperature of a Dyson V8 battery in MAX mode. The battery can run until it is exhausted. The surface temperature of the battery reached 69°C. I guess if the battery has power, the temperature will continue to rise. However, according to the battery specification, the maximum discharge temperature should not exceed 60°C. Is there any problem with using the battery in this way? In addition, the specification says that the charging temperature should not exceed 50°C. I charged the battery when the battery temperature dropped slightly to 65°C, and Dyson's firmware actually allowed charging. Friends, do you think there are any risks in Dyson's product design like this?

[IanB]

It's a lightweight, portable vacuum cleaner, so it is probably intended for quick cleanup jobs. I doubt it is expected to see continuous, heavy-duty use.

That said, I suspect the effect of high temperatures on the battery will be to shorten the overall cycle life and lessen the capacity.

So you can punish the battery if you are prepared to pay the price of earlier battery replacement.

[EPAIII]

And a greater risk of spontaneous combustion.

If the battery can easily exceed the published specifications with just a quick and simple test, then I think it really should have a temperature sensor built in.

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That said, I suspect the effect of high temperatures on the battery will be to shorten the overall cycle life and lessen the capacity.

...<snip>...

[NiHaoMike]

I read somewhere that for fast charging, the ideal temperature is a good amount above room temperature. But not that high, maybe the problem is that with the case removed, the temperature sensor is getting cooled down so that it thinks it's cooler than it really is?

[turenabo]

It only takes 6 minutes to reach this temperature in MAX mode, and as far as I know, many people are used to using MAX mode.

[turenabo]

There is a thermistor inside the Dyson V8 battery as a temperature sensor. My thermocouple is attached to the side of this thermistor.

And a greater risk of spontaneous combustion.

If the battery can easily exceed the published specifications with just a quick and simple test, then I think it really should have a temperature sensor built in.

...<snip>...

That said, I suspect the effect of high temperatures on the battery will be to shorten the overall cycle life and lessen the capacity.

...<snip>...

[turenabo]

In MAX mode, the temperature rises quickly, reaching 69°C in 6 minutes. The sensor of the V8 battery is attached to the battery shell, and my thermocouple is also attached to the side of the sensor, so there should be no temperature difference. The temperature rises very quickly, and I think the plastic shell has little effect on this temperature rise.

I read somewhere that for fast charging, the ideal temperature is a good amount above room temperature. But not that high, maybe the problem is that with the case removed, the temperature sensor is getting cooled down so that it thinks it's cooler than it really is?

[Siwastaja]

69degC is not high enough to indicate any real danger. Also because it's that hot only for some minutes on a temporary basis, it isn't that detrimental to the lifetime.

Many li-ion cell datasheets indeed specify 60degC maximum operating temperature but I'm pretty sure that a manufacturer could, at least under NDA, get more detailed descriptions about short peak temperatures which can exceed this longer term limit.

~70degC is still in the range which does not significantly reduce battery lifetime short term and which pretty much minimizes internal resistance and maximizes current output. I would not be surprised to see such temperatures inside many power tool battery packs during normal use, even well known brands. It's not a panic level yet but on the other hand, it's awfully close to a level where you would like to start limiting the power to prevent any further temperature rise.

Good that they have a sensor. Maybe it triggers at 75 degC, maybe at 80, I don't know, but it can't be much more than that. By increasing ambient temperature by, say, 10-15degC and then run the same cycle, you could test what happens.

[Siwastaja]

I read somewhere that for fast charging, the ideal temperature is a good amount above room temperature. But not that high, maybe the problem is that with the case removed, the temperature sensor is getting cooled down so that it thinks it's cooler than it really is?

I actually ran some cycle life tests on multiple specimen of the same cell and it was quite revealing to see that "the higher the better" held surprisingly far, I mean, going beyond the manufacturer's max 45degC charge temperature spec I was still seeing improvements in cycle life by using even higher temperature up to 50degC or so.

But managing the internal heating and uniformity of the temperature is the key. In my tests, the cells were thermally coupled to a heated aluminum bed. So while 50degC sounds like a lot, the arrangement also sucked out internally generated heat from the cells preventing them from going much higher than said 50deg.

For calendar life, high temperature is bad, but for fast charging, you want to heat beyond room temperature. Luckily, most of the charging damage is done near top, so fast charging empty cell heats it up suitably allowing to keep up the rate. But this all requires care in BMS design and thermal management, EV manufacturers are finally starting to understand this.