Dyson battery charging and discharging temperature limits

[turenabo]

Last week I posted a post about the temperature limits of Dyson battery charging and discharging. I said that the battery can still discharge at 69℃ and charge at 65℃, which shocked me. Today I will update my test data. In addition, I also added a power meter to see the power during charging. I found that Dyson set the discharge cut-off temperature at 70℃. After 70℃, it still has a blue light but stops discharging. When the temperature drops below 70℃, it can discharge normally. Charging is very interesting. Through the power meter, you can see that the battery light will flash at any temperature as if it is charging, but it will only be truly charged when the temperature drops below 40℃. I tried to increase the temperature continuously from 70℃. When it was increased to 80℃, the red light flashed. When I lowered the temperature, even to room temperature, the red light still flashed, indicating that the BMS was permanently locked. Can any friends come to analyze why Dyson overuses the battery when discharging and derates when charging? Is it designed like this based on feeling? Or is there sufficient experimental data basis?

[tom66]

"Dyson are rubbish" is the short answer! 

My best guess is they found that the batteries do get very hot under discharge - a stick vacuum may be pulling 120-150W with peaks of 200W depending on the model.  If the vacuum runs for 30 mins on a charge, and they're using normal 18650s in, say, a 5S configuration, then they are potentially pulling 8A continuously from those little cells.  They are pushing the cells to the limit, but their consumers demand lightweight, high performance products, and they build them cheaply.

If you look at EV batteries for a comparison, all EVs begin to rapidly power limit around 50°C on discharge, and charging is normally limited to around 50-55°C.  The best charging speeds are available between 40-50°C so battery preheating usually targets around this point when using rapid DC charging stations. 

As for charging, degradation occurs more rapidly as you go above 50°C when charging, so it could be an issue for warranty claims.  This is especially an issue as the battery may be charged at 50°C and then left at 100% whilst still hot, which accelerates degradation further.

[Siwastaja]

Discharge cut-off of 70degC and charge cutoff of 40degC sound completely normal, nothing crappy here. Also permanently disabling if 80degC is detected during charge sounds like a good safety feature. I find absolutely nothing abnormal or alarming here, sounds like good engineering.

For some reason, each time Dyson is mentioned on this forum you receive a lot of posts saying Dyson is crap. I have no idea about their general quality but given this datapoint however their BMS engineering sounds very decent.

[tom66]

Discharge cut-off of 70degC and charge cutoff of 40degC sound completely normal, nothing crappy here. Also permanently disabling if 80degC is detected during charge sounds like a good safety feature. I find absolutely nothing abnormal or alarming here, sounds like good engineering.

For some reason, each time Dyson is mentioned on this forum you receive a lot of posts saying Dyson is crap. I have no idea about their general quality but given this datapoint however their BMS engineering sounds very decent.

Disagree. 70°C is way too hot to cut out for a modern NMC/NCA lithium ion cell (which these vacuums almost certainly use.)  You will degrade the cell, especially if it is a sustained temperature.

I know a couple of people who worked at Dyson, the engineering culture is not what I would describe as positive.   They pretend they are the Apple of vacuums, but they don't seem to put as much effort into longevity as a company like Apple does.   

[Siwastaja]

70°C is way too hot to cut out for a modern NMC/NCA lithium ion cell (which these vacuums almost certainly use.)  You will degrade the cell, especially if it is a sustained temperature.

What logical fallacy. It's a safety cutout. Existence of safety cutout at 70degC does not degrade the cell as if the cell was at 70degC . If the battery pack frequently runs close to 70degC, especially for prolonged time, then that will be detrimental for cell health but there was nothing in the opening post suggesting that, on which I based by reply. In the other thread made by OP they mentioned they had to increase ambient temperature beyond normal to reach such temperatures, suggesting the design is OK.

Quite the opposite, cutoff being at 70 degC makes "it being a sustained temperature" logically impossible! If it operated close to 70degC in normal use, the cutout would trigger all the time and customers would be very dissatisfied by the interruptions.

You can't affect this operating temperature by choice of safety cutout. You affect the temperature by varying pack ESR (choice of cell type or capacity) or cooling design. Keeping the actual temperature low gives long operating life. Safety cutoff is then just safety cutoff.

[Xena E]

For some reason, each time Dyson is mentioned on this forum you receive a lot of posts saying Dyson is crap.

I have no comment on the BMS system, however I'm sorry to have to say that my two experiences of Dyson were not good.

Nearly £1800 worth of sorry.

A washing machine that damaged clothes and was a breakdown nightmare straight out of the box: it cost £1200.

After exactly two years of hassle it failed again flooding my utility room, I was told that the guarantee was out.

I was quoted £220 for a repair, so I replaced it with a cheap,  (Indesit, same £220 to buy outright), machine that has functioned satisfactorily for the last six years.

That followed the purchase of an upright corded vacuum cleaner that was impressive initially,  (hence the purchase of the WM), however the plastics deteriorated alarmingly and it basically fell apart at four years old, now have a Vax that has remained fault free also for six years.

After sales service was a joke. On the first breakdown, taking seven weeks to source a replacement part for the Washing machine. There was a flat refusal on the part of the seller to take the machine back and refund, Dyson said that they had no legal responsibility to do so.

Considering you can buy better at a fraction of the price then yeah, IMHO it's total garbage: wouldn't touch it with a bargepole ever again.

Regards,
X.

[turenabo]

Dyson V8 has two working modes, normal mode and MAX. In MAX mode, the power reaches 425W. The battery pack is 6S. If so, when the cell voltage is close to 3V, the current will be 23.6A. I found that the heating speed of the battery depends on the internal resistance of the cell. A relatively new battery cell has an internal resistance of 11mΩ. It has not reached 70℃ (room temperature 30℃) after working in MAX mode for more than 6 minutes, while a battery with an internal resistance of 16mΩ has reached 70℃ after working in MAX mode for only 4 minutes. I heard that some friends like to use MAX mode. If he works continuously every time, the battery temperature will reach 70℃ every time he uses it. In addition, I also found that since I tested the surface of the cell, the BMS can only test the surface of the cell. When the discharge is cut off at 70℃, the surface temperature of the cell will continue to rise to 73℃, which means that the temperature inside the cell may be higher than 73℃ before the cutoff.

"Dyson are rubbish" is the short answer! 

My best guess is they found that the batteries do get very hot under discharge - a stick vacuum may be pulling 120-150W with peaks of 200W depending on the model.  If the vacuum runs for 30 mins on a charge, and they're using normal 18650s in, say, a 5S configuration, then they are potentially pulling 8A continuously from those little cells.  They are pushing the cells to the limit, but their consumers demand lightweight, high performance products, and they build them cheaply.

If you look at EV batteries for a comparison, all EVs begin to rapidly power limit around 50°C on discharge, and charging is normally limited to around 50-55°C.  The best charging speeds are available between 40-50°C so battery preheating usually targets around this point when using rapid DC charging stations. 

As for charging, degradation occurs more rapidly as you go above 50°C when charging, so it could be an issue for warranty claims.  This is especially an issue as the battery may be charged at 50°C and then left at 100% whilst still hot, which accelerates degradation further.

[turenabo]

I am learning about BMS through Dyson batteries. My biggest question now is how to design my own BMS. Based on the battery cell specification, I should set the discharge and charge limit temperatures at 60 and 50 degrees Celsius, but after learning about Dyson's BMS, I am confused. I think Dyson's design makes sense, but I am also very clear that this is definitely not the standard answer. If the limit is set too high, it will affect the user experience, and if it is set too low, it will affect the lifespan and even safety. If I want to study this field more deeply, are there any good learning materials you can recommend? Thank you

Discharge cut-off of 70degC and charge cutoff of 40degC sound completely normal, nothing crappy here. Also permanently disabling if 80degC is detected during charge sounds like a good safety feature. I find absolutely nothing abnormal or alarming here, sounds like good engineering.

For some reason, each time Dyson is mentioned on this forum you receive a lot of posts saying Dyson is crap. I have no idea about their general quality but given this datapoint however their BMS engineering sounds very decent.

[Siwastaja]

What does the manual say about MAX mode? It is not uncommon to include short peak power features which can't be sustained for long, e.g. every EV on the market does this.

Remember in resistance calculations that increasing temperature also decreases the resistance so power dissipation decreases compared to room temperature value. At the same time higher temperature also means more power can be dissipated through the thermal resistances. This is so complex to model that practical measurements are often the best.

Really, the relevant questions to assess the design quality are:
1) What does the manual say about the MAX mode?
2) Under normal ambient temperature (say, maybe max 30 degC, if you have more than that in your home you have other problems), when used per manual, how hot does the cells become and for how long?

Also consider things like do you need to run full cycles back-to-back? I.e., is the vacuum cleaner good enough to get your "typical" household cleaning done in one discharge cycle, or do you need to instantly charge then start another cycle; if latter, how hot do the cells become on the second cycle? Or is the charging slow enough (with the forced cool-down to 40degC before starting as shown by the OP) that this does not matter? And if charging is slow and many cycles needed for usual cleaning, then surely the problem is more the crappy user experience than cell life?

[tom66]

The difference is, a performance EV* will be in 50% power derating by the time it gets to 50°C cell temperature, and probably 80% derating by the time it gets to 55°C.  That will stop cell temperatures going much above 55°C, whilst still allowing for short bursts of acceleration at full power as long as the pack is cool enough.  Because EV manufacturers offer 8 year, 100,000 mile warranties on their batteries and Dyson offers a 2 year warranty.

*Parameters from Tesla Model 3 performance LR.  A regular average-performance EV won't get anywhere near these temperatures.  Highest I've seen on my ID.3 58kWh is 45°C when hammering it in hot weather (30°C).

[Siwastaja]

Yeah, sure EV's implement more gradual power limiting, but really, a vacuum cleaner? I bet 90% of other manufacturers have no such cutout at all, or depend on stuff like 18650 PTC caps reacting at 100degC or so, then reverting back to higher than original resistance.

What is the duty cycle for a vacuum cleaner anyway? Are you really going to drive it against this thermal cutoff many times every day? I mean, even if you hit the limit say once a week in a more thorough cleaning, you would accumulate like 15 minutes * 52 = 13 hours of high temperature operation in a year.

Besides, jury's still out about the claims about cells degrading that significantly faster at 70degC. I have personally tested at 60degC and of course the capacitance fade and DC ESR rise increases manyfolds compared to room temperature but those tests still had to be run for months straight to see any degradation, how much do you think accumulated high temperature damage is a vacuum cleaner going to see?

Besides, high temperature damage is mostly combination of high temperature and high SoC, which does not happen on discharge cycle. The key question is how quick you allow charging before cooldown. And seeing the limit is 40degC here sounds quite conservative, it's close to charge temperature sweet spot (colder is worse, too).

Much more detrimental than short peaks of high discharge temperatures are long sustained (means, months of accumulated time, not minutes) temperatures, and high charge currents near 100% SoC or at temperature too low/high.

To me as a battery system designer the choice of these limits sound like they would prevent any serious degradative use pattern and transition it into poor user experience (I would hate it cutting off while I'm trying to clean). But does it trigger often in real life, that's the key question?