It's All About the Delta (and Temperature, but the BMS will Protect You) -- Test: Effect of temperature on battery cells life

[MickeyAO]

TL;DR you are missing out on actual data that is already summarized as much as I can while presenting the facts about the cells in your truck. I'm not going to summarize it further for you

So, I recently retired from the Institute (and received my final paycheck with my 8 weeks of vacation being paid out) and the exit documents I signed only talked about trade secrets, so I think it is safe to share some actual (normalized) data explaining what I've been talking about over the last couple of years on this forum.

For the last 14 years I have been testing Li-ion cells from all sorts of chemistries (LTO, LCO, LMO, LFP, NCA, NMC, and others that I can’t talk about, but hopefully you should be hearing about in 3 years if all goes well). My lab was more automotive centric, but I have certified camera battery packs for the space station, tested cells for medical implant devices, and even worked on the project for a 3mW pack for a mining company. I actually got my start in energy storage by working on the control system for a flow battery 16 years ago (my degree is in Computer Science, and what I was originally hired at the Institute for). Over the years I have learned how to kill cells (some clients wanted cells to fail) and how to make them last (virtually) forever, and purposely making them catch fire through abuse testing. I say this to show I’ve been at this awhile and know what I’m doing by observing real time data (and maybe a little patting myself on the back after 2 successful 20 years careers and retirements from the USAF and the Institute ).

You all have seen my advice on what is happening with the cells and how to make them last (virtually) forever. I'm sure you have seen all the naysayers who drove EVs for xx number of years and have seen data from the OBD port saying it doesn't matter. Here is some hard actual data.

About the methodology, a Taguchi L9 matrix with 3 charge currents, 3 discharge currents, 3 temperatures, and 3 delta SOC for cycling, with 2 samples per test. The delta is centered on 50% SOC. That means a 30% delta is between 35% and 65% SOC. Despite the varying conditions, all cells end up cycling just about the same kWh of energy throughput. It makes it very easy to see which of the conditions is most dominant in degradation. We are then able to build a mathematical formula that will calculate the lifespan for any type of duty cycle we feed into it. On the side, I ran my own test cycling between 80% (actual so 85% SOC on the display) and 50% SOC. Results closely tracked the cycle 30% delta at 25C as I suspected, but I did not graph it.

Calendar life is 3 temperatures and 3 storage SOCs. When you see 80% SOC on calendar life, it means it sits for 4 weeks at 80% SOC (actual) at the listed temperature.

Cycle life Reference Performance Test (RPT) is performed every week at 25C so we can compare the results across all test coditions. Calendar every 4 weeks at 25C. While we take pulse power and resistance for charge and discharge at 10% SOC increments (90% to 10%), the graphs we produce typically only show discharge pulse power and resistance at 50% SOC. I'm not going to bother showing those graphs. I will show capacity loss based on the test conditions. RPT0 (the first dot) was done before any of our testing started.

We do 10 RPTs for calendar, and 20 for cycle life. We were only at RPT8 for calendar life (RPT9 started on my last week) but have completed cycle life. If you see a test that suddenly has no more dots, then it had to be pulled from testing. In most cases, it was because the cell started to swell (produced off gassing that was trapped in the cell skin). In a couple of cases, the resistance just got too high for us to conduct the RPT…the voltage would shoot past Vmax at the first application of the pulse.

I've normalized the graphs, so you can't see the actual values. I will tell you that RPT0 was just under the rated capacity for the cell (we bought modules from a 3rd party that did some light vehicle testing before pulling the modules to sell to us), and the top of the graph is around 10 Ah above the rated capacity. I'll leave it to the reader to find the rated capacity for the SKE805A... hint, it's in the nomenclature of the name of the cell. BTW, I have 1st hand knowledge that you cannot replace a single cell in a module.

Calendar life started at 25C, 45C, and 55C. The early results from 55C and the previous results of the thermal stability test forced us to use a different temperature for cycle life (I threatened my colleagues that I would stop cycling anytime the cell hit 60C and let the cell cool back down). 10C was chosen as it helped the spread for the model, but I was arguing for 35C as a more real-world condition.

All testing was at actual SOC of the cell without a BMS, so you will have to convert with what the truck reports on the screen. This is accelerated testing, but I have not seen the model (it will be produced in about 10 weeks, and I won't have access to it), but this falls somewhat in line with what I have seen over the last 14 years of cell testing.

For cycling, the 2 dominate conditions on degradation were temperature (very dominate but the BMS should protect you from this) and delta SOC (almost as dominate). Minor degradation conditions were charge and discharge currents (high being the worst for both). Nothing here surprised me EXCEPT for the VERY low thermal stability point (about 35C or more below what I’ve seen in other NMC cells), the 1 calendar condition at 45C (I have never seen where 80% SOC outperformed lower SOCs and made a post about it), and 30% delta at 10C (Li-ion cells don't like being cold, but 30% delta should have been better on sample 2). As my mentor Bapi liked to tell tours, Li-ion cells like the same temperatures humans do.

All this said, your battery has a warranty for 8 years down to 70% capacity loss. This information is only a concern if you plan on keeping the truck for the long term or don't want to go through the hassle of a warranty replacement on the battery.

 

[Grumpy2]

Thank you.

 

[Maquis]

Good stuff. Thanks, Mickey.

 

[Tom Jensen]

Thank you. Interesting info and analysis.

 

[Mashedtators]

Thanks for sharing this. Great info

 

[Webbo85]

Congrats on your retirement! And thanks for the data!

 

[jetfixr1]

Awesome info. One tidbit often overlooked is the little note in the manual: “Set your preferred charging times to be at least 2-3 hours after your typical plug in time. This allows the battery to cool before charging begins.” This is aligned with your testing showing how temperature can affect the modules.

 

[Riverant]

I understand not diluting the analysis with a TLDR, but what is the actionable takeaway from this for us owners? I assume it’s in the charts but I admit to this being nowhere near my field of knowledge.

 

[lakeguy55]

I did read (twice) and will admit to only lightly grasping the content. But I plan to keep my truck for a long time. So based on this post and others by Mickey, my learnings are;

• Large delta charges (greater than 30%) are not good for the batteries.
• Ideally, 30% deltas are centered around 50% SOC. But given real world need for range, I charge to 80%, ideally starting the charge before I go below 50%.
• I keep it garaged so extremely low temps are rare for me, as are high temps given the NH climate. But the battery management helps here.
• I try to precondition while plugged in whenever possible.
• ABC, always be charging. I plug in whenever I'm home, only charging to the 80% level.
• The biggest impacts to battery health are charge delta and temperature.

 

[I <3 My Lightning]

Let me clarify for all of you. Blah blah blah, just drive the vehicle and enjoy it for at least 8 years and/or 100k miles. Then if the wheels fall off put it up on blocks and use it for house battery back up.

 

[jetfixr1]

I understand not diluting the analysis with a TLDR, but what is the actionable takeaway from this for us owners? I assume it’s in the charts but I admit to this being nowhere near my field of knowledge.

TLDR: 1. The most important issue and key takeaway is to avoid large temperature swings. The BMS should take care of this, and seems to do a fine job given the battery temp is always in the middle on my dash display. The original leafs with air cooled batteries degraded so quickly because a lack of a BMS and temperature regulation.

2. Avoid large charge/discharge cycles. It seems not as important as 1 according to Ford, since they say you can charge to 90% every day. These batteries love to be closest to 50%. You can take the one way % consumption of your commute and add it to 50%. If you use 15% to get to work, charge to 65%. By the time you get home, you should be at 35%. This keeps you closest to 50%. IMO, I just charge to 80 every night and 100 for roadtrips.

 

[Maxx]

I am mostly lost here, can one of you guys that understand these charts dumb it down for me? What is the difference between the two colors? If they are two samples going through the same test why are they so different? Also in the following why 50% SOC has sharper drop than 80%?

I am concerned about what behavior or conditions contribute to cell failure rather than just capacity loss. Can you guys tell from these charts if the two are directly correlated?

Can someone translate the following one to a 5th grade level?

Awesome info. One tidbit often overlooked is the little note in the manual: “Set your preferred charging times to be at least 2-3 hours after your typical plug in time. This allows the battery to cool before charging begins.” This is aligned with your testing showing how temperature can affect the modules.

I always wondered about this and the fact that battery likes to be at body temperature. In cold winter days, after a short drive, battery is slightly warmer than it was when I left and it is warmer than it will be after sitting on my driveway for three hours. So temperature wise it would make sense to charge immediately after I arrive home. Waiting may have something to do with cells balancing which I don’t quite understand. So I am still uncertain about which practice is better in winter.

 

[jetfixr1]

I am mostly lost here, can one of you guys that understand these charts dumb it down for me? What is the difference between the two colors? If they are two samples going through the same test why are they so different? Also in the following why 50% SOC has sharper drop than 80%?

I am concerned about what behavior or conditions contribute to cell failure rather than just capacity loss. Can you guys tell from these charts if the two are directly correlated?

Can someone translate the following one to a 5th grade level?

I always wondered about this and the fact that battery likes to be at body temperature. In cold winter days, after a short drive, battery is slightly warmer than it was when I left and it is warmer than it will be after sitting on my driveway for three hours. So temperature wise it would make sense to charge immediately after I arrive home. Waiting may have something to do with cells balancing which I don’t quite understand. So I am still uncertain about which practice is better in winter.

EDIT: I hit tab+enter too soon without the full explanation. Here is the full answer.

Each color represents one battery cell, pouch, etc. undergoing testing. OP does not say which ones they are. It could be a battery cell used in a Tesla vs a cell used in our Lightnings. It could also very well be a random benchmark to compare our cells to. I would assume we are looking at the battery pouches used in a Lightning, since this is the Lightning forum.

The x axis represents the testers version of charge and discharge cycles, OP refers to this as the Reference Performance Test (RPT). The charts show various conditions the cells are tested at: 10C, 25C, 45C and 55C. Along with that is the SOC the battery is tested at. The Y axis represents cell capacity. You can see as the testing is being done, those cells closest to 25C and 50% SOC lose the least amount of capacity. I do agree with OP, I would like to have seen these cells tested at 35C which would represent the closest real world conditions our trucks would likely encounter.

As far as waiting a couple of hours before plugging in, it is more likely meant for summer operation. Hot outside air temp, hot battery, and now you induce a charge which will likely spike cell temperatures higher than they already are. In the winter, it shouldn't be an issue. As a matter of fact, below certain temps the truck requests you to plug in right away.