Minimizing Battery Capacity Losses

[Pennsylvania RAV]

I'd be interested in everyone's feedback/knowledge on things that can be done to decrease the gradual battery capacity loss.
I know Nissan on LEAF sites states that running the battery from 20 to 80 to 20 to 80% SOC (etc. ) is ideal cycle. They also recommend
letting the battery cool before re-charging (ie. don't pull into garage and plug it in right away and start chargig.) Any other pearls
out there? Is it bad to just always keep it plugged in as soon as you pull in and unplug when you pull out?

I've also heard that capcity loss is a logarithmic function until some point at which there is catastrophic loss...at least that's how
it works in cell phone/lap top batteries. Applicable to EV battery pack?

What other ideas out there to maximize capacity retention?

Thanks!

 

[Kohler Controller]

I'm preparing to run tests on the Panasonic cells we think make up our packs using a standard United States Advanced Battery Consortium (USABC) EV battery life cycle test called Dynamic Stress Test (see pages 15 and 16 at http://avt.inl.gov/battery/pdf/usabc_manual_rev2.pdf)

Basically it runs the battery through a very simple driving cycle (charge and discharge pulses) which remove the energy from the pack in a way not to dissimilar from the way we drive. The highest load is an 80% max power pulse, which if I assume 129kW is maximum power the battery pack can deliver, I will program the cycler to give it a 103kW equivalent pulse (step 15 for 8 seconds). This corresponds to 23W per cell, assuming 4494 cells make up our packs. This means I will be testing under a DST485 profile. I will keep the cells at a constant 30C temperature. I will charge them at a 5 hour rate (C/5) which simulates the fastest charge these cells could ever see.

I will run 4 different scenarios.
1. Charge using Extended Charge, then remove all capacity (~41.8kWh), or 100% available Depth Of Discharge.
2. Charge using Standard Charge, then remove all capacity (~35kWh), or 100% available DOD.
3. Charge using Extended Charge, then remove the capacity needed to drive 50 miles (~16.6kWh), or 40% DOD.
4. Charge using Standard Charge, then remove the capacity needed to drive 50 miles (~16.6kWh), or 55% DOD.

I'm assuming the cells provide >2.95Ah, so 4494 cells gives a true total rated capacity of > 47 kWh. But Tesla is going to limit the top and bottom regions for safety margin, giving a little headroom for regen to occur should you need any right after a full extended charge. They have will keep a little room at the bottom (I believe it is around 8%) to keep the car's systems alive until it can be charged again.

I believe the Extended Charge profile charges the cells up to 96% State of Charge, while the Standard charge gets them to 83% SOC. End of discharge stops at 8% SOC. These are true SOC percentages related to the cell only, nothing related to the GOM or SOC bar graphs, etc.

I hope the results of the tests will let us know the affect that Extended Charge over Standard Charge and what affect of depth of discharge has on the cell (Tests 1 vs 3 and 2 vs 4).

This way I can find the sweet spot to make my pack last as long as possible under my driving conditions (50-60 miles per day, 8 months of mild weather and 4 months of hot weather).

The cells measure 33 mOhms (1kHz AC impedance) each and have a charge efficiency of ~93% (Wh out/Wh in).

Does anyone have any suggestions or feedback?

 

[AvLegends]

I'm preparing to run tests on the Panasonic cells we think make up our packs using a standard United States Advanced Battery Consortium (USABC) EV battery life cycle test called Dynamic Stress Test (see pages 15 and 16 at http://avt.inl.gov/battery/pdf/usabc_manual_rev2.pdf)

Basically it runs the battery through a very simple driving cycle (charge and discharge pulses) which remove the energy from the pack in a way not to dissimilar from the way we drive. The highest load is an 80% max power pulse, which if I assume 129kW is maximum power the battery pack can deliver, I will program the cycler to give it a 103kW equivalent pulse (step 15 for 8 seconds). This corresponds to 23W per cell, assuming 4494 cells make up our packs. This means I will be testing under a DST485 profile. I will keep the cells at a constant 30C temperature. I will charge them at a 5 hour rate (C/5) which simulates the fastest charge these cells could ever see.

I will run 4 different scenarios.
1. Charge using Extended Charge, then remove all capacity (~41.8kWh), or 100% available Depth Of Discharge.
2. Charge using Standard Charge, then remove all capacity (~35kWh), or 100% available DOD.
3. Charge using Extended Charge, then remove the capacity needed to drive 50 miles (~16.6kWh), or 40% DOD.
4. Charge using Standard Charge, then remove the capacity needed to drive 50 miles (~16.6kWh), or 55% DOD.

I'm assuming the cells provide >2.95Ah, so 4494 cells gives a true total rated capacity of > 47 kWh. But Tesla is going to limit the top and bottom regions for safety margin, giving a little headroom for regen to occur should you need any right after a full extended charge. They have will keep a little room at the bottom (I believe it is around 8%) to keep the car's systems alive until it can be charged again.

I believe the Extended Charge profile charges the cells up to 96% State of Charge, while the Standard charge gets them to 83% SOC. End of discharge stops at 8% SOC. These are true SOC percentages related to the cell only, nothing related to the GOM or SOC bar graphs, etc.

I hope the results of the tests will let us know the affect that Extended Charge over Standard Charge and what affect of depth of discharge has on the cell (Tests 1 vs 3 and 2 vs 4).

This way I can find the sweet spot to make my pack last as long as possible under my driving conditions (50-60 miles per day, 8 months of mild weather and 4 months of hot weather).

The cells measure 33 mOhms (1kHz AC impedance) each and have a charge efficiency of ~93% (Wh out/Wh in).

Does anyone have any suggestions or feedback?

All I can say is Thank You for doing the testing and research. Looking forward to your results!

 

[TonyWilliams]

Please do one cell at 50C temperature. Thanks.

 

[Dsinned]

I concur, "THANK YOU". Your test results will prove to be extremely valuable to all of us here!

I presume Tesla (and/or Toyota) did similar validation testing as well as calculating MTTF/MTBF predictions, before deciding on a 100,000 miles - 8 years battery warranty for our cars, but of course, we will never know the actual results.

Btw, how many cells will be used in each of your various test scenarios? Will each test use "fresh" (or brand new) cells, and are you using only Panasonic 18650"A" Li-ion cells, believed to be the type used by Tesla in our RAV4 EV battery packs?

 

[Kohler Controller]

Please do one cell at 50C temperature. Thanks.

I could, but what would that tell you? I realize that high temperature testing is often used as a proxy for speeding up life cycle decay. But since the RAV4EV thermal management system tries to keep the cells at or near 25C as best it can, I wanted to do a test that was specific to our use and application.

 

[Kohler Controller]

I presume Tesla (and/or Toyota) did similar validation testing as well as calculating MTTF/MTBF predictions, before deciding on a 100,000 miles - 8 years battery warranty for our cars, but of course, we will never know the actual results.

Btw, how many cells will be used in each of your various test scenarios? Will each test use "fresh" (or brand new) cells, and are you using only Panasonic 18650"A" Li-ion cells, believed to be the type used by Tesla in our RAV4 EV battery packs?

I don't think they picked the warranty, CA did. I think they just tested the cells to ensure they could meet that goal.

I'm planning on one or two cells for each scenario (depends on how many channels I can get access to). These will be fresh, brand new Panasonic NCR18650A cells.

 

[Dsinned]

You may be right about the terms of the warranty in CA. However, my CA sold VOLT's battery pack (16.5kW total capacity) is even better than Toyota's. My 2012 Chevy VOLT's warranty is 150,000 miles - 10 years. I think that is the best of any EV in the automotive industry. I am sure glad I waited till GM certified for AT-PZEV by CARB! :mrgreen:

Here's some additional info: http://gm-volt.com/2010/07/19/chevrolet-volt-battery-warranty-details-and-clarifications

Note, the above article has a few errors, namely, (1) the VOLT's longer (CA) battery warranty was first introduced in the mid 2012 model year, not with 2013 models, and (2) the VOLT's eligibility for the CA Rebate decreased in value to $1500 at some point. I don't believe it was ever as high as $5000 as stated in the article.

 

[cwerdna]

(2) the CA Rebate decreased to $1500 at some point. I don't believe it was ever as high as $5000 as stated in the article.

I only briefly looked at the article, but the CVRP was $5K initially for the early buyers of EVs like the '11 Leaf.

IIRC, it was lowered to $2.5K before the '11 Leaf model year ended. That made the '12 Leaf less attractive coupled w/the '12 Leaf's price increase (http://green.autoblog.com/2011/07/19/2012-nissan-leaf-higher-price-tag-standard-equipment/).

You can see posts/thread related to the $5K by Googling for site:mynissanleaf.com $5000 cvrp.

 

[TonyWilliams]

Yes, I got $5000 on my first LEAF.

 

[TonyWilliams]

Please do one cell at 50C temperature. Thanks.

I could, but what would that tell you? I realize that high temperature testing is often used as a proxy for speeding up life cycle decay. But since the RAV4EV thermal management system tries to keep the cells at or near 25C as best it can, I wanted to do a test that was specific to our use and application.

With the car sitting on a hot airport Phoenix parking lot for weeks at 50C plus at the asphalt, the battery cells will most likely be at that temperature.

The only alternative is always plugged in, or a bricked pack trying to cool it.

 

[TonyWilliams]

I presume Tesla (and/or Toyota) did similar validation testing as well as calculating MTTF/MTBF predictions, before deciding on a 100,000 miles - 8 years battery warranty for our cars, but of course, we will never know the actual results.

It's regulatory, which is why they all offer that as the minimum warranty.

 

[Kohler Controller]

I believe our RAV4EV cells are similar to those in the Roadster (just newer and of a higher capacity rating, 2.9 vs 2.2Ah?), I thought the forum would like to see this study.

"Plug In America study projects Tesla Roadster packs will retain 80-85% capacity after 100K miles"

http://www.greencarcongress.com/2013/07/pia-20130714.html

My cell cycling testing is currently around 20k simulated miles. I'll report the results when they cross 100k miles and compare to their study.

The full study can be viewed at
http://www.pluginamerica.org/surveys/batteries/tesla-roadster/PIA-Roadster-Battery-Study.pdf

 

[Ampster]

I bought one cell and was going to run it through a simple charge discharge cycle on a PowerLab that I have. Unfortunately my source for the cell wasn't very reliable and I got one that barely had 2amphrs of capacity.

I have subsequently read some battery threads on the Tesla Motors Club Forum that suggested that the cells used in the Model S are different from the ones that are commercially available. From what I recall Tesla and Panasonic working together, slightly changed the chemistry and perhaps also something about the anode to make the cells "automotive grade". One of the articles said that they kept the 18650 form factor in order to use all the same assembly tooling and this allowed them to achieve manufacturing efficiencies which brought the cost down.

Because of that information I gave up on being able to achieve any kind of meaningful analysis. It does sound like your mode of testing will be more scientific than my simple charge discharge cycle tests and I applaud you for that effort.

EDIT Here is the source for the above information:
Source:
Tesla Motors CTO talks future batteries and charging protocols
SAE International, 12-Mar-2013
We use a nickel-cobalt-aluminum (LiNiCoAlO2) lithium-ion chemistry for our battery cathode material. We don’t use a titanate, which has about half the energy density but is generally good at high charge rates. Some start-ups are using metal oxides; we fall broadly in that category. At this point we really have heavily customized that cell. We’ve totally custom-engineered that cell working jointly with Panasonic to create. It’s an automotive cell, tested to automotive standards. It doesn’t go into laptops anywhere. What keeps us in that general shape and size is the production and cost efficiency. We’re seeing price points that none of the larger-format cells are able to meet.

 

[TonyWilliams]

I believe our RAV4EV cells are similar to those in the Roadster (just newer and of a higher capacity rating, 2.9 vs 2.2Ah?), I thought the forum would like to see this study.

"Plug In America study projects Tesla Roadster packs will retain 80-85% capacity after 100K miles"

The Roadster and Model S/X are actively cooled. Rav4 EV is not. There will be a significant difference in a hot ambient environment Rav4 EV if the owner does not take all due precautions.

That might mean actually leaving the car in READY mode all day, or charging at the slowest rate possible on 120 volts to keep the TMS operating.

It only cools the battery on Rav4 EV with the car in READY or when charging.

 

[waidy]

The Roadster and Model S/X are actively cooled. Rav4 EV is not. There will be a significant difference in a hot ambient environment Rav4 EV if the owner does not take all due precautions.

That might mean actually leaving the car in READY mode all day, or charging at the slowest rate possible on 120 volts to keep the TMS operating.

It only cools the battery on Rav4 EV with the car in READY or when charging.

My Rav has active cooling. It did it again last night. When I walked into my garage, it has the buzzing noise. In additions, when I started it this afternoon, the AC turned on when I powered my Rav on. I never use AC, it is very annoying the active cooling turn on my cabin AC.

 

[user 340]

My Rav has active cooling. It did it again last night. When I walked into my garage, it has the buzzing noise. In additions, when I started it this afternoon, the AC turned on when I powered my Rav on. I never use AC, it is very annoying the active cooling turn on my cabin AC.

Interesting. How much range do you lose on the GOM in a day when your car is unused but doing active cooling?

 

[Ampster]

My Rav has active cooling. It did it again last night. When I walked into my garage, it has the buzzing noise. In additions, when I started it this afternoon, the AC turned on when I powered my Rav on. I never use AC, it is very annoying the active cooling turn on my cabin AC.

So that must mean it was plugged in to the charging cable to be making that noise.

 

[waidy]

Interesting. How much range do you lose on the GOM in a day when your car is unused but doing active cooling?

I lost 4-8 miles per day pending on weather.

So that must mean it was plugged in to the charging cable to be making that noise.

Nop. it makes noise even without charging cable plugged in.

 

[user 340]

Interesting. How much range do you lose on the GOM in a day when your car is unused but doing active cooling?

I lost 4-8 miles per day pending on weather.

So that must mean it was plugged in to the charging cable to be making that noise.

Nop. it makes noise even without charging cable plugged in.

That is really interesting! Losing 4 - 8 miles is probably a couple kWh, maybe enough for cooling if the car is in a temperate bay area garage. I suspect that leaving the car on (EV system on) will drain at a higher rate though, from the postings of others. You have 2 2012+ RAV4s, right? It might be interesting to compare their firmware versions. That said, I am skeptical that the firmware version we get through our Nav displays has much to do with the actual firmware of the car's EV components.