The Science and Math - Battery Charging

[PoorShah]

After weeks of bugging Porsche Manufacturing for answers to my EV battery questions specifically for my GTS

Gross Capacity = 93.4kWh, 396 cells
Usable Capacity = 83.7kWh, Porsche defines usable capacity by the following parameters:
- Discharge rate (
- Expected end user temperature operating ranges (where are the customers driving their cars)
- Depth of discharge
- Cutoff of voltages
- BMS electronics error
- Capacity fade compensation ( primarily based on manufacturers warranty. High compensations typically correlates to longer manufacture battery warranties)

Target upper limit = 75-80%, exceeding targets promotes:
- structural degradation
- dissolution of active materials
- loss of conductivity
- reduction of lithium potency
- in other words, increased heat and stress do to lithiums rate of energy absorption as it reaches its full absorption capacity.

so what is the true 80 upper limit?
Answer is 89.2%, calculated as follows:
- 74.7 is true 80% of 93.4kWh
- 74.7/usable = 89.2%

In a nutshell, 90% would be a good average upper limit for charging for those of you who asked what’s the highest you could reasonably charge daily without impacting performance and longevity. Porsche will recommend 80, as they should given it would be a far more conservative and safer percentage to ensure they can fulfill their 8 year HV warranty, and ultimately ensure longer battery performance.

if anyone can explain any of those terminologies with greater scientific depth and ease of understanding. Would love for you to share.

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[ciaranob]

Have a look at the two videos in this first post on this thread - lot of relevant info pertaining to above:

#1

 

[KLHubb]

Have a look at the two videos in this first post on this thread - lot of relevant info pertaining to above:

#1

Take a look at Dr. Dahn's videos (Dalhousie University, Nova Scotia) His discussion is very informative.

 

[ciaranob]

Take a look at Dr. Dahn's videos (Dalhousie University, Nova Scotia) His discussion is very informative.

Eh - that is what my link is to - am I missing something?

 

[azraeil]

I think the Taycan Manual recommends 85%

 

[ciaranob]

I think the Taycan Manual recommends 85%

I think you have a lot to catch up on . Have a look at the video links prev posted but if you prefer 85% no worries.

 

[W1NGE]

85% has always been the recommendation - high water mark on battery display on PCM, lower display, display in My Porsche App, target charge setting etc (not to mention the owner's manual.

 

[azraeil]

I think you have a lot to catch up on . Have a look at the video links prev posted but if you prefer 85% no worries.

Yeah, I followed the video by Prof Dahn. Now switching to lower the charging depth. Previously I charge from 30-85%, now charging everyday (79-85%).

Porsche says 85%, Dahn and a lot of other experts says 80%. Was doing 80% before and then went to 85% after reading the manual. Now thinking of maybe going back down to 80%

 

[utsteve98]

Maybe 85% versus 80% is recommended by Porsche because we have battery reserve that’s not accessible. 85% is really more like 77% of true battery capacity.

 

[Vim Schrotnock]

I've been charging my car overnight to 85% pretty much every day for almost 3-1/2 years, and have over 36,000 miles. I had two (of 33) modules replaced last August, and had a very informative discussion with the Porsche Battery technician. He guessed that one of the cells was probably bad from the beginning, and replaced the other out of an abundance of caution. I specifically asked him about my routine, and he said charging every day to 85% is absolutely fine. For you OBD aficionados, my OBD SOH is 93.47%, and mileage range at 100% is running 260-280 mi. depending on the temperature here in the winter.

 

[PoorShah]

I've been charging my car overnight to 85% pretty much every day for almost 3-1/2 years, and have over 36,000 miles. I had two (of 33) modules replaced last August, and had a very informative discussion with the Porsche Battery technician. He guessed that one of the cells was probably bad from the beginning, and replaced the other out of an abundance of caution. I specifically asked him about my routine, and he said charging every day to 85% is absolutely fine. For you OBD aficionados, my OBD SOH is 93.47%, and mileage range at 100% is running 270-280 mi.

Thanks for sharing. Great real life data points. Hoping if we have any battery issues, we are able to discover them early on. Since we only drive 2-3000 miles a year on this, we’ll likely go past the warranty before any big issues pop up.

 

[WasserGKuehlt]

Maybe 85% versus 80% is recommended by Porsche because we have battery reserve that’s not accessible. 85% is really more like 77% of true battery capacity.

Correct - 80% vs 85% of what - without an agreed-upon definition they're all "expert recommendations".

In addition to the points made by @Vim Schrotnock, I prefer 85% as it's close enough for an impromptu "charge to full" scenario, should I only have a few hours' worth of notice before a long trip. It's ~1hr to 95% (and another 2-3 hrs for 100%). I don't charge every day, so I'm not really worried about targeting 85 as the default.

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