What is the target SoC to initiate topping up to 85% in order to optimize battery life?

[Tooney]

After reading everything I still don’t get it. But to be honest I don’t want to understand either. Too complicated and too much constraints. The software of the car should be intelligent enough and do all the thinking about charging, leveling, battery health etc for me right? Like a push notification that I should approve or deny proceeding with charing when the car suggests it shouldn’t or something similar. Or even with some colors on the battery screen inside my car. With green, red and orange zones. If I charge further than x% and it is seen as bad that the battery turns from green to orange?

Dealing with topics like:

What is best charging procedure for battery longevity?​

Why has the range recently increased/decreased?​

Why is your car's energy efficiency so much better than mine?​

Has anyone else lost Porsche Connect signal?​

Why so few/did my car miss OTA updates?​

Why aren't my Taycan's charging profiles and timers working right?​

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[Dr Bob]

@Dr Bob I want to believe you, but I am having a hard time doing so with that profile pic

Absolutely! I got it as a nickname when I left university with a PhD centuries ago and its now my reg number on the Taycan. Trust me, I'm a Doctor........mmmmmmmm? I'm assuming people remember the muppets?

@Dr Bob
You state that bottom and top 10% are the limits one should not go over, but then introduce those different numbers of 25 and 85%, so called knees.
Which ones is it then: 10-90 or 25-85?

It is quite straightforward. Your batteries will last years (10 years plus???) if you never went outside 25-85 (ish)%. If you push into the knees then you will start to stress the batteries and the further you go into the knees the more you will stress them. Keep in the safe range if you arent doing more than 150 miles a day but dont be afraid to take more of the range - but dont do it everyday. If you are only keeping the car for 3 years then you can be more bullish but there do seem to be many battery failures reported on here which will be a combination of the use of the voltage knees and caused very likely by out of spec batteries.

Doesn’t the net capacity already addresses this key issue already? At least on the top end.

DCYL, Yes, it will help as when we see 100% SOC, the actual charge will be only 95% (I'm assuming there will be 5% at top and bottom) but you are still well into the voltage knees at 95%. But you are right, and that's why I am happy to charge to 100% overnight for a long run the following day.

It is actually a bit more complex from what I have written with a few other key things going on - but I dont want to get into more detail as it will loose 95% (or is it 85%????) of the readers.

 

[Tooney]

It is actually a bit more complex from what I have written with a few other key things going on - but I dont want to get into more detail as it will loose 95% (or is it 85%????) of the readers.

Aw, cmon. Some of us love more detail about this.

 

[MrB]

Absolutely! I got it as a nickname when I left university with a PhD centuries ago and its now my reg number on the Taycan. Trust me, I'm a Doctor........mmmmmmmm? I'm assuming people remember the muppets?


It is quite straightforward. Your batteries will last years (10 years plus???) if you never went outside 25-85 (ish)%. If you push into the knees then you will start to stress the batteries and the further you go into the knees the more you will stress them. Keep in the safe range if you arent doing more than 150 miles a day but dont be afraid to take more of the range - but dont do it everyday. If you are only keeping the car for 3 years then you can be more bullish but there do seem to be many battery failures reported on here which will be a combination of the use of the voltage knees and caused very likely by out of spec batteries.


DCYL, Yes, it will help as when we see 100% SOC, the actual charge will be only 95% (I'm assuming there will be 5% at top and bottom) but you are still well into the voltage knees at 95%. But you are right, and that's why I am happy to charge to 100% overnight for a long run the following day.

It is actually a bit more complex from what I have written with a few other key things going on - but I dont want to get into more detail as it will loose 95% (or is it 85%????) of the readers.

Feed my inner geek please, I’d love to know more if you can spare the time.

 

[Dr Bob]

Feed my inner geek please, I’d love to know more if you can spare the time.

Dr Bob was my hero.

I'll get back on the rest later.

 

[Vim Schrotnock]

Dr. Bob - a comment and some questions. It seems like we're overcomplicating things a bit here. I've been told a number of times by the Certified Porsche Battery Technician (one of 11 in the country, but not a PhD, and possibly not the final authority here) that charging to 85% each day is 'perfect'. Here's the process he outlines, and I'd appreciate your comments:

1. Charge to 85% using your home charger every night.
2. Every few months, drain the battery to 10-15% and let it sit overnight to allow the cells to 'balance'.
3. Recharge to 100% using home charger and let it sit overnight as well. (Some question here remains)

The thinking is that frequent small, slow charging is optimal, and you need to discharge/charge to the high and low ends periodically to allow the BMS to 'balance' the cells at these low and high voltages. He also stated that discharging to the low voltages will identify 'bad' cells that might not be apparent at the standard 'working' voltages. Your thoughts?

 

[Dr Bob]

The thinking is that frequent small, slow charging is optimal, and you need to discharge/charge to the high and low ends periodically to allow the BMS to 'balance' the cells at these low and high voltages. He also stated that discharging to the low voltages will identify 'bad' cells that might not be apparent at the standard 'working' voltages. Your thoughts?

I was about to try and write a response to MrB and Tooney with some more battery info, but let me first answer your question Vim, as that is the easy one.

Yes, the Porsche Tech is spot on. Agree with everything he is saysing. Let me try and explain why he is saying what he is saying.

The 85% advice is based on what I said in the earlier post. Operate if you can without going into the Knees of the voltage curve, ie keep the SOC less than 85% (ish). I say 'ish', because there is no hard number. That's the region when you know you are not in the knee. If you stick to 25% to 85% then very little is likely to go wrong. However, if things do start to go wrong, it is likely to start from an in-balance in the cells. I'll try and cover why you get an in-balance in a future post, but if you start getting an in-balance, you can not see that in-balance if you are on the voltage graph plateau (ie not in the knees). Because the cell voltage does not vary much wrt SOC in the 25% to 85% range, you cannot estimate SOC of each cell. To ensure the cells remain balanced, the instruction is to take it up to 100% SOC now and again (ie every few months). As you take it above 85% and get into the voltage knee, the car's BMS (battery management system) can see any in-balance from the individual cell voltages and start to correct the in-balance. Taking it up to 100% and keeping it there for a period (ie overnight) gives the BMS chance to complete the balancing, so is a good and important thing to do.

It is important though to understand why 'balancing' is needed to explain the 'take it to 100% then discharge to 10-15%' advice. In our cars, we have 1,000s of battery cells. Each has its own capacity. Let's assume each has a capacity of 10Wh. All together they give the Taycan battery capacity of 93kWh. There will be a specification for each cell that allows for small variation in capacity. I dont know the detail for the Taycan but lets say the variation is 9.999Wh to 10.001Wh. The key is when charging is to get all cells to 100% at exactly the same time. If cells A, B, C...etc all have a capacity of 10.000Wh but cell Z has a capacity of 9.999Wh then the BMS has to put a little less power into cell Z so when charging, they all get full at the same time. The problem then is that if the variation is much more and say cells A, B C etc are all 10.000Wh but cell Z is only 9.000Wh, you then have an issue. It is easy to balance at the top end to get them all to 100% at the same time but think then about what happens when you discharge to the bottom. Cell Z only has 90% of the capacity of the rest so could be at 0% SOC when the bulk of the cells are at 10%. Hence the voltage of cell Z will be much lower than the rest at the bottom. This is why the Porsche tech is suggesting taking the bank down to 10-15% as it then easy to see a low voltage cell – which as you say is not easily seen when out of the knees. If there are low voltage cells, the Tech can do something about it.

It's important to realise that you can only balance to get EITHER all the cells reaching 100% at the same time OR balancing to get all cells reaching 0% at the same time. You cant do both. EVs all balance to the top end to ensure you can get 100% SOC. The danger then is that if you have cells with widely varying individual capacities, then when you are balanced at 100%, you will not be able to use the full capacity at the bottom end. This is when the BMS starts reporting low cell voltages and modules need to be swapped out. Following Vim's Tech advice is the ideal way to manage the batteries.

I will pick up on another point though. The tech uses the words, 'frequent, small, slow charging'. That is the method of charging a Lithium battery with least stress. Much better than fast DC charging. I will write another note with more of the detail that MrB and Tooney were asking for and go into a bit more detail on this.

All excellent advice Vim.

 

[Donar]

Taking it up to 100% and keeping it there for a period (ie overnight) gives the BMS chance to complete the balancing, so is a good and important thing to do.

This is good to know. I do charge to 100% maybe once a month, because I need the range. But I never leave it there for a period. Normally I try to reach the 100% as close as possible to the moment I need the car.

I will try to do the drain to 10-15%, leave overnight, charge to 100%, leave overnight next time I need 100%.

 

[d00d]

I've surpassed 30K miles and have 95.5% SoH shown in Car Scanner Pro (carscanner.info), checked in warm weather.
At 26K miles it was 92.33% in cold weather.
Didn't check SoH when new, but why the recent increase, maybe it wasn't balancing properly in the cold?

Except for seventeen 30 minute <200kW DC sessions on the road, starting at 25% I've always used the timer function to 10kW AC charge to 100% before leaving.
The timer charges a little, stops, charges more, maybe this helps ameliorate the voltage changes?

Trips in the cold arrive at single digit SoC, warm weather around 15%.

 

[Tooney]

Yes, the Porsche Tech is spot on. Agree with everything he is saysing. Let me try and explain why he is saying what he is saying.
. . .
Taking it up to 100% and keeping it there for a period (ie overnight) gives the BMS chance to complete the balancing, so is a good and important thing to do.
. . .
This is why the Porsche tech is suggesting taking the bank down to 10-15% as it then easy to see a low voltage cell – which as you say is not easily seen when out of the knees. If there are low voltage cells, the Tech can do something about it.
. . .
This is when the BMS starts reporting low cell voltages and modules need to be swapped out. Following Vim's Tech advice is the ideal way to manage the batteries.

Thank you for the post.
Since periodically charging to 100% and discharging to 10-15% and holding at that SoC is so important for HV cell balancing, why is that important advice not officially shared in owner manuals and other documentation?
--Porsche engineering is unaware of the procedure?
--Taycan BMS somehow handles the issues on its own without the need for owner intervention with the up to 100%/down to 10% procedure?
--Porsche engineering is opposed to the procedure or sees no need for it?
--Porsche believes formally communicating that procedure to owners runs a misuse risk of damage or error?
--Porsche agrees with the advice but has just neglected to include it in 4 years of owner manuals?

If this procedure helps prevent/reduce damage to the HV battery, I'd think Porsche has big incentive to communicate it to owners. Why haven't they done so?

 

[Dr Bob]

Aw, cmon. Some of us love more detail about this.

Feed my inner geek please, I’d love to know more if you can spare the time.

MrB and Tooney asked for more info - so here is some. Anyone not interested then look away now. My expertise on Lithium batteries is in their structure and chemistry rather than the electrical or Battery management (BMS) side. I guess the more detailed information that some peeps need is around why we say 'avoid the voltage knees' or 'operate mostly in the 25-85% range' and I'll try and tackle that point.

Let's start by looking at Lead Acid batteries. They work by converting lead to lead sulphate and back with charge/discharge but if left in the discharged state, the lead sulphate can form hard irreversible crystals and not be available for future use. Recharging to 100% after discharge (within a few days) is vital to retain capacity. Lithium batteries are very different. The cathode comprises inorganic 'crystals' – ie in a Li-ion battery you get a crystal structure containing oxides of Nickel, Manganese and Cobalt which is then doped with Lithium ions. In use, these lithium ions move to the anode (graphite) when charging and then move back when discharging. Over the majority of the charging range the voltage of the cell doesn't change much but at the top and bottom of the range, the voltage change is significantly higher. During charging, the amount of current the cells will accept reduces as the cell's internal resistance rises, to the point where as you reach nearly 100%, the current flow is very low in comparison to the high charging rate at an SOC of 30-50%. To explain what is happening in chemistry terms is very complex and I cant even start to explain it in electrical terms, but I like to try and use a very simplified analogy.

Think of the battery cell when charging as a large room with a reception desk all the way across the middle. Behind the desk is a wall full of shelving (ie the stainless steel stuff you get from Costco). Between the desk and shelving are team of 20 people. On the shelves are hundreds of empty egg boxes. When the charge starts, the lithium ions turn up as eggs. The eggs are brought in and put carefully on the reception desk. The job of 20 people is to transfer the eggs from the desk to the egg boxes – carefully. There are 93kWh of eggs and only 93kWh of egg boxes ie one box for each egg and no more.

So at the height of the charge, at say 30-40% SOC, the eggs arrive at a very rapid rate and the 20 people can transfer the eggs to the boxes very quickly as most boxes are empty. The temptation will be to use the nearest boxes first! By the time the SOC gets to 80%, the boxes are becoming fuller and the 20 people have to start thinking about where to put them so the rate they can transfer the eggs slows down considerably – so they put restriction on how many eggs can be brought into the room. By the time the SOC gets to 95%, there are not many empty eggs boxes left. The empty ones are hidden at the back and only individual eggs can be placed. Of course the people are under great pressure to work fast so that's when things go wrong. Someone drops an egg. That will never be available again. Someone knocks a box on the floor. That cant be used again. The people are fighting over a vacant egg box. The ability to store and use the eggs (the lithium ions) is reduced.

Just the fact that the eggs have to be put in box (ie the lithium ions into hole in the electrode structure – either anode or cathode, dependent on charge or discharge) will stress the battery as it gets to full on either electrode. You can then go further and look at what else is going on. Mechanical damage to the electrodes happens – ie boxes get knocked off the shelving – due to say thermal damage. As we charge our batteries from 20% to 100% using a fast DC charger, considerable heat is generated and rapid changes in temperature can damage the structure by thermal cycling. Although Porsche aim for near 40°C on charging, heat cycling is perhaps one of a batteries biggest enemies. Charging slowly allows the eggs to be boxed with far less stress. Rather than just the effect of heat, rapid charging requires more eggs to be placed in a shorter time. Chemical reactions in general take place twice as fast with every 10°C rise in temperature. So if we end up at 45-50°C, what else can go wrong? There is more chance of 'non-optimal placement' and that then could lead to issues when full. I do worry a bit when I see charging rates of over 250kW (3C) and what that is doing to the battery mechanical or chemistry integrity. You better be sure you batteries were made properly!

The analogy paints a picture that things will go wrong at the top and bottom of charge and the main outcome of this is that some cells will reduce in capacity and/or change in internal resistance. A change in internal resistance will then change the cells charging rate and hence ultimately it's capacity. As the cell's capacity changes relative to the other cells, we now have a cell in-balance and the need for the BMS to try and bring the cell's SOC in line with the others at the 100% SOC level, giving issues then at the bottom of the charge curve. Ultimately as some cells loose capacity, the variation of voltages at the bottom of the charging curve (the 10-15% SOC range) mean that certain cells cant be used as they will be at 0% charge when the bulk of the cells are at say 10%. This is flagged up by the BMS as failed modules and it's a visit to the service centre.

I'll finish with a few comments on measurement of SOC. There is no way to measure how many eggs there are on the shelving! We can measure how many eggs come into the room and how many go out – but we cant tell what % of boxes are full. For our car then, you cannot determine SOC from the cell voltages. You can estimate it once you are in the voltage knees but it is dependent on discharge rate and other factors. The way to track SOC is to charge to 100% then count the charge going in and out. Over time however the in/out measurement can get skewed (as it is not easy to accurately measure the in/out – see 'Peukert’s exponent' on the web). That is another reason to charge back up to full every few months.

Good luck with your batteries guys!

 

[Dr Bob]

Since periodically charging to 100% and discharging to 10-15% and holding at that SoC is so important for HV cell balancing, why is that important advice not officially shared in owner manuals and other documentation?

I dont agree this is "so important".
Cell balancing will occur as you get in the top voltage knee so it is valuable to do but not vital. If you never charge above 85% your batteries will last for years and years so why would Porsche tell people to charge to 100%. My previous post I hope helps in understanding why the 85% or the 100%. The reason to go down to the 10-15% is to see if the voltages are bad rather than balance as I dont think you can top balance the bank when at the bottom (...I agreed with VIm's note earlier - that going to 15% was good but it was to identify voltage rather than balance).
It would be impossible for Porsche to explain all of this to the average reader. Most people dont even understand the advice of the 25%-85% so what hope of explaining the best way to treat the batteries. It is a shame more Porsche agents dont use Vim's Tech advice. Charging is a very complex story and there are so many different versions of it.

 

[KarlFreund]

Great info, folks! I've been lazy and have been AC charging to 100% whenever I need to charge. Any idea how much this has reduced my battery life? (Only some 14,000 miles on the CT4) I'm probably going to upgrade to a '25 soon and want to take better care of the new baby! Thanks!

 

[Tooney]

Great info, folks! I've been lazy and have been AC charging to 100% whenever I need to charge. Any idea how much this has reduced my battery life? (Only some 14,000 miles on the CT4) I'm probably going to upgrade to a '25 soon and want to take better care of the new baby! Thanks!

Opinions differ.
Nobody knows.
Can check HV battery state of health (SoH) with OBD device and car scanner software.

 

[Dr Bob]

Nobody knows.

Agree with Tooney. Nobody knows.
What I do know is that you have as good a chance as any if you follow the ideas above. If you dont you may have a long battery life but you will be far more exposed if there are inherent defects in your battery cells.

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