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

[4thPcar]

I'm sure there is a best practice but all I've seen is"don't wait until its near zero."

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[Fun TC Driving]

We just saw another, recent scientific study come up with 75% SOC is the daily max we should use for best long term battery life.. Coincidentally that is what I have been using the last few months (except of course the night before a long trip).

On the other end I typically charge at least some at night if the next day would take me to below 25% SOC when I return home.

 

[Fun TC Driving]

Here is an interesting SOC video imo.

 

[4thPcar]

Here is an interesting SOC video imo.

This is great, thanks for sharing!

 

[KenR]

Thanks for sharing the video. I am confused. I have recently had a Qcells battery storage system fitted in my home and the installer told me that the batteries are designed to be fully charged/discharged each day. The battery technology is Lithium Ion NCA (Samsung SDI) and although EVs and homes are somewhat different, this leaves me wondering how they can give me a 15-year warranty on the system. Any ideas?

 

[Tooney]

Q: What is the target SoC to initiate topping up to 85% in order to optimize battery life?
A: Opinions/data vary. Nobody knows.

PS Taycan owner manual says charge to 80% for daily use, 85% max if ambient temp more than 86F/30C:

Use the timer function to program a high-voltage battery charge state of approx. 80 % for daily use of the vehicle without any long-distance driving. ​

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When the charge state is below 5 %, connect the vehicle to the power grid following operation and charge the high-voltage battery. ​

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If required, a battery charge of 100% can be programmed before starting long journeys.​

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At ambient temperatures above 86 °F (30 °C), connect the vehicle to the power grid following operation and charge the high-voltage battery with alternating current (AC) to a maximum charge state of 85%. Use the timer function.​

 

[mattmatt]

Q: What is the target SoC to initiate topping up to 85% in order to optimize battery life?
A: Opinions/data vary. Nobody knows.

PS Taycan owner manual says charge to 80% for daily use, 85% max if ambient temp more than 86F/30C:

​

Mine (in the UK) says 85% for daily driving

• Use the timer function to programme a high-voltage battery charge level of approx. 85 % for daily use of the vehicle without any long-distance driving.

 

[Dr Bob]

I am a battery scientist and sometime comment on these charging questions. We tend to do most of our work with Li - NMC systems but I do also have lots of user experience of LFP's.
Li batteries are quite happy charging and discharging, even up and down to the limits but the big issue around reaching the limits ie the bottom 10% and the top 10% is that the voltage is changing much faster in these ranges with discharge and charge and you MUST NOT go over 100% (ie continuing to charge when you get to the top critical voltage - ie the 100% point) and you MUST NOT try and discharge as you get to 0.0V. The voltage change rates are faster at the top and bottom as you are in the voltage knees - and when you are in these regions, the faster changing voltage can change the individual charging rates to each cell - so the cells can go out of balance. You can only balance at the top end. If out of balance then you can be charging a cell that is already at 100% SOC.
One thing the car manufacturers do is to have the 100% and 0% SOC set at say 95% and 5% of the actual battery so you have a buffer each end which will then help with out of balance cells.
The answer for us though is easy. First of all dont charge in the voltage knee (top) and dont discharge in the voltage knee at the bottom - so if you operate between 25% SOC and 85% SOC you cant go wrong and you'll never need to be balancing. That however is not practical in a car as you want the full range. I do this on my boat batteries (LFPs) and they are as good now as they were when installed 7 years ago - and have never have needed balancing (I dont have auto balancing like you do on a car).
With the car, if you want to go on a long trip then sure, charge to 100% but dont do it everyday, and avoid DC charging to 100% as AC slow charging is much more gentle on the cells. Sure, you can go down to 0% - and I would do in an emergency, but charging at 25% means you will never be in the bottom knee. The more you operate in the knees the more stress will be put on the batteries - but I expect these batteries to last 8 years (maybe a 20% loss in capacity) if treated well (and that includes excursions into the knees now and again). I do worry if people charge to 100% every time especially if DC charging - and I do worry that these batteries dont like charging at 3C at high temps (ie over 270KWh charging).

 

[Watchesandburgers]

For most EVs like the Taycan, keeping charge between 10%-90% (roughly) will ensure best/longest battery life. I'm not a scientist, but I've been an EV driver for 11 years (Tesla, now Porsche). Studies I've read suggest that 100% charging is bad for the battery, and battery life is better if charged to 90% and better still if charged to 80%. My Taycan suggests daily charging to 85% and that seems very reasonable. On the bottom end, 10% and 20% is totally fine, and try not to get to 0% SOC. Because no two battery systems are exactly the same, it's hard to get exact data, but this is has been my experience and it's served me well.

 

[Dr Bob]

Because no two battery systems are exactly the same, it's hard to get exact data,

You will never get exact data. No two systems are exactly the same. What is needed, is an understanding of why going up to 100% or down to 0% is detremental - and that is not easy to get for someone who is not 'skilled in the art'. Each battery cell is a unique structure and micro changes in internal resistance will cause cells to charge/discharge at different rates. Those micro changes are more likely at the extremes of the knees. That then starts to cause issues in the voltage knees. To many 'papers' by so called experts just regurgitate sometimes inaccurate information without teaching the reader what is important.
Your comment about "10% and 20% is totally fine " is not absolutely correct. It is 'fine' most of the time but not all the time (at least not the 10%). The reason 'why' is important. Keep out of the knees if you can.

 

[4thPcar]

I am a battery scientist and sometime comment on these charging questions. We tend to do most of our work with Li - NMC systems but I do also have lots of user experience of LFP's.
Li batteries are quite happy charging and discharging, even up and down to the limits but the big issue around reaching the limits ie the bottom 10% and the top 10% is that the voltage is changing much faster in these ranges with discharge and charge and you MUST NOT go over 100% (ie continuing to charge when you get to the top critical voltage - ie the 100% point) and you MUST NOT try and discharge as you get to 0.0V. The voltage change rates are faster at the top and bottom as you are in the voltage knees - and when you are in these regions, the faster changing voltage can change the individual charging rates to each cell - so the cells can go out of balance. You can only balance at the top end. If out of balance then you can be charging a cell that is already at 100% SOC.
One thing the car manufacturers do is to have the 100% and 0% SOC set at say 95% and 5% of the actual battery so you have a buffer each end which will then help with out of balance cells.
The answer for us though is easy. First of all dont charge in the voltage knee (top) and dont discharge in the voltage knee at the bottom - so if you operate between 25% SOC and 85% SOC you cant go wrong and you'll never need to be balancing. That however is not practical in a car as you want the full range. I do this on my boat batteries (LFPs) and they are as good now as they were when installed 7 years ago - and have never have needed balancing (I dont have auto balancing like you do on a car).
With the car, if you want to go on a long trip then sure, charge to 100% but dont do it everyday, and avoid DC charging to 100% as AC slow charging is much more gentle on the cells. Sure, you can go down to 0% - and I would do in an emergency, but charging at 25% means you will never be in the bottom knee. The more you operate in the knees the more stress will be put on the batteries - but I expect these batteries to last 8 years (maybe a 20% loss in capacity) if treated well (and that includes excursions into the knees now and again). I do worry if people charge to 100% every time especially if DC charging - and I do worry that these batteries dont like charging at 3C at high temps (ie over 270KWh charging).

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

 

[Jasper4S]

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?

 

[iamai]

even up and down to the limits but the big issue around reaching the limits ie the bottom 10% and the top 10%

@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?

 

[DCYL725]

N

You will never get exact data. No two systems are exactly the same. What is needed, is an understanding of why going up to 100% or down to 0% is detremental - and that is not easy to get for someone who is not 'skilled in the art'. Each battery cell is a unique structure and micro changes in internal resistance will cause cells to charge/discharge at different rates. Those micro changes are more likely at the extremes of the knees. That then starts to cause issues in the voltage knees. To many 'papers' by so called experts just regurgitate sometimes inaccurate information without teaching the reader what is important.
Your comment about "10% and 20% is totally fine " is not absolutely correct. It is 'fine' most of the time but not all the time (at least not the 10%). The reason 'why' is important. Keep out of the knees if you can.

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

 

[DCYL725]

@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?

Every set of battery is different so to be safe stick with 25-85 I guess ? Though 10-90 on net capacity is essentially 20-80 on gross….

oh the confusion.

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