How-to Recalibrate your SoH.

[Gru]

I had a look at other forums (Kia, Ford) about the SoH the BMS delivers. It looks like this State of Health (?) is more a State of Homogeneity (?).

My experience : SoH was at 90.3, did 1000 mi on DC, it went down to 90.2. I then charged from 70% to 85% (AC) and it went down to 88.8% right after this single short charge ?.

I did again 1000 mi last week on DC, and a re-calibration (AC) today: SoH went back from 88.8 to 90.9% ?

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[Tooney]

I had a look at other forums (Kia, Ford) about the SoH the BMS delivers. It looks like this State of Health (?) is more a State of Homogeneity (?).

My experience : SoH was at 90.3, did 1000 mi on DC, it went down to 90.2. I then charged from 70% to 85% (AC) and it went down to 88.8% right after this single short charge ?.

I did again 1000 mi last week on DC, and a re-calibration (AC) today: SoH went back from 88.8 to 90.9% ?

This reminds me of the story about the building manager who kept getting complaints from workers in an office that the office temperature was too hot or too cold.
He had a dummy thermostat installed so the office workers could make adjustments (which had no effect on the HVAC unit.) That stopped the complaints.

 

[Tooney]

https://en.wikipedia.org/wiki/State_of_health#SoH_evaluation
Since SoH generated by a BMS is arbitrary, owners do not really know if the SoH value generated by the Taycan BMS is calculated the same arbitrary way for all Taycan models or for different model years. "Arbitrary" does not necessarily imply "consistent".

As apparent SoH declines, perhaps the BMS 'readjusts' the relative weights of parameters measured to "boost" the reported SoH value.

How would an owner, or anyone without access to the BMS software, know?

 

[D00notD00d]

The SoH measurement method used by Porsche when determining SofH is shown below.
The pre-amble mentions that temperature and charging/discharging behaviour (?) affect the calculation
From this, the SofH is a snapshot rather than an absolute value and may vary.
This becomes critical only if the manufacturers warranty milestones are in doubt (80% at 3 years, 70% at 8 years, up to 100k miles).
It is unclear to me what part (if any) the ‘unused’ capacity plays in the calculation and the purpose of this.
Charging time efficiency? Battery longevity?
If I remember correctly the waiting time referred to is a minimum of 6.5 hrs.

 

[Tooney]

Let's assume that "net capacity value of the high-voltage battery" described in preceding post is SoH. Stated above that this value is continuosly calculated by BMS.
We know that battery chemistry of post-2020 Taycans differs from 2020 Taycans (and 2025 models have different battery chemistry than earlier years).

Do the BMS in Taycans in all model years calculate SoH the same way?

If external temperature affects SoH calculation accuracy, how is that standardized?

Is the value that Porsche reports for SoH as (in)accurate as the miles per kwh for the total of all trips reported in the Porsche app?

How is Porsche's system for calculating SoH calibrated? Is Taycan SoH measurement accuracy reviewed by any regulatory agencies and/or tested according to industry standards?

If the reported SoH is consistently skewed high or low, who benefits?

 

[Gru]

I'm afraid we won't get answers to all these very relevant questions...
For sure, it's not standardized: i can't remember which forum (Kia, Ford, Mercedes?) where SoH are near 95% after 50k mi.

 

[whitex]

Some sort of standardization of SoH would be a great for maintaining EV resale values. Unfortunately I don't think even the manufacturers have a solid grasp on this concept today, i.e. some objective metric that would indicate the current wear or left life on a battery. I think metrics such as number of charge-discharge cycles, while nowhere near perfect indicator of battery, might end up being the key metric for a while - similar to a mileage on an ICE engine. So, for the foreseeable future, this will likely go the way to estimating engine health, where a car mileage and mostly subjective evaluation from a mechanic is all that used ICE car buyers get today.

 

[CaliPorsche]

Some further pre and post Data.

My car is 2020 4S, 22 inch Mission E, 24k miles, Lead Feet - vast majority of charging v. abusive on High Speed EA Chargers throughput its life, Car kept mainly in HOT climes, including 3 summers in California desert often at 100-120 degrees Fahrenheit (not used much in summer).

Process
1) ran down to 4% - left for 72 hours at that - carscanner "pre" readings taken.
2) charged at home using PWCC set at 40A or 8.8kw from 4% to 96%, left for 8 hours overnight.
3) Next Morning "post" readings taken.

Gained over 2% in SoH. Range at guessometer is about 7% higher than I would normally see - we will soon see in the real world.

Below first image is pre - second is post.

 

[Tooney]

@CaliPorsche Thanks for the information.
Has your Taycan had any HV battery modules replaced?
Ever had any yellow or red electrical errors?

 

[CaliPorsche]

@CaliPorsche Thanks for the information.
Has your Taycan had any HV battery modules replaced?
Ever had any yellow or red electrical errors?

yes - covered elsewhere here but 3 of the 33 modules were replaced in March this year

 

[Tooney]

yes - covered elsewhere here but 3 of the 33 modules were replaced in March this year

Ah, so it is likely that the modules with higher voltages in your diagrams are the ones that were replaced. And, if the dealer followed TSB, those modules have post-2020 battery chemistry.

 

[CaliPorsche]

Ah, so it is likely that the modules with higher voltages in your diagrams are the ones that were replaced. And, if the dealer followed TSB, those modules have post-2020 battery chemistry.

exactly .....

 

[Berti123]

Hi,

been following this issue as an Etron GT driver bc I too have low SoH and therefore also low capacity. 3yo, 25000miles, 86 SoH, 76kWh tops.

Trying to do the routine from the OP. What I did find out after letting it sit for 8h at 20% and 3% SoC, there is no rebalancing happening at these SoCs. Delta is like 30mV and 50mV and it barely tightens.

This is also suggested in this paper from Audi:
https://github.com/electrichasgoneaudi/etron-issues/issues/86

Now trying to charge at the lowest rate(1.3kW), see if it balances at these low SoCs and with these low rate it has more time.
Hope to hear from my Taycan brothers what they think of these findings

 

[SergeyIndy]

Hi,

been following this issue as an Etron GT driver bc I too have low SoH and therefore also low capacity. 3yo, 25000miles, 86 SoH, 76kWh tops.

Trying to do the routine from the OP. What I did find out after letting it sit for 8h at 20% and 3% SoC, there is no rebalancing happening at these SoCs. Delta is like 30mV and 50mV and it barely tightens.

This is also suggested in this paper from Audi:
https://github.com/electrichasgoneaudi/etron-issues/issues/86

Now trying to charge at the lowest rate(1.3kW), see if it balances at these low SoCs and with these low rate it has more time.
Hope to hear from my Taycan brothers what they think of these findings

One, I do not see anything wrong with 85.85% SoH reading at the age and mileage and 76kWh total calculated usable battery, but it is on the lower side. The warranty threshold in miles is 37.5k and SoH of 80% or below.

Two, the rebalance is meant to align the cells in their individual SoH so the total after the rebalance is more than before may be by 1% or 2%.

Also, what is your charging protocol as it may be contributing to slightly lower SoH. Specifically, what is your SoC variations between charges and how much AC vs. DC at what rates are you using, what is the ambient temps you are driving in, and do you have Battery Friendly option checked on or off.

See an example from my most recent reading and calculations after rebalancing:

https://www.taycanforum.com/forum/t...ation-data-by-taycan-owners.12815/post-350013

 

[Berti123]

Thanks for the quick answer!

Mainly I am charging AC with 8kW at home in the range of 25-80% SoC. Austria has mild summers up to 33°C and mediocre winters to -6°C. I'd say less than 10% is DC charging.

I suspect highly varying SoH bc I see highly varying SoC per cell of up to 3-4% at low SoC, combined with high voltage delta.

My method already seems to show success:
-Yesterday night 3% SoC and about 50mV voltage delta.
-10h letting it sit without plugged in only gained to ~45mV (this shows that there is nothing happening at these low SoCs, just like the Audi paper says)
-having it charged at only 1,3kW for 2h shows already only 23mV delta at 6% SoC, This is already the best delta I have ever had at these low SoCs.

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