Counterintuitive evidence regarding EV Batteries

[JackSparrow]

Perhaps there are other opposing studies that contradict this but here’s the summary I’ve distilled from this:

Intermittent hard acceleration is good for battery health.

DC fast charging vs Level 2 is not materially different for battery health, although DC fast charging can be detrimental for a very hot battery (probably applicable to a high ambient temp environment).

The rest time at a high state of charge should be minimized as rest time at high state of charge is associated with degradation. (In excess of about 80%). That’s likely what the Taycan’s timer feature is all about.

Constant low current demand cycles will tend to accelerate battery degradation.

Repeated deep discharge (study used an 85% depth of discharge) is negative for battery health.

EV batteries should average a 200,000 mile or so usable life.

Please feel free to correct me if I am misstating the implications of these studies or if you have other recent data that has some other conclusions.



https://www.nature.com/articles/s41560-024-01675-8
https://thedriven.io/2025/04/10/ev-...-than-previously-thought-new-study-shows/amp/
https://cleantechnica.com/2023/11/0...fast-charging-on-electric-car-battery-health/

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[f1eng]

My guess is that age will also be a factor, because the chemicals may deteriorate with time.
It may even be a bigger influence than mileage, which makes a certain amount of sense for items which wear due to friction, but even then a high mileage engine which has run warm for a large proportion of its life, may have less wear than a low mileage engine which did a lot of short (cold oil) journeys.
Running gear and transmission (and driver's seat...) are probably the only parts of the car whose deterioration is strongly influenced by mileage.

 

[slothinker]

I've heard charging to 85% is OK as Porsche seems to use both those numbers. That's what I've been doing. Hope you're right about all these take-aways. Thanks for the research!!

 

[violuma]

I've heard charging to 85% is OK as Porsche seems to use both those numbers.

It might be worth keeping in mind that the 85% that we see isn't truly 85% of battery capacity (AFAIK), as there is "invisible" headroom managed by the battery management software.

 

[Gino]

Perhaps there are other opposing studies that contradict this but here’s the summary I’ve distilled from this:

Intermittent hard acceleration is good for battery health.

DC fast charging vs Level 2 is not materially different for battery health, although DC fast charging can be detrimental for a very hot battery (probably applicable to a high ambient temp environment).

The rest time at a high state of charge should be minimized as rest time at high state of charge is associated with degradation. (In excess of about 80%). That’s likely what the Taycan’s timer feature is all about.

Constant low current demand cycles will tend to accelerate battery degradation.

Repeated deep discharge (study used an 85% depth of discharge) is negative for battery health.

EV batteries should average a 200,000 mile or so usable life.

Please feel free to correct me if I am misstating the implications of these studies or if you have other recent data that has some other conclusions.



https://www.nature.com/articles/s41560-024-01675-8
https://thedriven.io/2025/04/10/ev-...-than-previously-thought-new-study-shows/amp/
https://cleantechnica.com/2023/11/0...fast-charging-on-electric-car-battery-health/

Great information. It actually makes a lot of sense. I try to keep my charge between 50% & 80% when I’m away and only charge above 85% when I know I’ll need the extra range.
It shouldn’t matter if you charge slowly at 120V or 220/240V but charging fast where battery temperatures could get higher than Porsche would like is understandable to degrade the battery.
Also driving the battery down to less than 15% on a regular basis would appear to also be not ideal for battery health but how often is too often? Once a week, once a month or once every 3 months as I do to rebalance the HV battery?
If I get 200K miles out of my battery I’ll likely be dead by then since I only put 6K to 8K miles a year on my Taycan.
That’s 25+ years in my case!

 

[chaz]

Degradation is higher with fast charging, that’s why after a certain % the charging rate decreases..so the problem has been solved (example, when I was a kid, old rc models, using rapid charge was a curse for life degradation!)
With lithium introduced, on mobile phones and then elsewhere, the common rule has always been no need to fully discharge/recharge the battery and don’t keep it very low/high in % and recharge it whenever you want so same principle applies to a car!

I've always heard that the charging speed decreases due to the time it takes the electrons to find a home. The "parking lot" example is used to show that in the beginning it's very easy to find a spot but as more cars park, it takes much longer to find an open space.

 

[Gino]

I've always heard that the charging speed decreases due to the time it takes the electrons to find a home. The "parking lot" example is used to show that in the beginning it's very easy to find a spot but as more cars park, it takes much longer to find an open space.

Yes, as you get closer to 100% the charge rate is extremely low.
The big issue with DC fast charging is all the heat that is generated. In a hot climate that can put a lot of stress on the battery cells structure.

 

[JackSparrow]

It might be worth keeping in mind that the 85% that we see isn't truly 85% of battery capacity (AFAIK), as there is "invisible" headroom managed by the battery management software.

Good point. Rereading the study, it’s not clear whether they used a specific battery management system, any indication of buffer size ( reserved cells kept aside to stand in for failed cells ), and specific variations in charge rate current/Voltage.
They generically used a current discharge rate C-rate to represent the current necessary to discharge the pack in one hour.
So C/2 (2 hour rate of discharge) was considered the highest rate.

 

[hifi239]

It might be worth keeping in mind that the 85% that we see isn't truly 85% of battery capacity (AFAIK), as there is "invisible" headroom managed by the battery management software.

A lithium ion battery should never be fully charged or discharged. The PB total capacity is 79.2 kWh with 71 kWh usable, the PB+ total capacity is 93.4 kWh with 83.7 kWh usable. When you charge from 0 to 100% you have increased the stored energy by usable amount of kWh multiplied by the SOH percent. But even so, it is bad for the battery to sit above the 85% displayed charge level in hot weather.

By and large, "managed" means the software won't let you charge above or run below the usable energy range, which is really below or above certain voltage levels. There have been suggestions that as the SOH decreases, the management software could eat further into the guard range to maintain energy capacity, but I think that is a myth until someone shows otherwise. Range is king and why wouldn't Porsche provide the fullest possible range when new?

 

[Gino]

A lithium ion battery should never be fully charged or discharged. The PB total capacity is 79.2 kWh with 71 kWh usable, the PB+ total capacity is 93.4 kWh with 83.7 kWh usable. When you charge from 0 to 100% you have increased the stored energy by usable amount of kWh multiplied by the SOH percent. But even so, it is bad for the battery to sit above the 85% displayed charge level in hot weather.

By and large, "managed" means the software won't let you charge above or run below the usable energy range, which is really below or above certain voltage levels. There have been suggestions that as the SOH decreases, the management software could eat further into the guard range to maintain energy capacity, but I think that is a myth until someone shows otherwise. Range is king and why wouldn't Porsche provide the fullest possible range when new?

Porsche can’t necessarily stop you from driving the battery to zero but they can set the 100% charge to be actually at 90% so the life of the battery will not be threatened by over charging to 100% of actual capacity.
Porsche has told me from both the dealer & corporate that charging to 100% is ok when you need it but not to charge to 100% and leave it that way for long periods.
That would make sense.
I keep the battery in my iPhone topped up to 100% every day overnight until the morning when I get up and use it all day with it plugged in the car where it commonly gets back to 100%.
My iPhone battery does not last me more than 4-5 years before I have to replace it because it just won’t hold enough charge any longer.
I know the battery chemistry in my iPhone is different than in my Taycan but I have to believe the battery management system is more sophisticated than in my iPhone since Apple only has to have their batteries last 3-4 years max before a customer upgrades to another phone or buys another battery like me since I keep my phones for at least 5-6 years with one battery change.
Porsche, by law needs to have their batteries last 8 years or 100K miles so I would expect they would balance range with the usable energy in the battery so people don’t treat their HV batteries the same way they treat an iPhone battery.
I wouldn’t be surprised if 100% is not really 100% but 100% of the energy stored in the battery they let you tap into.
Driving it to zero charge is not something they can do much about but even when the car stops at zero with no miles left I wouldn’t be surprised if the BMS software says the battery is at zero miles, tells the car not to move but there is actually still 10% or more left in the battery to protect the life of the battery.
Otherwise I don’t see how their batteries could last 8 years if you kept it charged to 100% every day.
Unfortunately I love my Taycan too much to take that chance so I only charge to 100% maybe twice every 3-4 months when I go on a long trip where I need the highest range on the way out and then on the way back.
But my normal use I stay between 30% & 85%.
So I will find out how long my battery will last where I charge it 100% 6-8 times per year, never leaving it charged to 100% for more than 12 hours before each trip and only driving 6K to 8K miles per year.
I’m anticipating my battery could outlive me with such a low use case.
I would love to learn more about how Porsche’s BMS works and their strategy to insure they meet & exceed the warranty requirements set by law.
I’m surely not pressing the envelope on my Taycan…

 

[hifi239]

Porsche can’t necessarily stop you from driving the battery to zero

Why can't the Porsche BMS prevent you from discharging to zero volts? 0% displayed charge is not zero battery volts (it better not be), and 100% displayed charge is not the highest voltage possible for the Lithium batteries (that better not be either).

 

[Gino]

Why can't the Porsche BMS prevent you from discharging to zero volts? 0% displayed charge is not zero battery volts (it better not be), and 100% displayed charge is not the highest voltage possible for the Lithium batteries (that better not be either).

That’s what I would expect Porsche to do. Zero miles range on the screen should not be actually zero volts on the battery or at least I hope so.
When I last rebalanced my battery, I drove it to 2 miles remaining but the OBDII readings still showed remaining charge on all the cells but it was not close to zero.
This tells me the BMS is keeping a percentage of the battery capacity in reserve to insure the battery health is not compromised by actually going to zero.
I’m not even sure if my iPhone actually discharges all the way to zero volts before it shuts down.
I would be very intrigued to hear from someone with the knowledge of how BMS software is used to protect batteries but also how it’s used to extend battery life to meet warranty requirements.

 

[joefig44]

I find charging to 60% and then recharging at 40% works for me and my low mileage. I then charge to 85% and that gets me more than enough for most of my regular longer trips.

I'm just unsure if it's better for the battery?
a. do as above 60-40% and back up every 3 days, OR;
b. charge to 85% then drop to 40% and thus charge less often (maybe every 5 days)

 

[Gino]

I find charging to 60% and then recharging at 40% works for me and my low mileage. I then charge to 85% and that gets me more than enough for most of my regular longer trips.

I'm just unsure if it's better for the battery?
a. do as above 60-40% and back up every 3 days, OR;
b. charge to 85% then drop to 40% and thus charge less often (maybe every 5 days)

I guess we’ll find out…
We charge to 85% maybe once every week or two from 30% to 40%. I’m assuming my HV battery is on vacation so to speak and hopefully I will begin to see a clearer picture at the end of year four.
The first / years

I find charging to 60% and then recharging at 40% works for me and my low mileage. I then charge to 85% and that gets me more than enough for most of my regular longer trips.

I'm just unsure if it's better for the battery?
a. do as above 60-40% and back up every 3 days, OR;
b. charge to 85% then drop to 40% and thus charge less often (maybe every 5 days)

I find charging to 60% and then recharging at 40% works for me and my low mileage. I then charge to 85% and that gets me more than enough for most of my regular longer trips.

I'm just unsure if it's better for the battery?
a. do as above 60-40% and back up every 3 days, OR;
b. charge to 85% then drop to 40% and thus charge less often (maybe every 5 days)
[/QUOTE]
I guess we’ll find out…
We charge to 85% maybe once every week or two from 30% to 40%. I’m assuming my HV battery is on vacation so to speak and hopefully I will begin to see a clearer picture at the end of year four.
The first 2 years my Taycan was driven 30K miles and was charged to 100% at the Porsche dealer almost ever day. When I bought it as a CPO the SOC was a 90% capacity.
Almost 2 years later at 5K miles/year on average I don’t see any further battery range degradation but I think it’s too soon to make that determination.
The true test for me will be when I get to 60K miles which will be around January 2030 and the end of my 8 year warranty. If my range loss is at an additional 10% or less then I will feel very confident my use case will keep my range at an acceptable level for many more years to come.
At this point I’m unsure if battery degradation is linear based on total miles driven but I assume this is unlikely. I hope battery degradation over time/miles will become more gradual until it goes off a cliff sometime after 8+ years & 100K+ miles.
Only time will tell if my battery is still showing no signs of significant degradation with my low use case.
I hope I’ll be pleasantly surprised…

 

[joefig44]

I guess we’ll find out…
We charge to 85% maybe once every week or two from 30% to 40%. I’m assuming my HV battery is on vacation so to speak and hopefully I will begin to see a clearer picture at the end of year four.
The first / years

I find charging to 60% and then recharging at 40% works for me and my low mileage. I then charge to 85% and that gets me more than enough for most of my regular longer trips.

I'm just unsure if it's better for the battery?
a. do as above 60-40% and back up every 3 days, OR;
b. charge to 85% then drop to 40% and thus charge less often (maybe every 5 days)

I guess we’ll find out…
We charge to 85% maybe once every week or two from 30% to 40%. I’m assuming my HV battery is on vacation so to speak and hopefully I will begin to see a clearer picture at the end of year four.
The first 2 years my Taycan was driven 30K miles and was charged to 100% at the Porsche dealer almost ever day. When I bought it as a CPO the SOC was a 90% capacity.
Almost 2 years later at 5K miles/year on average I don’t see any further battery range degradation but I think it’s too soon to make that determination.
The true test for me will be when I get to 60K miles which will be around January 2030 and the end of my 8 year warranty. If my range loss is at an additional 10% or less then I will feel very confident my use case will keep my range at an acceptable level for many more years to come.
At this point I’m unsure if battery degradation is linear based on total miles driven but I assume this is unlikely. I hope battery degradation over time/miles will become more gradual until it goes off a cliff sometime after 8+ years & 100K+ miles.
Only time will tell if my battery is still showing no signs of significant degradation with my low use case.
I hope I’ll be pleasantly surprised…
[/QUOTE]
Mine is a 25 4S and bought it new. Planning to keep 10 years and that battery lasts that long.

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