85 % Battery charging - where does this recommendation come from?

JimBob

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Some more information.

Not all manufactures disclose the size of their buffers and not how much is at the high end or low end. Tesla uses a smaller buffer of around 4% and Porsche a larger buffer. Depends on the confidence the manufacturer has in their batteries and their risk of getting it wrong.

Lithium Ion cells, depending on chemistry can charge to about 4.2v +/- depending on chemistry. How you string them together in series and parallel will give you the 400v/900v architectures.

But you are restricted from charging to that voltage by the software, your SOC just says 100%. What that number is, is unknown. You are also restricted from discharging below a lower voltage level to prevent bricking the car. What that number is, is also unknown.

One additional point, is that Taycan batteries are made by LG Chem, of Chevy bolt fame. I used to think that with experience Porsche might some day expand the useable battery capacity as Tesla did. But not likely to happen.
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Porsche - in the Owners Manual and the Porsche Good To Know App.
The Porsche Connect app also says this when you change timer remotely.
 

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Some more information.

Not all manufactures disclose the size of their buffers and not how much is at the high end or low end. Tesla uses a smaller buffer of around 4% and Porsche a larger buffer. Depends on the confidence the manufacturer has in their batteries and their risk of getting it wrong.
So its safe to assume that 100% charge isnt 100% battery utilization then for Porsche either? Thats good to know if that is the case - and it would make sense that they create a buffer to protect the batteries integrity from premature failures or degradation inside the life of the mfg warranty.
 

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So its safe to assume that 100% charge isnt 100% battery utilization then for Porsche either? Thats good to know if that is the case - and it would make sense that they create a buffer to protect the batteries integrity from premature failures or degradation inside the life of the mfg warranty.
This is correct. The battery is a 93.4 kWh , but the usable portion is 83.7 kWh . Some real world tests and calculations have projected the real world usable portion is closer to 87kWh.

Either way Porsche ropes off roughly 7-11% of the battery. So if you are charging to 100%, you're really charging closer to 90%.
 

whitex

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Tesla use to restrict access to 15% of the batter capacity to the user and show it as 100% charge capacity. Has anyone ever broken down the math to determine if Porsche does the same or a version that holds some back to protect the battery?

Overall, they both have a vastly different approach to battery management, but Tesla has played with this a couple of times to magically added range with an update. In the same vein, they also take away range in older cars to protect battery life.

In general (I have had both) I think the Taycan is just better overall, including this critical aspect of car usage.
Tesla never did that. I've been driving Tesla's since 2013, took them apart and hacked their electronics, and what they do is they do reserve the bottom portion of the battery for the "anti-brick buffer" and different models reserved a different capacity, but all of that simple did not allow the battery to discharge below certain percentage, no top end charge limit. The only top end limiting Tesla did was in the following cases:
  1. Software limited batteries, specifically selling 70KWh or 75KWh as a Model S60, and the very few S40's which had a 60KWh battery in them. Those were always software upgradable to full capacity (I has S60D which I upgraded S75D for $2,000 and my 100% range increased to actual 100% battery). There was one batch in EU where they used up some old 85's and sold them as 75's, but no software unlock was ever offered.
  2. When some of the older cars, caught on fire while parked, Tesla realized some batteries started failing, so they over-the-air updated those cars to limit their max state of charge (reset 100% as you describe) and limited their max charging rates. Tesla either had a way to diagnosing which batteries went bad, or simply deduced it based on which experimental management strategy of BMS batch those cars were in.
Here are actual and usable capacities of early Tesla batteries as reverse engineered by a well known Tesla hacker, the numbers below come from his post here.
  • Original 60 - ~61 kWh total capacity, ~58.5 kWh usable.
  • 85/P85/85D/P85D - ~81.5 kWh total capacity, ~77.5 kWh usable
  • 90D/P90D - ~85.8 kWh total capacity, 81.8 kWh usable
  • Original 70 - ~71.2 kWh total capacity, 68.8 kWh usable
  • 75/75D - 75 kWh total capacity, 72.6 kWh usable
  • Software limited 60/60D - 62.4 kWh usable
  • Software limited 70/70D - 65.9 kWh usable
 
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whitex

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I don't think we know if that 15% is at the "top" or "bottom" - meaning is what you see as 100% full really 100% full or is it 85% full? What you see as 0% is it really 0% or is it 15%? Or maybe a mix - like 100% is really 93% and 0% is really 7%?
This is correct. The battery is a 93.4 kWh , but the usable portion is 83.7 kWh . Some real world tests and calculations have projected the real world usable portion is closer to 87kWh.

Either way Porsche ropes off roughly 7-11% of the battery. So if you are charging to 100%, you're really charging closer to 90%.
There are ways to reverse engineer a bunch of information. On Teslas for example, eventually people figured out how to snoop the CAN buses and be able to read the voltage on each battery module, and min/max voltages of individual cells even. However, there are also less invasive indirect ways, for example Lithium Ion batteries cannot be charged fast as they are getting full, so the charge rate as the car hits 100% reported SoC can tell how close you actually are to the top of the battery capacity.

I have not gotten my hands on a Taycan yet, but it is my understanding that charging doesn't slow down as much as Teslas between 98-100% SoC. We know from the hacking community that Tesla does allow their cars (at least when new) to actually hit 100% charge, so this would imply that Taycan does not. Out of curiosity, what is the battery plus charge rate between 98%-100% when on DC charging?
 

whitex

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Just wanted to add a practical for every day reason why not charge to 100%. If your battery is at 100% SoC, the recuperation has nowhere to put the power, therefore physical brakes will be used instead with Taycan's blended braking system (on a Tesla with single pedal driving it simply does not slow down when you take your foot off the accelerator while battery is full). Given that Taycan recuperation can deliver as much as 260KW, I suspect that the full 260KW is not available anywhere above 75% SoC, so again, your car ends up using up the brake pads. At least on a Taycan, the feel is supposed to be the same. I have to tell you, Tesla only has up to 60KW regen but when the battery is too cold or too full, it actually feels like something is wrong with the car because it drives so differently.

As for battery longevity, only Porsche would know but they don't have sufficient duration field data to tell so I suspect they are playing it safe. Tesla has tons of data, especially they they experiment on customers with different BMS strategies and learn which one works best (and if that hurts your battery, they will just limit the max SoC and/or max charge rate via over-the-air update so it doesn't fail before the 8 year warranty runs out). I've had 4 Model S's since 2013, I charged them all to 90% every day and they had very little degradation (~1%, most of it in the first 2 years). Some of the older cars Tesla added a precautionary measure to keep batteries charged above 75% cooler than in the past, which results in the cooling pumps running 24/7 if your car is charged above 75%, which uses energy and puts wear on the pumps.
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