So apparently, always charging to 100% is OK (?)

[Daniel]

The BMS takes some juice from the single cell group with the highest voltage and shunts it to the single cell group with the lowest voltage. Then it waits a bit and repeats the cycle. Eventually all groups are brought to within the tolerance set in the BMS - anything from one to a few millivolts.

Imagine this as a separate little module with separate wires to each cell - and that is in fact how it is often implemented, separate from the charger, which stuffs electricity into the battery pack as a whole.

From a technical perspective, there are several different ways to shuffle electrons from one cell to another (https://en.wikipedia.org/wiki/Battery_balancing) but basically it's all about taking from the highest voltage and sending to the lowest voltage.

Importantly, the maximum balance current is very low, which is why fast charging doesn't leave enough time for much balancing.

Audi Etron GT ( brother of the Taycan) balancing process

Source: @rs38 https://static.nhtsa.gov/odi/tsbs/2021/MC-10190946-0001.pdf

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[whitex]

Audi Etron GT ( brother of the Taycan) balancing process

Source: @rs38 https://static.nhtsa.gov/odi/tsbs/2021/MC-10190946-0001.pdf

It works the same on the Taycan. From my Taycan internet scrapebook:

 

[thinkbusiness]

Battery technology in general has already evolved significantly so you could charge the car to 100 and be OK. It’s not going to damage the car as such. It’s also better than to run it down to 0%. Studies have also shown that charging frequently is also better. That is why the general advice is to avoid to drop below 10/15 and not exceed 85/90. Doing frequent launches can definitely be a lot of fun (especially when you have a turbo plus) but they’re not helping battery life in the long run.

 

[rs38]

I did not read the full thread, but when Taycan is charged to 100% displayed, it's actually 97%.
So the upper margin is not huge and I would not keep the car fully charged any longer than necessary, especially in hot conditions.

 

[B61]

Good God no!

There's Jack Daniels for that!

Well, if one live in Half Moon Bay, no need JD for that

 

[DL_AU]

How can any ‘loss of capacity’ be measured? Would it be obvious, i.e the charging display no longer gets to 100%? If so, has anyone experienced this yet?

 

[daveo4EV]

How can any ‘loss of capacity’ be measured? Would it be obvious, i.e the charging display no longer gets to 100%? If so, has anyone experienced this yet?

I do not believe the display will ever not show 100% - but overall range can drop - if you notice less range - the battery can be measured by the dealer for it's current "true net" capacity - if it's lower than 80/70% vs. factory spec then warranty issues are on the table and would be discussed I'm sure.

it's always at Porsche's discretion as to how the warranty issue is "resolved" - and do not assume you'll get an entire battery replacement (but you might). Also you may not get "back" to 100% factory spec - they have the option to only service the battery to restore it to meeting warranty condtions - i.e. you're at 63% capacity with in 3 years - they could only service the battery enough to restore/repair/replace modules to get back to 82% - and that would meet warranty criteria.

 

[j.w.s]

Audi Etron GT ( brother of the Taycan) balancing process

Source: @rs38 https://static.nhtsa.gov/odi/tsbs/2021/MC-10190946-0001.pdf

That's an excellent find from the Audi docs! Thanks for sharing!

And frankly it's disappointing news.

It looks like the Audi/Taycan balancing is "passive" and "resistive" which means old-school technology and inefficient operation compared to active capacitative or active dc/dc balancing. And it says that it'll balance from 30% charge which I know from experience is good only for "gross" balancing and not fine balancing. And in the same doc it says that the maximum balancing current is 100 milliamps which is shockingly low. It could take days or weeks to remedy a signifcantly out of balance battery. Frankly I would expect better from the Porsche/Audi engineers.

By way of comparison, I have a self-built 14KW Lithium battery in my boat, with an active capacatative balancer that has a 4,000ma maximum balance current, starts balacing at around 90% charge, and keeps the cells within 0.001 volts indicated and 0.002 volts actual. It took over a day to get the battery initially balanced, and then maybe an hour per week keeping it that way. The Taycan, with its 100ma max probably takes days to get into gross balance and achieves fine balance probably never.

The very simplistic balancing scheme makes the extra unused battery capacity (93.4kwh gross, 83.7kwh usable) absolutely necessary.

Oh and a brief bit of Googling says (second hand info though) that Tesla also uses passive capacatative balancing, but starts balancing at 93% state of charge.

 

[rs38]

as far as I know, every(!) volume OEM is doing "only" passive balancing.

monitoring cell voltages with Carscanner App I regulary see deviations of all 198 2p cells at ~ 0.005V, which seems to be a good industry standard.

What do you think is wrong with balancing cells starting at 30% SoC or more?

 

[rs38]

Minimum DC charging is 50KW

there is no lower limit of DC charging.
there are plenty of (old) 20kW DC charger here

Porsche's warranty and best practices state somewhere in the manual: please do not regularly charge with more than 50kW. That makes sense that ~0.5C or even 1C at >20°C and <80% SoC will not harm a modern, managed NMC battery with silicon graphite composite anode.

 

[j.w.s]

as far as I know, every(!) volume OEM is doing "only" passive balancing.

monitoring cell voltages with Carscanner App I regulary see deviations of all 198 2p cells at ~ 0.005V, which seems to be a good industry standard.

What do you think is wrong with balancing cells starting at 30% SoC or more?

When I built the Lithium iron pack for my boat, I originally set the BMS to balance at 3.2v or higher. I found that it would spend time balancing there, then would need to balance again when nearing full charge, and then would balance again at mid charge. Basically "balanced" looked different at different states of charge, and the balancer was fighting itself.
I really cared about balance when nearly fully charged, so I switched to start balancing at 3.45V (I think - I'm not logged in to the BMS right now). After that I saw far less balancing happening, and the cells are staying nicely in balance.

Now EVs like the Taycan, with Lithium and not Lithium iron batteries, might be different etc. But I like active balancing and would guess that the next generation of EVs might switch to that in the hunt for greater efficiency.

 

[rs38]

- Charging speeds over 80% state of charge are significantly slower and not recommended for daily use.
- For regular fast DC charging usage, Porsche recommends charging at 50kW stations.

and some more info you will find in:

https://static.nhtsa.gov/odi/tsbs/2021/MC-10202963-0001.pdf

 

[Tooney]

@rs38 thanks for sharing the Porsche bulletin. I notice it advises when using the app to initiate DC fast charging, to authenticate the session with app prior to plugging the cable into the car.
Kyle, the out of spec youtube guy who does a lot of DC fast charging, also recommends that practice, at least for charging at EA sites in US.

 

[iten]

Does a 20KW DC charger generates more heat than a 21kw AC charger? I am concern because I usually charge with a 20-24kw DC Charger and wonder if it will create more damage then using a 3 phase AC charger.

 

[whitex]

Does a 20KW DC charger generates more heat than a 21kw AC charger? I am concern because I usually charge with a 20-24kw DC Charger and wonder if it will create more damage then using a 3 phase AC charger.

Nope. Same heat at the battery (which always sees DC). Slightly more heat in the onboard charger (which converts AC to DC) when using AC.

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