Dr Bob
Well-Known Member
- First Name
- Andy
- Joined
- Sep 16, 2023
- Threads
- 5
- Messages
- 78
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- 77
- Location
- Falkirk
- Vehicles
- Taycan Turbo
Sorry guys, maybe has been discussed previously but close to getting a GTS. Simple question to more experience owners, why not charge to 85% and not to 100% every time? Hear me out, on other cars (or electronic devices per se) like Tesla you dont want charge to 100% every time to preserve the battery, HOWEVER, Tesla allows you to use 100% of the battery capacity. On the Taycan only roughly 90% is available to the driver (dont have the exact numbers), remaining 10% are a buffer locked by Porsche.
That begs the question, even if you charge to 100% you never actually charge the battery capacity technically to 100% but only 90%. So why the caution?
If anyone is wondering why ask this question… live close to an EA charger and would charge the car for free…makes more sense to charge to 100% to have less trips to the charger.
Boy, is there some lack of understanding of charging our batteries on here! The myths from the internet are perpetuated and become the norm. Reading this thread gives the reader the impression that 100% SoC is fine but just dont keep it there that long – but is that seconds, minutes, hours or days? Guys (and Gals), you are really not understanding the issues of LFP (or Li ion) batteries.
I'm reasonably new to this forum (I bought a used Taycan 4 months ago) but have significant experience in Lithium batteries. I haven't read all the data in all previous threads on charging but can input from a external perspective. In my day job, I am technical director of a company involved in recycling Lithium EV batteries. This means recovering the important metals as oxides from battery black mass (ie Lithium, Nickel, Manganese and Cobalt) and then re-forming them into cathode materials. I do understand the chemistries of these batteries. I also have 5 years experience of fitting a 5 kWh bank of LiFePO4s (Winston Thunderskys) to a boat – which we spend a lot of time living on, which required me to build my own simplistic BMS to stop overcharging or sub zero temperature charging- so have first hand experience of what can and does go wrong when you charge up to 100% with LiFePO4s. Finally I have 3.5 years experience of charging a Tesla M3 with many charges overnight up to 100%.
Yes, holding an LFP battery at 100% for extended periods is bad for the battery but this is not the main reason why we should worry about taking our batteries up to 100%. Yes, if you want to store the batteries for 3 months, then discharge to 30% as Porsche tells you, but they are fine at 100% overnight if you are charging for a long run (if the batteries are really at 100%). The bottom line is that you do need to charge to 100% now and again but there's no way I would do it on a DC charger every charge, as per the OP's original question.
I'm now going to bore you with the two technical reasons why not! I'll try and keep it simple.
First up, you have to consider the chemistry of what is happening in the battery and what will go wrong as you get to 100% charge. During charging, the lithium ions on the cathode transfer to the anode and at 100% SoC are at the maximum the anode will hold. If the voltage from the charge source then is still higher than the voltage in the cell, further lithium ions will flow but there is nowhere for them to go and then all sorts of things start to go wrong with possibility of dendrites forming (which can short out cells), temperature increases and internal resistance changes etc. Overcharging from 100% is a bad, bad idea. LFP's are better than Lithium ion chemistries and are far less likely to suffer thermal runaways (ie fires) but they will still suffer irreparable damage if charged to over 100%.
OK, so what do we mean by 100% SoC. We really need to consider cell voltage, not SoC as it is not possible to measure SoC – you derive it from other measurements. LFP cells have a max allowable voltage of 4.2V. (Note: this is for typical LFP's – I have no idea what the voltage profiles are for Taycan cells but that's not important for this explanation) At that voltage, the cell is fully charged and the anode will not accept any more ions. You must not exceed 4.2V in any cell during charge. Now, this will be easy to manage if all your cells are exactly the same voltage during charging but unfortunately that never happens. Slight differences in capacity and internal resistance will mean you get some cells at slightly higher voltage and will enter the 'voltage knee' of the charge/voltage curve sooner than the rest and their voltage then shoots up and they reach the 4.2V (or 100% SoC) ahead of the rest. As soon as one cell reaches 4.2V you must stop charging. This is why when we see 100% SoC on the screen, the car is telling porkies. The real SoC will be lower to ensure no one cell ever goes over 100%. The issue then is how to keep all the cells nearly the same – and this is where balancing comes in. The BMS will control balancing but it doesn't happen immediately ie in seconds, but over a period of 4-24 hours. If a cell bank is well out of balance, then you could have some cells reaching 4.2V while some are still down at 3.6V. One important piece of information that is often ignored is that charging to 4.2V is likely to initiate balancing issues which then gives the BMS a lot of work to do and that could lead to damage. It is far better for the battery health to avoid charging into the voltage knee so reducing out of balance issues. On my boat, I have a 4kWhr LFP bank and in the 5 years I have had them, I have never charged above 85% and never had to re-balance any cells after the initial installation (I have no automated balancing). Taking cells to 100% SoC (whatever that means) risks getting cells over the 4.2V through poor balancing.
Another issue here is that our cells are top balanced, ie the balancing is done as we charge to 100% SoC, but what that means is that if cell capacities vary due to long term charging effects, they will not be balanced at the bottom end of the SoC curve and could be a long way out. Everything may look fine at 95-100% but at the end of discharge when some cells are at 10% SoC, some laggard cells could be at 0% and that can wreck a battery just as badly as overcharging. If you are worried, take you batteries up to 100% then discharge to say 10% (or 5% if you are brave!) and look at the cell voltages. That's where you may see big voltage deltas between the cells. Lot's of charging up to 100% will put pressure on the balancing and hence the need to watch the balance at the bottom of the discharge curve.
The second technical input why very frequent DC charging to 100% SoC is not good idea, is all around what happens during charge. You can define charge rates based on the capacity of your battery bank therefore charging a Taycan's 93kWhr bank at 93kW is a charge of 1C. Charge at 186kW and we have 2C etc. Charging at 3C is a frightening thought to the battery chemist!!!!, but that's what we are doing on these Ionity chargers! The temperature rises and the physical changes in moving ions around quickly cause massive mechanical stress which over time will take its toll on the battery structure. I sat in the car a few nights back on a long trip from Jockland to the Midlands (in the UK) watching the Ionity charger put in 270kW. That is just too fast. On my boat, I charge at C/3, ie 9 times slower than this and still see 10°C temp rises in 30 mins.
One thing I do notice is that the Taycan charges significantly faster than the Tesla ever did – I just wonder if the higher number of battery problems I see on this forum compared to what I saw on the Tesla forums is indicative of our super duper charging speeds. It does make me nervous! Overall though, we need to be able to charge fast – and that is great- and doing it now and again will not cause significant issues – and the Porsche warranty will support that. However, doing fast DC charging every charge cannot be good for the battery structure and likely a lot worse than keeping a bank at 100% SoC for 24 hours.
It is worth mentioning though that we have to take our cells up to over 95% now and again for the system to work. Cell balancing can only be initiated once you are in the voltage knee, so if you get balancing drift at mid SoCs when never going outside 25%- 75% SoC, you could be in trouble when you eventually do go high or low. The other thing is the SoC 'measurement'. SoC is not 'measured' by an instrument. It is calculated by integrating the amps passing in and out of the battery bank. Some clever math is required to do this accurately but this drifts over time (weeks/months) and the BMS needs to re-set the SoC itself which happens once you get well up the voltage knee during charging.
So to summarise, the advice to not always charge to 100% is not because these LFP cells dont like being at 100% but because of the inherent dangers of overcharging cells (caused by balancing issues) and not doing it on fast DC chargers all the time because of stress issues on the battery structure. Of course we all need to charge to 100% (and often on fast DC chargers) – that is what EVs need to do, but we need to understand the issues.
On my Tesla, in the 3 ½ years I had it, I estimate 50% of the charging was done on fast DC chargers with the remainder on 3kW and 7kW AC. Battery health was 90% and in the top decile when I sold it. Charging on the Taycan has been similar but these ''very' fast chargers do worry me.
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