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Discussion Starter #1 (Edited)
You MUST only charge your EV at 20-80% is a myth.
Let’s look at this with some common sense.
Fact: The EV battery is made from many cells, some in series and some in parallel.
Ignoring for a moment those in parallel, we know that cells in series will have their voltage added to build it to the 350V+ range.
Fact: A somewhat dead cell will show 3.5V, a somewhat part charged will show around 3.7V and fully charged around 4.2V.
Fact: the EV battery pack is somewhere around 350-400V, therefore it will take around 100 cells in series to make up that voltage.
So by simple deduction, the state of charge can easily be determined by the voltage of the whole pack. 350V depleted, 420V pretty much full. Anything in-between is very difficult to determine, hence the GOM, it is only a GUESS. While they say it’s based on past driving style, I think it’s just to cover up the inaccuracy of the SOC.
Fact: We also know not every cell is made exactly the same. Each will have a different voltage for Full and Empty. Not by much, we are talking small differences.
Fact: We know each cell will discharge at different rates. Some quicker than others. Not by much, we are talking small differences.
Fact: If you don’t charge the battery to 100% and leave it there for a bit longer, some cells will not attain their full charge, hence balancing the cells.
Following me so far?
We have stated that a battery voltage of 350 is pretty much empty. Each car will know when it’s time to stop. Might be 352V, might be 357V, each car is different.
What if one of the cells was kind of draining slowly and another cell was draining faster, due to their individual differences. Their respective voltage would be 3.7 and 3.5, agree?
Overall the 100 cells in series with an overall voltage of say 370V would not show up which cells are fully depleted and which are partially charged.
The more often you don’t charge to 100%, the more likely you will have a few depleted cells even when the battery pack is showing partially full.
Driving this partially full battery down to 10% might cause damage to those already depleted cells. With only the partially charged cells supplying power.
This is particularly bad if the current is reversed into the depleted cell. This happens with low SOC. Don’t forget we have 100 cells in series, some will be depleted. Now the partially charged cells will still try to deliver current forcing it through the depleted battery, this current would actually be in reverse.

Let’s talk about cells in parallel. This is actually a good thing, it averages out the weak cells and makes a cell pack more uniform which can withstand any one cell going to 0%. The good cells in parallel will bolster that weak cell and average out the differences.

The overall consensus of this post is to recommend that if you go down to a low SOC then you must charge your battery to 100% and then some. Basically don’t let any cell get fully discharged due to several partial charging sessions. We really don’t want those few really energetic cells that readily discharge quickly and become fully depleted and go into reverse current. You won’t know when this happens, only your gut feeling will tell you need to charge to 100%.

Now read the manufacturers manual, what does it say? Does it actually say you MUST only charge 20-80% daily?
Does it say you must not charge it to 100% every day?

So, to round up this post, who would you like to believe, the manufacturer with employees with degrees in battery science, who designed the battery, knows how it works and how you should maintain it, or the few individuals who have heard some old spouse tale over a camp fire saying they would only charge 20-80% on a daily basis because they have always done it, they swear by it, their dad and grandad always did it, so it must be consecrated. What is the science in this? When/if you become a grandad and tell your children of your EV charging habits, we will see which tale prevails.

If this post has changed your charging habits, we would like to know. Of course they will be those who will be even more determined to charge 20-80, let us know why.
 

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My understanding....
You do need to charge to 100% on a reasonably regular basis to balance the pack.
My manual recommends that you do that monthly.
However when a battery is at 100% it is putting maximum strain on everything including physically as the batteries do expand and that is known to affect lifespan.
So charge to 100%, just don't keep it there for long.
 

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I've been saying this for years. Why else does your car have a BMS? It's there to look after your battery. Heat and rapid charging are far more detrimental to your battery than the SOC.
 

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You MUST only charge your EV at 20-80% is a myth.
Let’s look at this with some common sense.
Fact: The EV battery is made from many cells, some in series and some in parallel.
Ignoring for a moment those in parallel, we know that cells in series will have their voltage added to build it to the 350V+ range.
Fact: A somewhat dead cell will show 3.5V, a somewhat part charged will show around 3.7V and fully charged around 4.2V.
Fact: the EV battery pack is somewhere around 350-400V, therefore it will take around 100 cells in series to make up that voltage.
So by simple deduction, the state of charge can easily be determined by the voltage of the whole pack. 350V depleted, 420V pretty much full. Anything in-between is very difficult to determine, hence the GOM, it is only a GUESS. While they say it’s based on past driving style, I think it’s just to cover up the inaccuracy of the SOC.
Fact: We also know not every cell is made exactly the same. Each will have a different voltage for Full and Empty. Not by much, we are talking small differences.
Fact: We know each cell will discharge at different rates. Some quicker than others. Not by much, we are talking small differences.
Fact: If you don’t charge the battery to 100% and leave it there for a bit longer, some cells will not attain their full charge, hence balancing the cells.
Following me so far?
We have stated that a battery voltage of 350 is pretty much empty. Each car will know when it’s time to stop. Might be 352V, might be 357V, each car is different.
What if one of the cells was kind of draining slowly and another cell was draining faster, due to their individual differences. Their respective voltage would be 3.7 and 3.5, agree?
Overall the 100 cells in series with an overall voltage of say 370V would not show up which cells are fully depleted and which are partially charged.
The more often you don’t charge to 100%, the more likely you will have a few depleted cells even when the battery pack is showing partially full.
Driving this partially full battery down to 10% might cause damage to those already depleted cells. With only the partially charged cells supplying power.
This is particularly bad if the current is reversed into the depleted cell. This happens with low SOC. Don’t forget we have 100 cells in series, some will be depleted. Now the partially charged cells will still try to deliver current forcing it through the depleted battery, this current would actually be in reverse.

Let’s talk about cells in parallel. This is actually a good thing, it averages out the weak cells and makes a cell pack more uniform which can withstand any one cell going to 0%. The good cells in parallel will bolster that weak cell and average out the differences.

The overall consensus of this post is to recommend that if you go down to a low SOC then you must charge your battery to 100% and then some. Basically don’t let any cell get fully discharged due to several partial charging sessions. We really don’t want those few really energetic cells that readily discharge quickly and become fully depleted and go into reverse current. You won’t know when this happens, only your gut feeling will tell you need to charge to 100%.

Now read the manufacturers manual, what does it say? Does it actually say you MUST only charge 20-80% daily?
Does it say you must not charge it to 100% every day?

So, to round up this post, who would you like to believe, the manufacturer with employees with degrees in battery science, who designed the battery, knows how it works and how you should maintain it, or the few individuals who have heard some old spouse tale over a camp file saying they would only charge 20-80% on a daily basis because they have always done it, they swear by it, their dad and grandad always did it, so it must be consecrated. What is the science in this? When/if you become a grandad and tell your children of your EV charging habits, we will see which tale prevails.

If this post has changed your charging habits, we would like to know. Of course they will be those who will be even more determined to charge 20-80, let us know why.
PHEW ....... I am tired now after reading all that ! o_O.
 

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I see a lot of rambling, facts and calculations but no peer-reviewed data. It is known that cycling Li-Ion batteries between 20 and 80% will significantly extend their lifespan compared to cycling them between 0% and 100%. However, there are many unknowns regarding any given EV battery and plenty of other facts to consider...for example:
  • It just isn't practical to only use 60% of your car's stated range;
  • Each manufacturer has a (usually unknown) buffer anyway so you won't know what 20-80% actually translates to;
  • Never charging to 100% is also bad for the battery;
  • Rapid charging is "bad" for the battery too;
  • Even in a worst case scenario (charging to 100% and depleting to 0% over and over), you're likely to get 250k+ miles before the battery is considered "dead" (<75% original capacity) anyway.
Basically for most typical drivers, it really won't matter and stressing about your battery staying in a certain range of charge to extend its life is pointless. If you really want to extend the lifespan of the battery pack beyond 250k miles, then you can just make minor efforts to:
  • Only charge beyond 80% one a month or when needed;
  • Avoid depleting below 20%;
  • Avoid rapid charging.
But even then it seems relatively futile: as stated above, the car won't let you fully deplete or over-charge the battery anyway.
 

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It is known that cycling Li-Ion batteries between 20 and 80% will significantly extend their lifespan compared to cycling them between 0% and 100%
This is what your BMS does for you. The BMS doesn't allow you to charge your car to 100% nor does it allow you to completely empty the battery. Say you have a 10 kWh battery. The BMS will allow you to put 8(ish) kWh into the battery and not let you run down to less that 2(ish) kWh. That's why you'll read folk saying that the car has a 10 kWh battery (6 kWh usable). That's the BMS doing it's job.

Of course non of this applies to cars without a BMS, but I can't recall any that don't.
 

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Or about how burning gas to charge an electric car is worse than running an ICE even though it isn't?
 

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Fact: A somewhat dead cell will show 3.5V, a somewhat part charged will show around 3.7V and fully charged around 4.2V.
The battery manufacturer chooses the cut off voltage. Based on the batteries I've worked with, 3.5V is not standard, so I'd be hesitant to call it a fact.. I think the lowest I've seen in our lab is 2.7V.

Fact: the EV battery pack is somewhere around 350-400V, therefore it will take around 100 cells in series to make up that voltage.
So by simple deduction, the state of charge can easily be determined by the voltage of the whole pack. 350V depleted, 420V pretty much full. Anything in-between is very difficult to determine, hence the GOM, it is only a GUESS. While they say it’s based on past driving style, I think it’s just to cover up the inaccuracy of the SOC.
That's why coulomb counting is useful - makes it much easier to predict state of charge than a pure voltage measurement (especially since a pure voltage method needs to be done with no load). Just needs a reset every now and then to remove drift.
 

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A battery pack in good condition will have less than 5mV between cells. The idea that one cell will be 3.5V and one 3.7V is wrong, unless the pack is literally knackered (or an original Nissan Leaf battery.)

Obviously, not everyone can work with a car with a battery in the range of 20% to 80%, but if you can avoid charging your vehicle above 80%, that will extend its lifespan somewhat. As you get to a higher cell voltage, cracking around the anodes have been noted, which increase with the frequency and duration for which a cell has been stored at a high state of charge. Lower state of charge is less bad, but the pack needs to be charged soon to prevent bricking. This has gotten so bad that Tesla has remotely updated vehicles to limit their top state of charge to prevent fires - this is known as "batterygate" and has many older Model S owners pissed off!

And while every manufacturer is different, most manufacturers make at least 90% of the battery capacity available. Tesla for instance let you charge to 4.2V - exactly 100%. The e-Golf goes up to 4.18V, same for the Golf GTE, which represents around 94%.
 

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Of course they will be those who will be even more determined to charge 20-80, let us know why.
The owner's manual for my i3 specifically says to not let the car sit at a low state of charge. If the car is going to sit for several weeks BMW recommend leaving the car plugged in to avoid running the batteries down. (prob. to avoid killing the lead acid 12V battery...)


LEAF 24/30 is very different. LEAF has a very small top safty margin. Storing LEAF at 100%, especially in hot climates is known to accelerate battery degredation.

On top of that the on-board charger and BMS have a serious flaw. The LEAF warranty menations that damage to the pack caused by starting charge when the battery is at or above 98% indicated is not covered by the warranty. Leaving the car plugged in is risky because a power glitch or cut will cause the car to start a charge cycle when the battery is allready at 100% indicated. This will damage the battery.




BTW this topic has been discuessed endlessly.
 

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To my mind, this whole debate pivots around whether or not you have your car on PCP and intend to keep it, or hand it back, at the end of the contract. All current EVs (to my knowledge) have a warranty covering the battery, of a period a lot longer than your PCP contract (most max contract terms are 4yrs; most warranties are 7yrs plus). So, if you're intending to hand the car back, keep it 100% charged, so you can obtain maximum use from your asset whenever you need to. As has already been pointed out, this is not 100% of possible battery capacity, as the BMS maintains the battery at safe levels to protect their warranty. If cars like the leaf have broken BMS which damage the battery, they should be called out separately (I think it's outragous that Nissan try to invalidate warranty if you plug your car in at the wrong time - I'd love to see that challenged in court).
For the rest of us (who haven't purchased a broken leaf) relying on the BMS and simply using the car as intended is I think the obvious behaviour. If you're intending to keep your car for longer than the warranty, it would be beneficial for you to protect the battery as much as you reasonably can. This means in general keeping the battery between 20% and 80%, and keeping charging swings as short as possible (in other words, plug it in every night and set the charger or car to charge to 80% if you have that facility in your car or charger). This will give marginally better battery longevety, but if you're keeping the car a long time, it will be a benefit.
 
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