What do people actually mean when they say the battery needs to be replaced?

[FH93]

I'm looking for info about what actually causes a battery to fail, or under what circumstances they need to be replaced as I was looking at a higher mileage 2021 M3 Dual Motor LR (110k miles as fleet car), but the overwhelming majority of people seem to reference battery degradation or range as being the issues. That's not an issue for me because even at 50% range it's more than I'll ever need.

I wanted to know what else actually would warrant a battery to 'need' to be replaced in a car. Do batteries just stop working? Do they need a minimum level of battery health to be able to function?

I ask because the one I'm looking at is £15k while the next cheapest is £17.5k with 92k miles on it because it's still under 100k. I can afford a £20k Tesla, but if I get the £15k one it'd be a lot better for me provided it doesn't just go kaput. Half of the people tend to say don't get one out of warranty because the battery might just die at any moment and the other half say not to worry because they've got cars going 150k+ miles and also people who have 300k+ miles, including the one guy with 430k miles on the original battery. I only do 15k-16k a year.

Any information is welcome.

 

[DuncanM]

I'm looking for info about what actually causes a battery to fail, or under what circumstances they need to be replaced as I was looking at a higher mileage 2021 M3 Dual Motor LR (110k miles as fleet car), but the overwhelming majority of people seem to reference battery degradation or range as being the issues. That's not an issue for me because even at 50% range it's more than I'll ever need.

I wanted to know what else actually would warrant a battery to 'need' to be replaced in a car. Do batteries just stop working? Do they need a minimum level of battery health to be able to function?

I ask because the one I'm looking at is £15k while the next cheapest is £17.5k with 92k miles on it because it's still under 100k. I can afford a £20k Tesla, but if I get the £15k one it'd be a lot better for me provided it doesn't just go kaput. Half of the people tend to say don't get one out of warranty because the battery might just die at any moment and the other half say not to worry because they've got cars going 150k+ miles and also people who have 300k+ miles, including the one guy with 430k miles on the original battery. I only do 15k-16k a year.

Any information is welcome.

IIRC, Tesla warranties the M3 LR HV power train and battery pack for 8 years or 120K miles (but don't quote me on that). The warranty is triggered if the battery fails completely or degrades below 70% during the warranty period. Stats show that ~1% or less of newer model EVs will need a HV battery replacement. The quick way to assess battery degradation is to note the indicated range at the indicated state of charge and compare it to the new car rating for the same model. If a new 2021 M3 LR indicated, say, 350 miles of range at 100% SOC and the M3 LR you're looking at indicates 315 miles of range at 100% SOC, then it has suffered 15% degradation. Note that Tesla bases indicated range on the USA EPA test cycle, not the EU/UK WLTP test cycle.

 

[FH93]

IIRC, Tesla warranties the M3 LR HV power train and battery pack for 8 years or 120K miles (but don't quote me on that). The warranty is triggered if the battery fails completely or degrades below 70% during the warranty period. Stats show that ~1% or less of newer model EVs will need a HV battery replacement. The quick way to assess battery degradation is to note the indicated range at the indicated state of charge and compare it to the new car rating for the same model. If a new 2021 M3 LR indicated, say, 350 miles of range at 100% SOC and the M3 LR you're looking at indicates 315 miles of range at 100% SOC, then it has suffered 15% degradation. Note that Tesla bases indicated range on the USA EPA test cycle, not the EU/UK WLTP test cycle.

Yeah I just looked it up and that warranty is correct. I'd thought it was 100k but that's for LR RWD but regular AWD is 120k, which is the one I'm looking at so I'd have about 10k miles left on the warranty which is better than nothing but that other one I was looking at which is £2.5k more expensive with 92k miles has almost 30k miles left on the warranty. I do 15k a year so the first wouldn't last me the year but the second would last me almost two. There's also some from £19k-£20k with 80k+/-3k on the clock which I could go for so I'm not sure which I should choose at this point.

 

[Rogueeconomist]

Almost all the battery replacements I've ever heard of for Teslas have been for serious failure, not for degradation (although degradation over time may contribute to serious failure). Serious failure means that the battery is basically unusable all of a sudden; the car either refuses to charge the battery or operate with it, or power and charging is so seriously limited that most owners would consider it unusable. Basically, what you should worry about is this kind of serious failure, not degradation.

In other words, batteries are not something that degrades gradually from 100% to 0% predictably over many years. In theory, battery cells might do that in the lab. In practice, what is more common is that there is something that fails within the battery (could be a set of cells, could be the cooling lines, could be corrosion, anything) that causes the battery management computer to regard the battery as unsafe to use, hence requiring repair or replacement.

Therefore, you simply cannot predict, e.g. for a battery that is already at 10% degradation and took, say, 5 years to get there that you have another 5 years to go to 20%, and nothing will happen in between, or anything like that. Not only is degradation not linear (it slows a lot after the first few years), it also says very little about whether your battery might have a serious failure, although if degradation is significantly faster than typical, it might be a warning sign.

The early Model S's, which are the ones that have sometimes had multiple battery replacements (due to an unlimited mileage warranty for early cars), tended to have replacement for serious failure and not because the battery degraded below 70%. The serious failure might have still occurred after several hundred thousand miles, but it was still unpredictable often from degradation alone. This was due to various manufacturing or design issues that have tended to be ironed out with the Model 3 / Y.

Due to this, I would base my decision on a used Tesla on getting the one with the most warranty and miles left in the battery if possible. If you need a lot of predictability in durability and costs, better to just get a new Tesla. That's what I'd lean towards when replacing my current Tesla, at least until viable aftermarket options exist for repairing and refurbishing Tesla batteries (some exist, but it's not a common service at all).

 

[FH93]

Almost all the battery replacements I've ever heard of for Teslas have been for serious failure, not for degradation (although degradation over time may contribute to serious failure). Serious failure means that the battery is basically unusable all of a sudden; the car either refuses to charge the battery or operate with it, or power and charging is so seriously limited that most owners would consider it unusable. Basically, what you should worry about is this kind of serious failure, not degradation.

In other words, batteries are not something that degrades gradually from 100% to 0% predictably over many years. In theory, battery cells might do that in the lab. In practice, what is more common is that there is something that fails within the battery (could be a set of cells, could be the cooling lines, could be corrosion, anything) that causes the battery management computer to regard the battery as unsafe to use, hence requiring repair or replacement.

Therefore, you simply cannot predict, e.g. for a battery that is already at 10% degradation and took, say, 5 years to get there that you have another 5 years to go to 20%, and nothing will happen in between, or anything like that. Not only is degradation not linear (it slows a lot after the first few years), it also says very little about whether your battery might have a serious failure, although if degradation is significantly faster than typical, it might be a warning sign.

The early Model S's, which are the ones that have sometimes had multiple battery replacements (due to an unlimited mileage warranty for early cars), tended to have replacement for serious failure and not because the battery degraded below 70%. The serious failure might have still occurred after several hundred thousand miles, but it was still unpredictable often from degradation alone. This was due to various manufacturing or design issues that have tended to be ironed out with the Model 3 / Y.

Due to this, I would base my decision on a used Tesla on getting the one with the most warranty and miles left in the battery if possible. If you need a lot of predictability in durability and costs, better to just get a new Tesla. That's what I'd lean towards when replacing my current Tesla, at least until viable aftermarket options exist for repairing and refurbishing Tesla batteries (some exist, but it's not a common service at all).

I can't afford a new Tesla right now, but I did toss up the idea of doing PCP and trading it back in afterwards and then using the guaranteed value to go towards at least a newer/better condition Tesla at the end of it, though it wasn't my first choice.

 

[Apprunner]

You could also look for LFP versions where low calendar age and high mileage still means significant life going forward. LFP calendar age is similar to NCA/NMC life but it has much more cyclic life meaning a highly used LFP pack still has many more life left.

Just a thought

 

[father_of_6]

As you mentioned, degradation isn't the primary concern - it's pack failure. An LR has something like 4,400 cells (like AA batteries). If even one of them goes "bad", the entire pack must be replaced.

It's a lottery. There's no way to know if either of those vehicles will experience a bad cell. Given *unlimited* time, I'd assume all batteries will eventually experience a bad cell... but for some packs that might take 40 years. Not sure anyone can answer this yet.

 

[jjrandorin]

I wanted to know what else actually would warrant a battery to 'need' to be replaced in a car. Do batteries just stop working? Do they need a minimum level of battery health to be able to function?

Failure to be able to be charged is the big one, not degradation.

This is likely not going to be a popular opinion, but I personally am not a fan of "I cant afford a new version of this car, so I am going to buy a super high mileage version and hope its ok".

If you cant afford a new version of the car, you probably cant afford to fix it if there is a problem either. It would be a lot better to buy a car closer to something you can afford thats reasonable as far as your budget. I am not intending to lecture, I just think "I cant afford..." means you should purchase something you can afford, and if the only version of your desired car that is in your budget is 100k miles, you should be looking at different cars (EV / ICE whatever).