LFP battery - how often is too often to charge?

[Max Spaghetti]

I find the recommendation to charge my RWD to 100% at least once a week a bit of a problem as I frequently don't need a charge for a few weeks at a time...

It feels a bit silly charging from ~80% to 100% due to some arbitrary requirement in the owner's manual so I don't!

Absolutely you shouldn't!

Charging to 100% calibrates the coulomb-counter in the BMS and gives it a chance to balance the cells - (cell balancing is only effective at a high state of charge in LFP batteries due to the nature of the charge curve).

If you are not driving often, a weekly charge to 100% will tend to have your battery sitting at or close to 100% permanently. Not good for the battery. LFP doesn't "hate" 100% as much as NCA/NMC, but it still dislikes sitting at 100%. It would, in fact, prefer sitting as close to 50% as you find comfortable for your usage case. (Look up storing a Tesla - the recommendation is 50%.)

I consider the recommendation to charge to 100% should be based on the number of cycles you are doing. Tesla has provided just one piece of advice, and doesn't want it to look complicated (bad PR).

If you are cycling the battery say 2 times/week (700-800km/week) or more, then sure, charge to 100% once/week.

But if you are only cycling the battery 2 times/month, then maybe charge to 100% once/month. And so forth.

That's my take on it. Don't expect Tesla to give you the perfect advice for every scenario. I personally drive around 110-140km (about 35%) per day, 5 days/week, so I set a nightly charge limit of 70% and cycle the vehicle 35%-70%. I almost never go below 20% SOC. I charge to 100% roughly fortnightly, and/or before any road trips or big days of driving. That works for me, I find it very easy to manage, and it gives me the satisfaction that I am at least trying to look after my (very expensive) battery.

 

[Quickst]

A battery that is cycled between 50-100% is better cycled between 20-70%. Both add 50%. Rather than charge high, it's better to shift the charge cycle to a lower SoC.

LFP - yes 100% is necessary for time to time but not necessary for every charging session.

LR batteries 100% only when required

 

[Max Spaghetti]

My layman's take:

LFP cells charge to 3.6V when full, and NCA/NMC cells charge to 4.2V when full.

This is why NCA/NMC cells degrade faster at 100%: The 4.2V potential encourages some of the Li ions to go and form naughty rogue molecules and then they never come back. So you end up with less "useful" lithium in your cells

The lower 3.6V potential in LFP at 100% is a lessor inducement for the Li ions to be naughty, but some of them still will. Lower the potential back to 3.2-3.3V and the battery will last longer.

 

[Quickst]

This is why NCA/NMC cells degrade faster at 100%:

I'm not sure we can attribute NMC faster cyclic degradation to difference in cell voltage. That there is a difference does not automatically mean it's the cause.

The faster degradation of cyclic life in NCA/NMC is only in circumstances where batteries are quickly cycled between 100 to zero and back to 100 at a charging /discharging cycle of 2hrs each direction. This is not something that most EV owners do. When was the last time a Long range battery was fully discharged in 2 hours then fully charged in 2hrs and the cycle repeated 1000times without stopping?.

 

[Max Spaghetti]

I'm not sure we can attribute NMC faster cyclic degradation to difference in cell voltage. That there is a difference does not automatically mean it's the cause.

I'm referring to calendar life, not cycle life.

The NMC/NCA battery will have a shorter shelf life sitting at 100% than an LFP battery. That is mostly due to the higher cell voltage.

This is in response to the question of how often should a little-used LFP battery be charged to 100%. I am just padding out my response of "not very often" with an explanation of why. LFP is not as bad as NMC/NCA due to the lower cell voltage, but neither like sitting on the shelf at 100%.

 

[Quickst]

The NMC/NCA battery will have a shorter shelf life sitting at 100% than an LFP battery. That is mostly due to the higher cell voltage.

Again I am unsure if we can correctly attribute the shorter shelf life of a Li-ion battery at 100% to be due to the higher cell voltage of its chemistry compared to another chemistry

If you have some research article that suggests that I'd like to take a look.

 

[Max Spaghetti]

Again I am unsure if we can correctly attribute the shorter shelf life of a Li-ion battery at 100% to be due to the higher cell voltage of a battery chemistry compared to another chemistry

You would be completely correct that cell voltage is not the only factor. Chemistry is certainly a factor. I may have misused the word "mostly".

Cell potential is, however, a factor. The lower cell potential of LFP at 100% reduces it's capacity/weight ratio but increases both it's cycle life and calendar life. Other advantages of the chemistry increase that life further.

We could get into electrolyte oxidation, electrolyte reduction, interphase growth, electrode balancing, the tendency of the various transition metals to dissolve, migrate and deposit on the anode, and other breakdowns in the lattice structure. But I was simply pointing out that the lower inherent cell voltage in LFP has the silver lining of assisting in long-term capacity retention, which can be increased further by limiting time spent at 100%.

I'm sure we disagree on nothing. I over-simplified and you picked me up on it... All good!

 

[Quickst]

The lower cell potential of LFP at 100% reduces it's capacity/weight ratio but increases both it's cycle life and calendar life

Again, I am not sure if that's correct.
In any case as NMC and NCA batteries are virtually never stored at 100% I suspect the difference in calendar degradation is minimal.

 

[coref1]

My main car is a M3 LR with NMC battery however a couple of weeks ago I took delivery of a base model MG4 with the 51kw lfp battery as a second car. Yesterday I messaged MG about charging the my Mg4 to 100%. They've replied this morning advising me to charge to 100% once or twice a month but only using AC charging. Reason given was only ac charging puts it into equalisation mode. The manual describes equalisation mode as "after a normal charging process the battery management system will enter a mode where it will it will attempt to equalise the charge of every battery cell." Interestingly Tesla seems to have removed their advice about charging the lfp battery to 100% however until recently they've said charge the lfp battery to 100% at least once a week.
As an increasingly confused 64year old I've got no idea what the answer is but I'm getting the feeling this subject is becoming like the age old discussion about how often you should change oil in a ice car. My only advice is "don't forget to enjoy driving your car".

 

[Quickst]

@coref1
The answer is to follow the manufacturer's recommendations. KISS principle applies

 

[leonk]

It's funny, I remember buying the Prius C in 2012 and freaking out about how long the battery would last.
When I replaced the battery in 2021 it wasn't a big deal at all.

When it comes to the MG, I specifically went with the 51 because of the LFP battery.
My aim is to not worry about it, plug it in when it needs it and if it has a problem within the 7 years of the warranty, that's MG's problem.

 

[hypnomedium]

How often is too often to top up the charge on a (new) 2022 M3 RWD w/LFP battery?

At the moment we're plugging it back in after even ultra short trips using as little as 1% of the battery. Typically this is twice a day.

It kind of sounds like this is the right thing to do from the Tesla website but it just feels kind of wrong. I wouldn't do it to my phone or other battery based device... Tesla site says "LFP Battery, Tesla recommends that you keep your charge limit set to 100%, even for daily use, and that you also fully charge to 100% at least once per week."

Which is followed by: "Tesla uses lithium ion batteries so there is no memory effect. This means there is no need to deplete the battery before charging. We recommend plugging in as often as possible."

 

[hypnomedium]

We have just purchased a Model Y. My husband was trying to find out about the battery charging as there did not seem to be anything in the owners manual.
It would appear that the original advice has been removed from the manual and possibly charging to 100% once a week or not?
We are still waiting for a reply.

 

[Tam]

We have just purchased a Model Y. My husband was trying to find out about the battery charging as there did not seem to be anything in the owners manual.
It would appear that the original advice has been removed from the manual and possibly charging to 100% once a week or not?
We are still waiting for a reply.

You are right. There's a change in the Owner's Manual.

In 2023, the Lithium Iron Phosphate instructions were there:

This year, 2024, it's deleted between the "Energy Saving Feature" and "Submerged Vehicle":

I have no idea what happened here. Tesla should have explained the deletion.

 

[Jules22]

I think too many were taking it too literally.

I just charge mine whenever it is cheap to charge or if it's getting down below 20% ish. Really don't have to overthink it.

 

[Tam]

I think too many were taking it too literally.

I just charge mine whenever it is cheap to charge or if it's getting down below 20% ish. Really don't have to overthink it.

Tesla explained the reason (BMS accuracy) to display an onscreen message for those who refuse to set LFP at 100%:

 

[Jules22]

Tesla explained the reason (BMS accuracy) to display an onscreen message for those who refuse to set LFP at 100%:

Yeah, unfortunately a lot of people took that to mean that they MUST ALWAYS charge to 100%, I've seen quite a number of people make that silly statement. Most also don't understand that to have the BMS calibrate fully it needs to go to 100% then down to 0 and back to 100%. I've even heard of people sitting at superchargers always charging to 100% because they think not doing so will damage the battery, which is lunacy.

 

[Tony Soprano]

Is it really people mistaking it, or Tesla wording it incredibly poor (if that's not actually what they mean?)? That's worded in a pretty strong and direct way, in my opinion.

Most people would read that and think they're doing something wrong if they don't have it set to 100%. Most people who buy these cars aren't doing deep research on what's best for batteries, they expect to just take the manufacturer's word for it at face value (which in my opinion, is fair enough).

I don't think it's a case of "people are idiots". Tesla (if their advice is wrong) are the ones who deserve the criticism.

 

[DonSi]

I also think it's just a case of managing an EV the same as an ICE. I know when I pull up to the servo, I just fill up the tank without giving it a second thought. Mostly I drive my EV the same way. I let the 'tank' drain to a point the I'm uncomfortable then I fill up. The only difference is I probably don't drain it quite as far as I did in an ICE car because my filling options are more limited.

 

[cafz]

I also think it's just a case of managing an EV the same as an ICE. I know when I pull up to the servo, I just fill up the tank without giving it a second thought. Mostly I drive my EV the same way. I let the 'tank' drain to a point the I'm uncomfortable then I fill up. The only difference is I probably don't drain it quite as far as I did in an ICE car because my filling options are more limited.

Whereas I treat my EV like my phone - I plug it in every night so it's ready to go in the morning (though I do usually have the charge limit set to 70% rather than 100%).