Frequent small charges vs. infrequent deep charges

[Boza]

I have an L2 charger at home and I use about 28kW per week driving around in the city. I could charge 28kW once per week or 4kW every day. All the other concerns aside, what would be better for battery longevity?

I read that cycles of charge is what determines the SoC but I am unclear if the depth of the cycle makes a difference. Is fewer, deeper discharges (I never go out of 20%-90% range) worse that more often, shallow discharges?

 

[ATPMSD]

Tesla recommends plugging in the car every night, but battery life seems to be best when the battery level is varied. So I suggest once a week is better than every day. However, I suggest it is even better to vary this: maybe 4 kWh one day, 28 kWh another day, then maybe 30-40 kWh+

Battery recommendations are all over the place, so I am sure you will get many more recommendations.

 

[EVEnergy]

Plug it in every night. The "low" AC power used in charging from your house isn't enough to compromise the battery. It's the DC fast charging that affects the battery.

 

[Boza]

Do small charges (e.g. 55% to 60%) daily “count” against the charge cycle count or only full discharge-charge cycles do?

 

[Big Earl]

A full charge is a full cycle. Two half charges are a full cycle. Four quarter charges are a full cycle.

The only thing to be aware of is shallow charging causing the state of charge calibration to drift over time. The car needs to periodically sleep at various states of charge to get an accurate estimation of full pack capacity.

 

[jjrandorin]

Do small charges (e.g. 55% to 60%) daily “count” against the charge cycle count or only full discharge-charge cycles do?

It doesnt matter, really. A "cycle" for this battery is 100 -->0--->100. Doesnt matter if you do that in one charge of 0-100 or if its 10 charges of 10% or if its 20 charges of 5%. There is no benefit to "running it down to charge it back up" which is what you are asking.

So it doesnt really matter as far as cycle count goes. It also doesnt matter for car usage, other than the fact that you dont want to stay at > 90 for too long, or at a low state of charge for a long time.

The "plug in" recommendation is at least partly because they dont want people to be like "I ran the car down to 30%, now I have an emergency and need to use the car and its going to take HOURS !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"

 

[Boza]

So, it is the energy that goes through the battery, not how it goes through it (if we keep it between 20% - 90% and L2, of course).

 

[hgmichna]

Yes, exactly.

If you want to extend the lifetime of the battery to the ultimate max, try to keep it between 40% and 60%, i.e. charge it whenever you can to 60%.

Keeping it between 20% and 80% will reduce its lifetime a little bit, compared to 60%. Fast-charging and charging to 100% also nibbles at the battery's capacity, but unless you do that very often, it's not a good reason to worry. Try to avoid charging it to 100% and then letting it sit there, fully charged, for extended periods of time. That's harmful.

And don't forget, the car is there to serve your needs, not the other way around.

That said, it also depends on the type of battery. If you have an LFP battery (Lithium-iron-phophate, used in the Standard Range RWD Model 3 Made in China), you can charge to 90% without any severe side effects, and you should even charge it to 100% once a week.

 

[Boza]

I have the old LiPo battery in my 2021 MS LR+. I know it is a bit of nit-picking but I am curious about the engineering/chemical aspects of EV.

Interestingly, the range estimator was more accurate when I was doing small daily charges vs. now, when I switched to weekly charges. There is also the V11 in between, though. I will switch back to small daily and see if that will improve the accuracy.

 

[Boza]

Since I am getting such good responses, let me ask another nit-picking question that I could not get a clear answer to

What would be the optimum charging current? I.e. one that does not extend the charging time too much and does not require active cooling.

I was experimenting a bit and it seems that at 75’F ambient temperature ~24A achieves that sweet spot. I could not hear the cooling pump starting but I was not there to monitor it throughout the charging cycle.

 

[ATPMSD]

Battery wise, anything below the battery size is fine, for example if you have an 85 kWh battery than anything less than 85 kW is fine. So even 240v @ 48A = 11.5 kW is a non-issue.

As to cooling, this seems like more trouble than it is worth, at least for me.

 

[Big Earl]

I have the old LiPo battery in my 2021 MS LR+. I know it is a bit of nit-picking but I am curious about the engineering/chemical aspects of EV.

Interestingly, the range estimator was more accurate when I was doing small daily charges vs. now, when I switched to weekly charges. There is also the V11 in between, though. I will switch back to small daily and see if that will improve the accuracy.

Which energy estimator?

The battery icon that you can toggle between miles and % is more of a battery meter/gauge that displays either a % of maximum battery capacity (accounting for degradation) or units of energy expressed in EPA rated miles. It doesn't estimate anything more than the amount of energy in the battery, and does not adjust for recent driving habits.

You're getting less range because it's colder now than it was a few months ago, not because you've changed your charging habits.

 

[Big Earl]

Since I am getting such good responses, let me ask another nit-picking question that I could not get a clear answer to

What would be the optimum charging current? I.e. one that does not extend the charging time too much and does not require active cooling.

I was experimenting a bit and it seems that at 75’F ambient temperature ~24A achieves that sweet spot. I could not hear the cooling pump starting but I was not there to monitor it throughout the charging cycle.

The optimum current for level 2 AC charging is whatever completes your charge cycle the fastest. If you can charge at 48 amps, do it, as it shortens the amount of time the charging session takes, thus minimizing the energy required to keep the car's systems running. Faster charging sessions are both more efficient and minimize run time on coolant pumps and computers. You may have other limitations like how much extra capacity you have in your electrical panel, what sort of installation you have (NEMA 14-50 vs Wall Connector), etc.

 

[Boza]

Is it possible to charge with only passive cooling?

 

[Boza]

Which energy estimator?

The battery icon that you can toggle between miles and % is more of a battery meter/gauge that displays either a % of maximum battery capacity (accounting for degradation) or units of energy expressed in EPA rated miles. It doesn't estimate anything more than the amount of energy in the battery, and does not adjust for recent driving habits.

You're getting less range because it's colder now than it was a few months ago, not because you've changed your charging habits.

It is a possibility but I doubt it. I live in a fairly consistent weather and have fairly predictable out of town trips.
The range estimator used to be pretty accurate; now, it fluctuates quite a bit. Could be firmware related too.

 

[Big Earl]

Is it possible to charge with only passive cooling?

I've never seen AC charging use active cooling (compressor). At most it'll run the cooling pumps and the radiator fan, but even that is very rare.

 

[MY1stEV]

I am not a battery expert, but I did supercharge at a Holiday Inn Express once.

There is something confusing that can happen if you do a lot of constant small charges within a narrow range. Your battery's estimated range at 100% charge will decrease, and people perceive this as battery degradation. It's not.

Tesla's have a battery management system (BMS) that tries to optimize the life of your battery. When your car has been sleeping for several hours, the BMS kicks in and performs some calibrations. The accuracy of this calibration depends on how many data points the system has at different states of charge to work with. If the bulk of the data points are all clustered around a higher percentage, the BMS's estimate will not be that accurate. (Long car trips where you go from high to low to high states of charge have no effect on this because the car is not sleeping so the BMS is not calibrating).

So, as stated in answers above, it's the number of complete charge cycles your battery goes thru that determine its lifespan, not your charging pattern. However, your charging pattern can affect the BMS and its ability to correctly estimate the capacity of your battery.

 

[MontyFloyd]

Slow charging is better (L1), when you have time.
Keep battery cool, but not freezing (most regulate automatically, but helps to have a cool garage anyway)

I would not charge if around 70% unless going long tip.

 

[Big Earl]

Slow charging is better (L1), when you have time.
Keep battery cool, but not freezing (most regulate automatically, but helps to have a cool garage anyway)

I would not charge if around 70% unless going long tip.

"Better" how? It's less inefficient and I've also come across references (a Jeff Dahn presentation, IIRC) that long continuous charges and discharges can, over time, degrade the battery more than shorter charges and discharges since the battery has less time to rest.

 

[MontyFloyd]

"Better" how? It's less inefficient and I've also come across references (a Jeff Dahn presentation, IIRC) that long continuous charges and discharges can, over time, degrade the battery more than shorter charges and discharges since the battery has less time to rest.

Fundamentally it due to heat. The hotter, the faster the degradation.
Cooling systems have limits, the very core is not cooled, the heat has to pass though entire battery to sink.

Still open question how much the matrix breaks down during charge/discharge cycles.

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