Run low, charge high periodically to "exercise" cells?

[Ponderosa]

Sorry if this has been addressed elsewhere, but can't find specific answer. I have 2023 M3/ LR / DM
I normally leave it plugged in when not in use. My normal usage does not bring the battery below 60%. I charge to 81% (personal quirk, I like it to read 80% when I leave my driveway.)
A Tesla tech told me today that I should occasionally (perhaps every 6 months) let the battery run down to about 10%, then charge to about 90%.
This, he said, would "exercise" the cells, like a person who sits a lot getting up and stretching.
Do others agree? Good, bad, no difference?
Thanks!

 

[fhteagle]

As I understand it, the BMS for BMC cells should see high SOC (91+%?) and less than 20% or so, AND let it sit for 1-3 hours with no loads (no HVAC, sentry, etc) so that true cell balance readings can be taken and reacted to. I try to accomplish this around 4-6 times a year, in combination with special trips that need those big SOC swings anyway. The rest of the time, small SOC changes and lowest storage SOC with enough reserve for an unexpected trip is my strategy.

LFP cells, same thing except the high side target is 100% and do so more often (somewhere between weekly and monthly), but don't let it sit at 100%.

 

[jjrandorin]

The high level answer is that doing what you say may make the numbers on the battery maintenance system (BMS) read better, but wont really change ACTUAL battery degradation. To change actual degradation rates, you will want to charge to a lower percentage than 80% on a regular basis (the number is <60% to impact calendar aging ).

For more than that super high level answer, you can look at any of the threads on battery degradation (there are probably 100+ threads on the topic on TMC). Specifically, you can search for posts from a couple of users, @AAKEE or @AlanSubie4Life.

 

[Ponderosa]

Thanks

fhteagle and​

jjrandorin​

 

[E90alex]

There is no need to specifically do that regularly unless you suspect the BMS is out of calibration. You’ll likely hit different charge levels just with regular usage and road trips etc.

If you normally only use ~20% of the battery then consider setting a lower charge limit for daily use, which will be healthier for the battery in the long run. The lower the better, but don’t set it so low that it causes any inconvenience. The magic number for the type of battery in your car is below ~60% if you can manage that.

For example I only use 10-15% daily so I set my charge limit to 50% which is more than enough to last me 2-3 days in case I forget to plug in or can’t charge for some reason one night.

 

[AAKEE]

Sorry if this has been addressed elsewhere, but can't find specific answer. I have 2023 M3/ LR / DM
I normally leave it plugged in when not in use. My normal usage does not bring the battery below 60%. I charge to 81% (personal quirk, I like it to read 80% when I leave my driveway.)

Most answers already given.

For longevity you would like to have the battery below 60% as much of the time as possible.

The cells do not need to be stretched. Ever, really.

Low SOC and small cycles reduce the degradation.

The technique to ”stretch” is to help the BMS to estimate the battery capacity better.

A Tesla tech told me today that I should occasionally (perhaps every 6 months) let the battery run down to about 10%, then charge to about 90%.
This, he said, would "exercise" the cells, like a person who sits a lot getting up and stretching.
Do others agree? Good, bad, no difference?
Thanks!

So in principle you cause slight extra wear to the battery when you ”stretch”.
It will not be very bad, just as charging to 100% is not very bad and discharging to very low numbers is not very bad either.

For most people the normal usage will be enough for the BMS to judge the capacity.
If the range suddenly drops and seems clearly lower than expected a BMS calibration could be done to help the BMS.

 

[Rocky_H]

@Ponderosa The thing is, there are two opposite things you can do that are at cross purposes with each other.

For the best actual health of the battery cells long term, you can use them in a very small range in the middle area of the battery, like 40-50%.

But doing that doesn't give the battery management much visibility to where the actual top and bottom energy levels of the pack are, so with that lack of information over time, the estimations get fuzzy and drift off. So to tune in that reading, every once in a while, you could let the car sit at a low state of charge and a high state of charge to recalibrate that. It's not doing anything helpful for the cells, but it may make the driver feel better.

 

[AAKEE]

@Ponderosa The thing is, there are two opposite things you can do that are at cross purposes with each other.

For the best actual health of the battery cells long term, you can use them in a very small range in the middle area of the battery, like 40-50%.

Cyclic aging is very very small during normal operation.
We can reduce the cyclic aging by using smaller cycles and do these at lower SOC.


10% DoD series.
Lowest degradation is 35-55% SOC in this test.
On a Tesla display this is around 32-53% displayed SOC due to the buffer.

Even using the worst range of these (5-15% true SOC or 0-10% displayed) give 17% loss in 3000FCE wgich will be around 1.200.000 km or 750K miles.
In other words, around 0.3% cyclic aging per year if you drive about as much as people mostly do.

So trying to reduce degradation only by hunting the best SOC range to cycle in will not help much.

So, if anyone would like to reduce the degradation rate, it is calendar aging we need to adress.

Using the correct approach will reduce the calendar aging by half, and the best part is that the cyclic aging will benifit as well (but as it anyway is a minor part, the difference will not be big in the cyclic case).

 

[Ponderosa]

Thanks, @AAKEE. So, If I understand your post, and your others on calendar aging, one can minimize both calendar and cyclic aging by maintaining a relatively narrow SOC < 60%.