How to take care of your battery without stressing: advice for new owners.

[PhysicsGuy]

Interesting. Thanks. My table in the first post is consistent with that. The value I have for 85% is 4.02 volts. I added that here.
4.11 volts is 92%
4.02 volts is 85%
3.97 volts is 80%
3.69 volts is 50%
3.50 volts is 20%
3.4 volts is 12%

Looks like there is a pretty linear region between 50% and 85%. With all my Li-ion batteries except Tesla I just use voltage measurement to determine state of charge. It seems to be much more accurate than BMS estimates.

 

[PhysicsGuy]

Everything between about 78% and 87% rounds to 4.0 volts. (That is why I prefer having a 3rd sig fig.)

 

[smatthew]

Interesting. Thanks. My table in the first post is consistent with that. The value I have for 85% is 4.02 volts. I added that here.
4.11 volts is 92%
4.02 volts is 85%
3.97 volts is 80%
3.69 volts is 50%
3.50 volts is 20%
3.4 volts is 12%

My bad - missed that! Thanks for that.

 

[camalaio]

The voltage differences are small, but interesting to me. What cell chemistry are your numbers for? There are probably more than 20 or 30 different 21700 size cells on the market now. I am not sure if the exact one that Tesla uses for the M3 is available for purchase. I think my numbers may have been for an MJ1 chemistry.

Numbers from the car itself with a relatively healthy battery around 25C (no outlier bricks in capacity or voltage, very minor degradation at 36,000km, low imbalance throughout discharge, low imbalance under heavy load - it seems I've won the lottery). I'm not really sure on the specifics but to my understanding it should be the same for all current Model 3s.

Maybe the good state of my pack is contributing to these being off, but it's also the only voltage numbers I know of correlating what we see (% on the display/app) to cell voltages. Everything else seems to be stating a percentage referring to something else, e.g. different cells, different discharge range, etc.

EDIT: To be very very very clear, this is based on CAN data which we could all be misinterpreting. I'm fairly certain that's not the case, but it could be.

EDIT: Perhaps it's worth mentioning, I don't know what "percent" is normally referring to. In the car I want to assert it almost definitely means percent of remaining watt-hours. I know via the RC community, sometimes percent is reported via remaining amp-hours, which does not perfectly follow watt-hours nor is an accurate indicator of total energy at a given point (but will be approximately correct).

 

[Zoomit]

Maybe the good state of my pack is contributing to these being off, but it's also the only voltage numbers I know of correlating what we see (% on the display/app) to cell voltages.

Your CAN bus data matches Bjorn Nyland’s very closely.

Here’s a screen capture from one of his videos that shows average cell voltage at 3.99v [383/96] while at 72% SOC and discharging only 1A.

 

[camalaio]

Your CAN bus data matches Bjorn Nyland’s very closely.

Here’s a screen capture from one of his videos that shows average cell voltage at 3.99v [383/96] while at 72% SOC and discharging only 1A.

View attachment 563492

Oh neat, thanks for capturing that. Glad to have it verified with at least one other (and in a completely different country).

I've been trying very hard to think of why this might differ from @PhysicsGuy 's numbers, but I'm still not super clear on why that might be. Probably related to the cutoff voltage with the bottom buffer vs. usual cutoff voltage. I've been thinking of getting data for that, but don't want to discharge my car that much in the name of science (I'm too happy with the fact I've mostly kept it above 10% )

 

[SeaNile]

Ahh.. the Osborne effect. Simply - forgoing today for the future possibilities is a never ending battle. Just jump in with both feet and enjoy every second of it!

(and PS - I don’t ever worry about the heat or AC. Comfort first....)

Keep in mind that for some people - many people on this board - efficiency and utilization is a game. They enjoy it. I don’t. I just want to drive the damn car. ;-)

Y decreased by $3000 and battery day announced for Sept 22. I'm definitely not buying before Sept 23 now!

 

[Gasaraki]

I have gotten a great deal of benefit from this forum, but I have become concerned that people are getting unsound advice here about how to take care of your battery. Specifically, there are recommendations of things you can do to increase your range that I think are not actually good for your battery.

Taking care of your battery[1,2] is simple and does not require a Ph.D.. I keep my battery between about 30% and 70% for everyday use and then charge to about 95% before I go on a longer trip. You can charge your battery to 90% everyday if you prefer to do that. It is no big deal. I would recommend avoiding going below 15% or 12% unless you are really in a pinch. That is pretty much it. Charging speed is not too important; Tesla knows what they are doing at superchargers. There is one other small thing. Acceleration comes at a small cost. Accelerate all you want, but realize the it diminishes the longevity of your battery very slightly because you are drawing current from your battery at a very high rate[3].

Things that concern me here include recommendations that people charge to 95% and drain to 5% three times for the purpose of increasing your range. This can make the range you see on your display higher, which is ironic because doing those three cycles definitely decreases the capacity of your battery in kW-h (kilo Watt hours). This is just the way the chemistry and physics of the cells in your battery work. There is no way around it. Your battery has a finite number of cycles. It is a large number, but it is finite. You have just wasted 3 of them for the momentary pleasure of seeing a higher number on your screen. This may be called “touching the shore”, which is not only bad for your cells, but has a theory behind it that is unsound because there is a simple relationship between state of charge and voltage that does not change as cells age. The thing that changes is the number of kW-h of energy that your battery stores.

Additionally, going to a high or low state of charge for the purpose of balancing is, for the most part, unnecessary. Tesla is able to monitor the degree of imbalance among your 96 groups of cells. I think they would let you know if that becomes problematic. They monitor intermediate voltages between groups that we cannot see. If your groups are slightly out of balance, the worst thing you can do is go below 12%, because that exposes your weakest groups to bullying by the stronger groups (I.e., getting current pushed through them that they can’t handle well).

It is pretty simple. Charge above 90% only just before you go on a trip. Endeavor to never go below 12%. Sustained acceleration is not free.

Oh, and here is the most important thing that I almost forgot. Set your range to percent, not to miles or kilometers. That will create a good mindset for paying attention to your batteries very simple needs and avoid range obsession. If you need to know how far you can drive, assume 10% will give you at least 25 miles[4].

————

1. Your battery is made of cells. It starts with groups of 46 cells in parallel; which establishes a high current capacity. Your battery, in a model 3, is a string of 96 of these groups. It has a high voltage, basically 96 times 3.7 volts at 50% charge and no load. (Be careful who you take advice from. Tesla techs are not generally Li-ion battery experts.

2. There is a relationship between voltage and percent charge at the cellular level, and therefore at all levels. For example:
4.11 volts is 92%
3.97 volts is 80%
3.69 volts is 50%
3.50 volts is 20%
3.4 volts is 12%
Going below 12% is bad for your cells.

3. For normal highway driving your battery would drain to zero in about 3 hours. That means it is drawing current at a rate called C/3. When you accelerate, Tesla allows you to draw about 5C. That is a high rate of current draw, but it is not a big deal because it is for a very brief time. In general, drawing current from a Li-ion battery at more than 2C tends to reduce the battery's longevity.

4. Range table.
10%. 25 miles
20% 50 miles
30% 75 miles
40% 100 miles
…
80% 200 miles

Or, more optimistically:
10%. 30 miles
20% 60 miles
30% 90 miles
40% 120 miles
…
80% 240 miles
The truth will probably lie somewhere in between depending on local conditions, including wind. I get noticeably more range going south than north on 101.

I mostly agree with this.

 

[Gasaraki]

Ooohh, and you were doing so well...

I agree with all of this about the charging levels, except for telling people to set the display for % and doing that in bold font. That's your preference you are projecting on people. I do agree that some people really can't handle seeing "rated miles". I've seen plenty of people foaming at the mouth, losing their everlovin' minds because the rated miles don't always equate 1 to 1 with real distance miles driven, and they absolutely just can't handle that without throwing a fit. It's like an OCD thing or something because they see that as "wrong". So yes, they need to switch to % to just not see that or think about that so they can stay calm. But humans don't think about real distances of places they go in percentages. People who are more laid back and not bothered with it can be comfortable just knowing that "rated miles" are always ballpark higher than real miles, and 84 miles of real driving today needs probably 120ish rated to comfortably do that.

But that is reopening what should have been an unrelated and overly debated can of worms. Thank you for the recommendations about charging levels.

What? So your gas tank displays in miles instead for percentage full?

 

[dmurphy]

What? So your gas tank displays in miles instead for percentage full?

In every gas car I've owned in the last 15+ years? Yup. Always on the dash display - "Distance to empty... XX miles". Nissan, Ford, GMC, Buick, Cadillac - had all of those, and all have a similar display. Haven't bothered looking at a gas gauge in decades.

 

[Rocky_H]

What? So your gas tank displays in miles instead for percentage full?

As @dmurphy pointed out, for a lot of gas cars, yes, they do exactly that. This technology to estimate remaining range has been available for some time.

But now to answer the question for myself, since you asked me, our other car is a 2005 Honda Civic Hybrid, and no, it does not display in miles. It's more like a little bar graph, so more like fullness percentage.

Now here's the further answer, because it addresses the blatant assumption you just revealed (and especially with giving me a dislike). You have already jumped to the conclusion that however gas cars do it is better. Why do you think that? I consider that percentage fullness kind of thing archaic, old, deficient, and less informative, and my old gas car just doesn't have anything better than that. The measurements of the rated miles in Teslas are more precise detail.

 

[jjrandorin]

In every gas car I've owned in the last 15+ years? Yup. Always on the dash display - "Distance to empty... XX miles". Nissan, Ford, GMC, Buick, Cadillac - had all of those, and all have a similar display. Haven't bothered looking at a gas gauge in decades.

Mine too, although I have been driving BMWs for the past 15 years or so, and they have "miles to empty" as one of the choices in the display, which I always set them to the minute I get them.

There is the regular gas gauge, which I pay little to no attention to, and then the "miles to empty" which I do pay attention to (talking gas cars here). I am also one who (like many others) keeps their tesla in "miles" instead of "percent" as it relates to range, because "miles" makes more sense to me.

 

[KenC]

Mine too, although I have been driving BMWs for the past 15 years or so, and they have "miles to empty" as one of the choices in the display, which I always set them to the minute I get them.

There is the regular gas gauge, which I pay little to no attention to, and then the "miles to empty" which I do pay attention to (talking gas cars here). I am also one who (like many others) keeps their tesla in "miles" instead of "percent" as it relates to range, because "miles" makes more sense to me.

I learned the hard way that BMW "miles to empty" really is miles to empty! Picked up a Eurodelivery 330xi in Munich, drove down to Italy, then up towards Paris. Just outside of Grenoble, 14 miles to empty, before getting on the autoroute, I was going to stop at the gas station, but I forgot, and missed it. Before I could get off the autoroute, my "miles to empty" hit zero, and it stopped. Where was the buffer?!?

Thank goodness, eurodelivery includes road service. Ran about a mile to a yellow call box, shouted in broken french to the person who answered, and amazingly, 15 mins later, I saw a service truck pass on the other side of the highway. I ran back to my car just as the service truck pulled up, and put in 5 gallons. No charge! Yay! So, yeah, "miles to empty" really means empty, no buffer.

On my Model 3 I use %SOC.

 

[jjrandorin]

I learned the hard way that BMW "miles to empty" really is miles to empty! Picked up a Eurodelivery 330xi in Munich, drove down to Italy, then up towards Paris. Just outside of Grenoble, 14 miles to empty, before getting on the autoroute, I was going to stop at the gas station, but I forgot, and missed it. Before I could get off the autoroute, my "miles to empty" hit zero, and it stopped. Where was the buffer?!?

Thank goodness, eurodelivery includes road service. Ran about a mile to a yellow call box, shouted in broken french to the person who answered, and amazingly, 15 mins later, I saw a service truck pass on the other side of the highway. I ran back to my car just as the service truck pulled up, and put in 5 gallons. No charge! Yay! So, yeah, "miles to empty" really means empty, no buffer.

On my Model 3 I use %SOC.

I have gotten to the "---" line before, without running out, but yeah I also got stuck once about 500 feet from the entrance to a gas station, because I ran it too low ( was running late to work that morning, said "Ill fill up on way home", then wanted to get home and said "ill fill up at the station by home", and 500 feet from the driveway to it... "sputter..."

Stuck there, feeling stupid, in my 1.5 year old Estoril Blue 435 Msport, with people pointing at me like I was the "silly bmw driver" on the side of the road... lol. after a few minutes, a good samaritan pulled over and helped me push it to the station. I never let it get below 1/4 tank after that.

 

[gavine]

I get noticeably more range going south than north on 101.

That's because south is downhill (at least in the northern hemisphere)

 

[Runt8]

Things that concern me here include recommendations that people charge to 95% and drain to 5% three times for the purpose of increasing your range. This can make the range you see on your display higher, which is ironic because doing those three cycles definitely decreases the capacity of your battery in kW-h (kilo Watt hours). This is just the way the chemistry and physics of the cells in your battery work. There is no way around it. Your battery has a finite number of cycles. It is a large number, but it is finite. You have just wasted 3 of them for the momentary pleasure of seeing a higher number on your screen.

I mostly agree with you, but my one nitpick is with the above. If you're doing extra driving just to run the battery down and recalibrate the BMS, then yes, you are "wasting cycles." If you were going to drive anyway and you skip recharging for a while in order to run the battery down, then those cycles aren't really wasted - you used them to get where you needed to go. It's the same number of cycles to drop down to 55% twice as it is to drop to 10% once. And charging to 95% isn't really a big problem as long as you don't leave it there.

So yes, driving from 95% to 5% a few times might be infinitesimally worse for the battery, but it's hardly the disaster you make it out to be (or maybe I misread your tone ).

My advice for new owners - don't charge above 90% unless you need the extra range and you're going to leave right afterwards, charge whenever you get home, and most important, stop worrying about it!

 

[Rocky_H]

but it's hardly the disaster you make it out to be.

He never "made it out to be" anything like a "disaster". That is you who is doing the exaggerating.

 

[Runt8]

He never "made it out to be" anything like a "disaster". That is you who is doing the exaggerating.

He obviously thinks it's a big enough deal to specifically call it out and write a large paragraph about it.

 

[Rocky_H]

He obviously thinks it's a big enough deal to specifically call it out and write a large paragraph about it.

That is just explaining that it's doing a little bit of harm for no benefit. It's not bad to explain that fact. That does not imply that someone thinks it is a "big deal".

 

[rafacq]

When I got my S I used miles to empty. Within two months I changed it to % SoC and definitely find it more to my liking. I charge to 90% at home but do not charge again until my battery is down to about 50%. I only use Superchargers on long out of town trips. My car is a 2015 S85D and after 5 years it has lost about 1.5% range, about 266 miles vs. the original 270. I’m very happy with how my battery is aging.