There seems to be some misunderstandings lately about rated range, the Battery Management System (BMS), and battery degradation. If you feel like reading about this, let me attempt to explain this in a different way, that I hope will clarify this phenomenon for you. I am NOT speaking out against battery degradation BTW, I believe it to be real. But I do think there are measurement errors in the way that some folks determine that degradation.
Here goes:
Let's go sailing.
Imagine yourself setting out on a sailing adventure. But for this adventure, you decide to go "old school" and navigate using the stars, the sun, and a time reference...just how they did it back centuries ago. For this trip, you have no GPS to pinpoint your position for your journey, but you have some tried a true ways to make pretty good estimates using those celestial elements.
For this trip you will be sailing exactly 1,000 miles across the ocean. From your home port to the far shore is exactly 1,000 miles. Got it?
Let's say you set off and sail for many weeks. Using the celestial methods, you determine one day that you are halfway across the ocean. Then for whatever reason, you decide to turn around and sail back home. You turn the ship around and sail for home. Heading this new opposite direction, you sail for weeks.
Now heading back home, many weeks later, you calculate that you are 90% of the way there when you again determine that you want to turn around and head back to the original destination! So you turn the ship around and head back the original direction towards that far off land.
Sail that way for many many more weeks and continue to update your position using our above celestial methods.
Let's say that along this leg, you suddenly encounter some doldrums (areas without any wind). For those days, here and there, you simply sit and drift...barely moving at all. Some days you get a little distance travelled, but others there are no helpful winds.
Using the sun and stars...where are you? How can you determine how close you are to either shore? This back and forth, along with some drifting and doldrums is going to make your time and speed calculations very difficult to pinpoint your exact position. Is the far shore 397 miles away? Maybe it's 390? Your best guess based on the noon sun and an inaccurate clock allows you to calculate that perhaps you are 390 to 400 miles away...but you can't be sure. (That's how they did it back then by the way). Plus, all that time drifting and sitting stagnate has foiled any time/speed calculations you might have done.
Bottom line: Without seeing either shore for a good length of time...you cannot be certain of how close you are to either shore.
So this is how the Tesla BMS works folks. It's running a best guess algorithm and guessing what battery range you have (State of Charge/SOC) based on voltages and your back and forth across the charging spectrum. Is that 90% you are charging to really 90%? When you drive all day and get to 30% SOC, is that really 30%?
Lemme answer that for you...it's not. It's the BMS's best guess at that moment.
Did you happen to sit in traffic, or at stop lights along your last drive? If so, you are covering zero distance, while there is slight drain on the system (doldrums). The BMS has to account for that too.
This is why Tesla engineers, battery experts, Bjorn, and others, highly recommend what I call "touching a shore" or getting your battery to 0 or 100%. This gives the BMS a known point to base is calculations on. Make any sense? Furthermore, to get a really accurate test of your true battery health via range predictions, you have to do like Bjorn talks about and charge to 100% and then immediately drive to zero percent with no hard accelerations, no stops (doldrums), no hills, constant speeds, etc. Truthfully, it can't be done.
I hope my analogy helps you understand a little bit better on some of the things going on with the BMS algorithm and therefore why suddenly, one day, you charge up to 80% or 90% and the number next to the green battery icon on the dashboard is different that what you're used to seeing.
Here's Bjorns fantastic video again, I really recommend you watch it to truly begin to get a feel for some of the "stuff" that is going on inside the Tesla BMS brain, and WHY accurate measurements are needed before claiming "battery degradation."
Thank you.
Here goes:
Let's go sailing.
Imagine yourself setting out on a sailing adventure. But for this adventure, you decide to go "old school" and navigate using the stars, the sun, and a time reference...just how they did it back centuries ago. For this trip, you have no GPS to pinpoint your position for your journey, but you have some tried a true ways to make pretty good estimates using those celestial elements.
For this trip you will be sailing exactly 1,000 miles across the ocean. From your home port to the far shore is exactly 1,000 miles. Got it?
Let's say you set off and sail for many weeks. Using the celestial methods, you determine one day that you are halfway across the ocean. Then for whatever reason, you decide to turn around and sail back home. You turn the ship around and sail for home. Heading this new opposite direction, you sail for weeks.
Now heading back home, many weeks later, you calculate that you are 90% of the way there when you again determine that you want to turn around and head back to the original destination! So you turn the ship around and head back the original direction towards that far off land.
Sail that way for many many more weeks and continue to update your position using our above celestial methods.
Let's say that along this leg, you suddenly encounter some doldrums (areas without any wind). For those days, here and there, you simply sit and drift...barely moving at all. Some days you get a little distance travelled, but others there are no helpful winds.
Using the sun and stars...where are you? How can you determine how close you are to either shore? This back and forth, along with some drifting and doldrums is going to make your time and speed calculations very difficult to pinpoint your exact position. Is the far shore 397 miles away? Maybe it's 390? Your best guess based on the noon sun and an inaccurate clock allows you to calculate that perhaps you are 390 to 400 miles away...but you can't be sure. (That's how they did it back then by the way). Plus, all that time drifting and sitting stagnate has foiled any time/speed calculations you might have done.
Bottom line: Without seeing either shore for a good length of time...you cannot be certain of how close you are to either shore.
So this is how the Tesla BMS works folks. It's running a best guess algorithm and guessing what battery range you have (State of Charge/SOC) based on voltages and your back and forth across the charging spectrum. Is that 90% you are charging to really 90%? When you drive all day and get to 30% SOC, is that really 30%?
Lemme answer that for you...it's not. It's the BMS's best guess at that moment.
Did you happen to sit in traffic, or at stop lights along your last drive? If so, you are covering zero distance, while there is slight drain on the system (doldrums). The BMS has to account for that too.
This is why Tesla engineers, battery experts, Bjorn, and others, highly recommend what I call "touching a shore" or getting your battery to 0 or 100%. This gives the BMS a known point to base is calculations on. Make any sense? Furthermore, to get a really accurate test of your true battery health via range predictions, you have to do like Bjorn talks about and charge to 100% and then immediately drive to zero percent with no hard accelerations, no stops (doldrums), no hills, constant speeds, etc. Truthfully, it can't be done.
I hope my analogy helps you understand a little bit better on some of the things going on with the BMS algorithm and therefore why suddenly, one day, you charge up to 80% or 90% and the number next to the green battery icon on the dashboard is different that what you're used to seeing.
Here's Bjorns fantastic video again, I really recommend you watch it to truly begin to get a feel for some of the "stuff" that is going on inside the Tesla BMS brain, and WHY accurate measurements are needed before claiming "battery degradation."
Thank you.
