It's the Batteries, Stupid!

[malcolm]

Sure but even on "Max" settings they are still keeping something in reserve. With the larger pack they can keep more in reserve, so that even on "Max" the pack sees a shallower discharge.

Hmmmm. I'm not sure. This from Martin, November 2006

There is a huge difference in cycle life between a 4.2V/cell charge (defined by the manufacturers as “fully charged”) and a 4.15V/cell charge. 4.15 volts represents a charge of about 95 percent. For this reduction of initial capacity (5 percent), the batteries last a whole lot longer. Unfortunately, further reduction of charge has a much smaller benefit on cycle life.

Understanding this tradeoff, Tesla Motors has decided to limit the maximum charge of its cells to 4.15 volts, taking an initial 5 percent range hit to maximize lifetime of the pack. We also limit discharge of our battery pack to 3.0V/cell and will shut down the car when the batteries reach this level. Limiting our charge rate is less of a compromise, since the wire size and availability of very high current outlets limit us much more than the batteries do at this point.

Tesla Motors - think

If I understand this correctly, there is a limit on the benefit gained by reducing the depth of discharge. You reduce the maximum range but batteries don't last significantly longer than they would normally.

So you might as well run larger capacity cells at the same old 95% setting - what Tesla refers to as "Max".

Presumably other factors like average battery pack temperature have a greater impact but are harder to deal with since energy is required to reduce the temperature below ambient.

 

[JRP3]

That is interesting. Most charts that I've seen for different batteries show increased battery life at 80% discharge over 95% but that may not apply to the specific battery used in the Tesla.

 

[WarpedOne]

Unfortunately, further reduction of charge has a much smaller benefit on cycle life.

If change from 100% DOD down to 95% DOD give 10 times more lifetime and 95% to 80% only means 20% more lifetime, those 20% more is much smaller benefit than first 10time improvement but it still could mean the difference between 5 and 6 years of lifetime.

244 miles for 5 years or 205 miles for 6 years? Hmm, decisions ...

I guess we won't hear the detailed battery operating parameters for some time to come. They are Tesla Motors' trade secret. Almost everybody claimed (and some still do) it cannot be done at all, Tesla Motors went ahead and did it. I understand they burned through quite a few millions to come up with params that give the best result. Why publish them and give the competitors a free headstart?

 

[SteveF]

Can one of you engineers please help me understand this a bit better? From Martin's referenced post:

"Understanding this tradeoff, Tesla Motors has decided to limit the maximum charge of its cells to 4.15 volts, taking an initial 5 percent range hit to maximize lifetime of the pack. We also limit discharge of our battery pack to 3.0V/cell and will shut down the car when the batteries reach this level. "

OK, if the max charge starts at 4.15 volts per cell, and they limit discharge to 3.0 Volts per cell, isn't that just a discharge of 1.15 volts per cell? Or was he saying they limit the discharge to 3.0 volts FROM 4.15, meaning the remaining charge is 1.15 volts per cell? If the latter is true, then by my calculations 3.0/4.15 is about a 72% discharge, so where are the 80% and 95% discharge figures coming from that you folks are talking about?

If you can phrase your reply so that a non-engineer can understand it, I'd appreciate it!

 

[WarpedOne]

You are assuming empty cell would have 0 V, 50% charged cell would have 4,2 * 50% = 2,1 V and fully charged cell would have 4,2V. It is not so, voltage does not drop linearly with discharge. Besides that, Li-Ion batteries must not ever be completely discharged as they would explode or at least be irreversibly damaged - charging would not be possible anymore. That's why all of them have some electronics which monitors the voltage and cuts off discharging when the charge level drops below set limit. Higher that limit and longer (more cycles) the batteries would last.

Same with charging and overcharging. If they are charged too high they can explode or get irreversibly damaged. That same electronics prevents overcharging also.

Here is a typical chart of voltage (white line) of a battery during discharge:
http://www.zeva.com.au/tech/LiFePO4/images/PHET-discharge-2.6ohm.jpg

It is a LiFePo battery so max is 3,4V and not 4,2 but general curve shape is still informative and valid. On the left side you can see that at first voltage quickly drops and than stays almost constant until only about 20% of charge is left where it again starts to drop faster. Over about 70% of charge interval the voltage hardly changed at all - for this LiFePo battery it changed for about 0.1V only - from 3.2V down to 3.1 V. Charge state went from about 95% down to about 25% but voltage only dropped for about 3%.

Li-Ion batteries don't like being charged to the max and they dislike being completely discharged even more. This "disliking" shows through drop in capacity after a few such cycles. You go through 1000 100% discharges and the battery will only hold say 30% - 50% of its initial charge as when it was still new. But if you treat it kindly (do not charge it to the max and never empty it) and you may find that after 1000 such incomplete discharge cycles the battery could still keep over 80% of its initial charge as when it was still new.

Now, with tweaking these parameters - hove high to charge it (up to 90%, 91%, 95%, ...) and how low to let it discharge (down to 30%, 25%, 20%) - you can increase its lifetime without compromising the capacity too much. For example you decrease max range with one charge for about 20% (only allow for 80%DOD) but more than double or triple the available cycle count before the battery dies on you. In this way you increased the total distance given battery will last i.e. you maximized the miles/$ for given battery.

 

[SteveF]

Thanks, WarpedOne, that helped my understanding a lot. Now I see what Martin meant about limiting the discharge to 3.0 Volts.

Much appreciated....

 

[TEG]

To go along with what WarpedOne said, also keep in mind that you need some minimum voltage to operate the vehicle effectively. If you were able to let the batteries drop down to a fraction of a volt you wouldn't likely have enough to get reasonable performance anymore. So that "3V=empty" isn't just because of battery life. If the control electronics expect between 3V & 4V per cell they can do a lot more than if they had to come to grips with cells that sometimes drop to near 0V.

 

[SteveF]

Allrighty then....:biggrin:

 

[TEG]

Tesla Motors unfazed by Big Three's electric offerings - Detroit Auto Show | Concept Cars, New Vehicles, Rumors, Pictures, Videos - MLive.com

Tesla Motors unfazed by Big Three's electric offerings - MLive.com
...Edson also said that Tesla produces it's own lithium-ion batteries in California...

Huh? I think they are Asian sourced commodity cells...

 

[doug]

Tesla Motors unfazed by Big Three's electric offerings - MLive.com
...Edson also said that Tesla produces it's own lithium-ion batteries in California...

Huh? I think they are Asian sourced commodity cells...

The author of the article is likely confused by the nomenclature Zak is using. In the video, Zak is using the word battery to refer to what most might call a battery pack. The cells make up the battery, so in this nomenclature, it's accurate to say "Tesla produces it's own battery in California." But it probably is more clear to say that "Tesla assembles it's own battery pack with proprietary technology in California. This battery pack uses Asian commodity Li-ion cells."

 

[Palpatine]

Huh? I think they are Asian sourced commodity cells...

They are commodity cells from 6 different suppliers. But they are all put together in the Tesla battery design in California just a few miles from Menlo Park.

The R&D on the battery had nothing to do with the cells. It was how to put it all together and cool the system so it will last. The design, assembly and packaging are all MADE IN THE USA.

It is like buying raw iron ore from Australia, but the real steel production is done in China.
Tesla buys the raw lithium cells, but the real work is done in California.

 

[TEG]

The R&D on the battery had nothing to do with the cells.

I would tend to disagree with that. I don't think all possible cells (including some super cheap Chinese made ones) are necessarily "up to snuff". Part of the R&D is at least "qualifying" the quality of a particular type of cells from a particular manufacturer. Given that the ESS ("Battery Pack") cells are not easy to replace, and could possibly cascade failures, I think Tesla expects to have high quality cells with a very low failure rate. Also their ability to stand up to the abusive discharge curves of the Roadster could eliminate some cells from the running.

"Assembled in the USA" would go over better than "Made in the USA" for such things. Or maybe "Made in the USA of foreign made materials".

iMac sticker:

Avoiding 'Made In China' labels not an easy task - Page 2 - Tech Support Forums - TechIMO.com

 

[TEG]

It is like buying raw iron ore from Australia, but the real steel production is done in China.
Tesla buys the raw lithium cells, but the real work is done in California.

I would agree with you if Tesla bought raw lithium from overseas, and made cells from the material, but in this case they are buying completed, working cells that could be used in multiple applications. Just because they bundle them and put them in a big box doesn't mean they made them.

How about something like this?:

Do you say the batteries were Made in the USA, if they made that holder in the USA, and inserted Asian made cells?

I guess you could argue that the Tesla ESS is "so much more" than just 6831 cells in a box, but I still think it is misleading to say that "Tesla made the battery". Tesla made the box, and the control electronics. They assembled/intergrated someone else's batteries into it.

I would even be OK with them saying "Tesla makes its' own battery packs in California...", but Tesla doesn't make the "lithium-ion batteries".

 

[Joseph]

"They are commodity cells from 6 different suppliers."

This means the pack is made up of cells from six different manufacturers?

Wouldn't that be bad because then the each of the six manufacturer's cellswould be slightly different, and thus making it more difficult for the BMS to manage the cells?

 

[Palpatine]

"They are commodity cells from 6 different suppliers."

This means the pack is made up of cells from six different manufacturers?

Wouldn't that be bad because then the each of the six manufacturer's cellswould be slightly different, and thus making it more difficult for the BMS to manage the cells?

They are all based on the same standard. From what I remember being told, any given Tesla battery pack could have lithium cells from more than one supplier.

"Commodity" typically means they are all functionally the same. There is really no difference in the product.

That is why Tesla picked this particular cell for their battery pack. They are mass produced by many suppliers, so Tesla is not held hostage by any one supplier. Also, these are MAJOR companies producing these cells. They do all of the testing on the cells before Tesla receives them. Tesla only takes delivery of cells that meet a certain standard for quality.

 

[graham]

New Li-ion battery maker in the U.S?
John Doerr Mentions Kleiner’s Stealthy Lithium Ion Battery Startup - NYTimes.com

it’s worth pointing out that Doerr mentioned a new “stealth mode” lithium-ion battery maker. He says the unnamed startup “creates stable, durable lithium ion batteries with higher effective storage capacity” that can power electric vehicles “twice as far, and eventually three times as far, to over 100 miles before recharging.”

Doerr also gave a few clues on the company’s origins and future plans — supposedly the startup was found “outside the U.S.,” but is building manufacturing plants in the Midwest and will ship batteries at the end of the year.

 

[JRP3]

that can power electric vehicles “twice as far, and eventually three times as far, to over 100 miles before recharging.”

100 miles per charge? Wow, that's amazing progress, great investment

 

[doug]

New Li-ion battery maker in the U.S?
John Doerr Mentions Kleiner’s Stealthy Lithium Ion Battery Startup - NYTimes.com

it’s worth pointing out that Doerr mentioned a new “stealth mode” lithium-ion battery maker. He says the unnamed startup “creates stable, durable lithium ion batteries with higher effective storage capacity” that can power electric vehicles “twice as far, and eventually three times as far, to over 100 miles before recharging.”

What are they talking about here? Do they mean the company is in "stealth mode" or is Doerr talking about the Fisker Karma's Stealth Mode. I think the author of the article might have been confused. Currently the Karma's battery only range is said to be 50 miles. So improving it's "stealth mode" battery could allow it to go "to over 100 miles before recharging" or engaging the gas generator. That's the only way this makes any sense.

 

[graham]

No, I think the author meant "stealth mode" as in startup company which is working on something secret. I think the "100 miles" comment was a bit of fluff from the author that means nothing.

If Kleiner-Perkins are investing in it there is a good chance that what they have is as good or better than what is on the market today. The "100 miles" would have made more sense if he said something like "100 miles for 250 lbs of batteries" or something that we could make a reasonable comparison with. But I don't think we were the intended audience.

 

[JRP3]

No, I think the author meant "stealth mode" as in startup company which is working on something secret. I think the "100 miles" comment was a bit of fluff from the author that means nothing.

Except the “twice as far, and eventually three times as far, to over 100 miles before recharging.” part was in quotes, so presumably what the guy actually said, not what the writer made up. I do agree it's a startup company, but I really hope their technology is far beyond 100 mile range.