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/...rge-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....

[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."