Tesla Model S 18650 Cell Test Data

[okashira]

For those who are not aware, I have acquired (http://endless-sphere.com/forums/viewtopic.php?f=9&t=67661) a full Model S battery pack, that's basically in new condition. I have disassembled the pack and removed individual cells.

For your pleasure, I present the first individual cell test results seen publicly on the internet of Tesla Model S cells, using a 4-wire programmable DC load; cell was charged to 4.20V with a 50mA cut and let to rest for 45 minutes.

I will also update this thread soon with some in depth pics of the inner workings of the pack.

I included a common e-bike cell, a new(from fast-tech) Panasonic NCR18650PF (which is rated for 10A cont.) as a comparison.

(My annoying DC load stopped the 10A run at 2.9V for some reason, so I added some extrapolation [dotted line] based on prior tests to 2.5V.)

During the 10A run, I reduced current to 3A for 83 seconds to allow the cell to cool a bit. I did this to point out a couple of interesting, and quite useful performance characteristics of these cells:
-DCIR falls significantly at higher temperatures. This helps to self limit thermal build up of the... as the cell heats up, it tends to actually produce less heat!
-Panasonic NCA cells exhibit significant "surface discharge," that is, they perform well in bursts, where DCIR is lowered for a short period of time after a rest.

Here is a zoom of both effects:

-These cells would run great if bursted to 15A for ~5 seconds, we're talking nearly 50W from this tiny, very high capacity cell.
-Because they are so dense, they have ~350% of the energy capacity per volume when compared to LiFe cells, yet they can still compete fairly well with LiFe cells on a power/volume basis.

I would rate these cells at up to 10A continuous, for up to 12 minutes at a time from full charge, then reduce current to prevent overheating.

 

[riba2233]

Great data, and great cells! Thanks! 8)

Can you do some comparison with BE cell?

 

[liveforphysics]

Awesome to see this data public. Thank you for taking the time.

 

[savaoaknyc]

Do you have any temperature numbers at 10a discharge?I guess, the cell reaches around 60 degree ceslius.Thanks for the info.

 

[okashira]

So I just finished my first torture test of one of these cells (much more to come) by accident.

I set up another 3A discharge on the cell I had just run at 10A to see how it held up.
It actually picked up even more capacity! However, in addition, my DC load decided this time to not shut off. Not only did it overdischarge below 2.5V, it kept going to 0V, and kept pulling current at 0V for three more hours!!:



The voltage reading is a separate 4-wire voltage sense, so the cell voltage was really reading 0.020V or less. Now my DC load isn't good at measuring very low current, which it was pulling once it hit 0V, so it reality, it had pulled a little more then shown... maybe 3.35 - 3.4 Ah.
Surely this cell is now trash? That's what they say about cells over-discharged this badly - dispose of them safely. Good thing it wasn't a HK Lipo in my living room...

Finally, after 3 hours it had shut off, and 10 hours after that, and the voltage of the cell had recovered on it's own to 1.8V
So, I decided to give it about 20mA for a while, bumped it up to 50mA, stopped a few times, and eventually charged all the way to 4.2V proper. The cell seemed ok. Held a charge, no self discharge, no heat, nothing:

Now, surely the cell was damaged in some way... maybe the capacity was severely reduced, or DCIR would go way up...

Holy crap, I was right about the durability of the cells!! It lost only ~1% capacity, and after going to 2.55V, it recovered to 3.32V, telling me it had a little more in it, too.
That sure does make me feel a little better about taking them all the way to 2.5V under normal usage....

 

[flathill]

Wow I wonder if these cells have "Zero Volt" technology. I would be interested to see more data on this. Maybe store a couple cells for a few weeks at zero volts and then retest. Thanks

Quallion makes 18650's cells here in california for military/space apps....I'm sure SpaceX (Tesla) knows them

Quallion’s patented Zero-Volt technology enables a lithium
ion battery to be completely discharged to an inert state, stored
in this condition for an extended period, and then recharged
without any permanent damage to the battery or reduction in capacity
or performance. Applied to electric vehicles, this technology
can enhance the safety of first responders and auto technicians
who risk electrocution from contact with an energized high
voltage system, and it can reduce warranty costs of replacing
batteries that are destroyed as a safety precaution. Zero-Volt
also enables long term storage and shipping of vehicles with
batteries in an inert state, eliminating the risk of fire or explosion
that has caused fatalities and property damage when lithium ion
batteries have combusted in cargo planes during shipment or in
storage facilities following crashes.

Zero-Volt technology relies on manipulating individual electrode
potentials within a lithium ion cell to allow deep discharge
without inflicting damage to the cell. Quallion has identified
three key potentials affecting the Zero-Volt performance of
lithium ion batteries. First, the Zero Crossing Potential (ZCP) is
the potential of the negative electrode when the battery voltage
is zero. Second, the Substrate Dissolution Potential (SDP)
is the potential at which the negative electrode substrate begins
to corrode. Finally, the Film Dissolution Potential (FDP) is the
potential at which the SEI begins to decompose. The crucial design
parameter is to configure the negative electrode potential to
reach the ZCP before reaching either the SDP or the FDP at the
end of discharge. This design prevents damage to the negative
electrode which would result in permanent capacity loss. Figure
1 shows a schematic of the voltage profile during deep discharge
of Quallion’s Zero-Volt cells.

Electric vehicles have also suffered from batteries being
destroyed by deep discharges when they remain unplugged for
extended periods of time. Recently some vocal Tesla roadster
owners complained of batteries that “bricked” after weeks of
storage due to the parasitic drain of the vehicle’s electronics,
requiring a battery replacement costing $40,000 and not covered
by the vehicle’s warranty. Whether attributed to an inattentive
owner, a faulty charge cord or some other cause, this is a very
expensive problem which Zero-Volt technology can eliminate.

This was in 2012. Maybe Tesla got the message.
http://www.quallion.com/images-pdf/BatteryPowerZeroVolt52012.pdf

 

[okashira]

Interesting. in my "experiment", I did a pretty heavy load of 3A (which it pulled all the way down to 0.25V) which is only 0.08 ohm (~1 ohm at normal voltages) so the cell was pretty stressed (probably heated up quite a bit, I was sleeping at the time)
If I did 72 ohm or much lower current down to 0V, perhaps it wouldn't hurt it at all. I'll try it later...

 

[AngusR]

Interesting accidental experiment. More torture tests to come sounds promising.

 

[silentflight]

Wow, this is great info- thanks for posting. I've been waiting to see these discharge curves for literally years. The cells are even better than I had thought.

I wonder what energy and power density will be for the cells produced at the gigafactory...

 

[llavalle]

Great info.

Were you able to record temperature of the cell during testing?

1-Ambient
2-Starting temp of cell
3-End temp of cell when running @ 10amp (maybe time VS temp would be good)

Thanks!

 

[Nobuo]

Wow I wonder if these cells have "Zero Volt" technology. I would be interested to see more data on this. Maybe store a couple cells for a few weeks at zero volts and then retest. Thanks

Quallion makes 18650's cells here in california for military/space apps....I'm sure SpaceX (Tesla) knows them

http://www.quallion.com/images-pdf/BatteryPowerZeroVolt52012.pdf

Tesla choose this cells and that would be a very good reason for avoiding being destroyed a thousands cells packs if any user decide/forget to charge during some weeks at empty capacity of "3V"
I don't know if panasonic Pf are "Zero Volt", but it would be not the first time the cells are credited to be maintained on storage time at 0V and then revive with no major issues.
http://endless-sphere.com/forums/viewtopic.php?p=969572&display_history=true

 

[Doctorbass]

These are really interesting test.

I am interested to know the nominal voltage at various current draw..

this would gibe the Wh output for various current discharge.

and as well the Wh in heat loss...

From what i see in your graph, If you are sure that the measured voltage is the voltage AT THE CELL TAB without any V loss i contact resistance etc (a true 4 wires kelvin connection) the nominal voltage look like to be 3.6V at 1A and 3.25V at 10A witch is about 35 miliohm.

(V (nom) calculated with median Ah and not meduian Wh)

Doc

 

[okashira]

These are really interesting test.

I am interested to know the nominal voltage at various current draw..

this would gibe the Wh output for various current discharge.

and as well the Wh in heat loss...

From what i see in your graph, If you are sure that the measured voltage is the voltage AT THE CELL TAB without any V loss i contact resistance etc (a true 4 wires kelvin connection) the nominal voltage look like to be 3.6V at 1A and 3.25V at 10A witch is about 35 miliohm.

(V (nom) calculated with median Ah and not meduian Wh)

Doc

I can repost the data with watt hour on the x-axis to make your calculation easier.

 

[flathill]

Wow I wonder if these cells have "Zero Volt" technology. I would be interested to see more data on this. Maybe store a couple cells for a few weeks at zero volts and then retest. Thanks

Quallion makes 18650's cells here in california for military/space apps....I'm sure SpaceX (Tesla) knows them

http://www.quallion.com/images-pdf/BatteryPowerZeroVolt52012.pdf

Tesla choose this cells and that would be a very good reason for avoiding being destroyed a thousands cells packs if any user decide/forget to charge during some weeks at empty capacity of "3V"
I don't know if panasonic Pf are "Zero Volt", but it would be not the first time the cells are credited to be maintained on storage time at 0V and then revive with no major issues.
http://endless-sphere.com/forums/viewtopic.php?p=969572&display_history=true

Very interesting. That does seem to support the theory that Panasonic has either licensed or made their own "Zero Volt" tech but until we know for sure I would not recommend discharging to 0V on purpose until we know for sure. Not being able to brick a 40 thousand dollar battery pack would be very useful to Tesla. They have to know about Quallion. Maybe this is how they "customized" the cell for their use. Negative terminal scored vent and zero volt tech = automotive grade 18650? The PF cell is used in the Rav4EV pack. It appears the BE cell is used in the Model S. Both PF and BE have been discharged to 0 Volts without being damaged (but we don't know for sure)

 

[riba2233]

Rav4 used PD, not PF.

PF and BE has both been discharged to 0 without damage, we do know

 

[flathill]

it appears the cell has not been damaged but we dont know for sure. it is common for a cell to recover but it is still recommended the cell be scrapped. you cant tell just by looking at capacity.

 

[liveforphysics]

Most every production EV cell can go to 0V for some amount of time and be recharged and function. The copper foils and some other layers get dissolved as they sit at 0v, but it's a slow process that often takes days to be very noticeable.

Titanate cells have always been able to go to 0v discharge and be fine, as the anode potential holds ~1.7v while the cell terminals read 0v. Over the years occasionally someone claims to have re-discovered that as some feature they made rather than an intrinsic property of the anode.

 

[riba2233]

it appears the cell has not been damaged but we dont know for sure. it is common for a cell to recover but it is still recommended the cell be scrapped. you cant tell just by looking at capacity.

Well, it's both capacity and internal resistance. It is a little harder to check cycle life thought :lol:

 

[flathill]

Most every production EV cell can go to 0V for some amount of time and be recharged and function. The copper foils and some other layers get dissolved as they sit at 0v, but it's a slow process that often takes days to be very noticeable.

Titanate cells have always been able to go to 0v discharge and be fine, as the anode potential holds ~1.7v while the cell terminals read 0v. Over the years occasionally someone claims to have re-discovered that as some feature they made rather than an intrinsic property of the anode.

The Quallion "Zero Volt" cells were shown to suffer almost no loss in capacity after 2 years in storage at 37C with the cell shorted with a 70 OHM resistor.

Not all their cells of the same chemistry are "zero volt" enabled. Enabling just means "configure the negative electrode potential to reach the ZCP before reaching either the SDP or the FDP at the end of discharge. This design prevents damage to the negative electrode which would result in permanent capacity loss"

I've had many cells that show almost full capacity after being shorted with a large resistor and being left there for a couple days, but I doubt they could survive in this state for two years. Are you saying any cell that shows no capacity degradation after being brought down to zero volts and kept there for a few days can safely be stored at zero volts?

See http://www.quallion.com/images-pdf/BatteryPowerZeroVolt52012.pdf

"Quallion’s Zero-Volt design is not limited to
a particular chemistry or set of materials"

Zero-Volt also enables long term storage and shipping of vehicles with
batteries in an inert state, eliminating the risk of fire or explosion
that has caused fatalities and property damage when lithium ion
batteries have combusted in cargo planes during shipment or in
storage facilities following crashes.

 

[liveforphysics]

Nope, I'm just saying it's a slow decay for modern EV cells, so most tend to have minimal damage from short periods.

Just suggesting the Quallion thing may be a Titanate anode, which naturally provides the benefits they mentioned.

 

[flathill]

The "Zero Volt" enabled cells from Quallion have a LNCAO cathode with a graphite anode

93% energy retention after 9500 cycles (40% DOD)

.02V cell voltage difference after 11,000 pack (8S) cycles with no balancing circuit

See file:
View attachment 10_Elec_Therm_Char_QL015KA_Pack_HNakahara.pdf


Telsa saw the light years ago with NCA but all the old guard want to use NMC because of the inherent calendar fade. We at the point now that some cells are "too good"

https://archive.org/details/PlannedObsolescenceDocumentary

"Did you know that the lifetime of light bulbs once used to last for more than 2500 hours and was reduced on purpose to just 1000 hours? Did you know that nylon stockings once used to be that stable that you could even use them as tow rope for cars and its quality was reduced just to make sure that you will soon need a new one? Did you know that you might have a tiny little chip inside your printer that was just placed there so that your device will break after a predefined number of printed pages thereby assuring that you buy a new one? Did you know that Apple originally did not intend to offer any battery exchange service for their iPods/iPhones/iPads just to enable you to continuously contribute to the growth of this corporation?

This strategy was maybe first thought through already in the 19th century and later on for example motivated by Bernhard London in 1932 in his paper Ending the Depression Through Planned Obsolescence. The intentional design and manufacturing of products with a limited lifespan to assure repeated purchases is denoted as planned/programmed obsolescence and we are all or at least most of us upright and thoroughly participating in this doubtful endeavor. Or did you not recently think about buying a new mobile phone / computer / car / clothes / because your old one unexpectedly died or just because of this very cool new feature that you oh so badly need?"

 

[okashira]

Yup that is why tesla allows 96% DOD on the Model S. They have even stated they got to the point where the battery would last "too long" thus they only do active cooling above 55°C and allow a full 100% charge and all the way down to ~4%.
The cells will do at least 4000 cycles to 70% capacity with a 70% DOD http://repository.osakafu-u.ac.jp/ds...2014900048.pdf
55C long term would rape a leaf/volt/LifePO4 pack ...

I will do the resistor test soon and store some at 0V for a while, sorry been caught up with other stuff...

 

[flathill]

http://repository.osakafu-u.ac.jp/dspace/bitstream/10466/14150/1/2014900048.pdf

Yeah I will be looking for a granny or grandpa owned P85+ in a couple of years when the basic warranty expires after 4 years and the price drops. The average driver never uses even half the 85kw pack in a day so the battery will still be cherry. Any P85+ is going to be a collector, but the P85+ with autopilot is the rarest of them all

 

[okashira]

Ok... got the tester working with English software.
I can program up to 10 steps, so I can simulate supercharging pretty easy and accurately to see how the cells handle it.
The tester works pretty well, but I have a bunch of critiques.... It's nice and accurate, was only off by 0.001V from my Brymen BM867s, which is very accurate. Each channel can be calibrated individually for current and voltage as well.

Here is what the heat exchanger tube in a module looks like:

I have not measured exactly, but the cells are less then 1mm apart in parallel banks. For series separation, it's like 1-1.5 mm.
Even across the heat exchanger tube, the cells lines are only spaced by only ~4mm. The heat exchanger tube is also 1.) wrapped in Kapton 2.) Wrapped in some unknown secondary tape 3.) Finally, wrapped in some grey thermal material.
The contact between the cells and the tube is quite poor - make no mistake, it is not used for "active cooling," due to any sort of load. It's only for environmental temperature control. Heating more often then cooling.
The width of the "ID" of the tube is only lke 1mm or less. The cooling flow rate is surely quite low.



Also started the 0V test. Used 25 Ohm on two cells and we'll see if they were damaged after several days storage at 0V.
They were at 0.65V last night.

 

[okashira]

0V test now at

0.0792V after ~31 hours
I should have set the multimeter to log to see just how much capacity comes from the cells all the way down to 0V. :-/