adaboost
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On cell specifications and battery charging characteristics

Thu Apr 21, 2011 7:38 pm

I know that Li-Ion battery packs of Leaf are from Automotive Energy Supply Corp (AESC). But there is little information on cell and battery specifications. Does anyone have an idea about it?

What are nominal voltage (V) and capacity (Ah) of a cell?

How many cells are in series and how many series of cells are in parallel for a battery pack?

Whether a constant current and constant voltage (CCCV) charging scheme is applied to the battery pack? Then what is the SOC around the CC and CV transition point, e.g., 80%?

Whether typical charging data are available, i.e., dynamics of voltage, current and SOC along the time axis during a full charge cycle (level 1 and 2)?

Many thanks.

WiseGuy
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Re: On cell specifications and battery charging characterist

Wed May 04, 2011 11:59 pm

You can find the detailed cell specifications at http://www.eco-aesc-lb.com/en/product.html

Leaf uses the 33.1Ah cell, two in parallel, and 96 in series (192 total). Nominal pack voltage is then 360V, and total installed capacity is 192*33.1*3.8/1000 = 24.15kWh. This should end all speculation wether the 24kWh pack size is useable or installed - it's installed.

The cells are kept below 4.2V for safety and life reasons. During charging, the cell will reach 4.2V before it's fully charged. Then you have to do constant voltage charging to push the last bit of energy into the cell without exeeding 4.2V. However, EV batteries are not used from 100%SoC (i.e. 4.2V). Ususally the cells are charged to 4.1V max. This is to allow some charge acceptance ability for regen when the battery is "full", and also prolongs the life. If charging happens at a slow rate, you may be able to charge at CC and reach 4.1V without needing to go into CV mode. It's all a function of the rate of charge, and internal resistance of the cells. That's why fast charging don't get you to 100% charged.

Herm
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Re: On cell specifications and battery charging characterist

Thu May 05, 2011 1:54 am

WiseGuy wrote:You can find the detailed cell specifications at http://www.eco-aesc-lb.com/en/product.html

Leaf uses the 33.1Ah cell, two in parallel, and 96 in series (192 total). Nominal pack voltage is then 360V, and total installed capacity is 192*33.1*3.8/1000 = 24.15kWh. This should end all speculation wether the 24kWh pack size is useable or installed - it's installed.



Not so fast WiseGuy, note the weasel words: "Rated capacity (0.3C) 33.1 Ah".. that means they may have taken out 15-20% out of the actual capacity for life prolonging purposes.. 0.3C means a discharge rate of 3.3 hours at a power output of 7.2kw.. from the Tesla charts that corresponds to a speed of 43mph with no AC or Heat running...

The actual modules use a 2S2P configuration, and then all 48 modules are in series.. each module has 2 cells in parallel, connected to 2 more parallel cells in series.. each module has 4 cells, a capacity of 66.2Ah and an average voltage of 7.6V.. BTW, two of these modules would make a nifty starting battery for a conventional ICE car, if you could keep them charged properly.

Herm
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Re: On cell specifications and battery charging characterist

Thu May 05, 2011 2:26 am

I just checked again, AESC is now actually providing discharge data for their cells, this is new, here:

http://www.eco-aesc-lb.com/en/products/ ... rmance_BEV

Look at the high energy (means high capacity) cells for BEVs.

If you integrate the area under the blue line (for 1/3C discharge) you will end up with the actual capacity of the cell at 1/3C.. normal industry practice is rating cell at 1C but lets not nitpick.. so someone should get busy, blow up that image and start counting squares that fit under the blue line. I got a headache and my eyes got blurry :)

BTW, according to the specs the cells alone weigh 153kg, or 337lbs... I believe the Nissan pack is supposed to weigh 600lbs so these MAY BE the specs for the next generation of cells to be used in the Leaf (sometime in the next 2 years).. the nickel-manganese spinel lithium-ion chemistry they have promised in the next generation Leaf, supposedly twice the capacity for the same weight. Either that or the battery case plus BMS weighs 260lbs, seems pretty heavy for a steel case.

WiseGuy
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Re: On cell specifications and battery charging characterist

Thu May 05, 2011 2:46 am

Normally the cut-off voltage is 3V, not 2.5V as in the graphs. You really don't get any more energy out from 3V to 2.5V. Note how the capacity slightly increases between 1C and 1/3C rate. The rated capacity of 33.1Ah is very close to the maximum Ah you can extract from the cell, so total installed capacity in the Leaf is definitely 24kWh, nothing more.

P.S. You'll find that by using the nominal voltage of 3.8V and multiplying that with the rated capacity is very close to the exact Wh value you'll get from integrating the graph.

AndyH
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Location: San Antonio

Re: On cell specifications and battery charging characterist

Thu May 05, 2011 11:29 am

WiseGuy wrote:Normally the cut-off voltage is 3V, not 2.5V as in the graphs. You really don't get any more energy out from 3V to 2.5V. Note how the capacity slightly increases between 1C and 1/3C rate. The rated capacity of 33.1Ah is very close to the maximum Ah you can extract from the cell, so total installed capacity in the Leaf is definitely 24kWh, nothing more.

P.S. You'll find that by using the nominal voltage of 3.8V and multiplying that with the rated capacity is very close to the exact Wh value you'll get from integrating the graph.

Not quite.

The charts show the full capacity range within the voltage limits used for these tests. The long-life voltage range - and thus capacity - is narrower.

When operated within the NARROWER CONSUMER RANGE - the limits selected for long battery life - we have more than 24kWh of installed capacity. The FULL or ULTIMATE capacity of the cells and thus the battery is larger.

A real-world example - used because I've all the details. I'm running receiving inspection on a batch of 12Ah LiFePO4 cells. Within the voltage limits that provide 3000 full charge/discharge cycles - 3.6V to 2.5V - the cells provide 11.4Ah. Capacity exceeds the rated 12Ah when cycled between the maximum 4.2V - 1.5V range.

The consumer capacity for the battery in the Leaf, when it is new, is a bit more than 24kWh.

AndyH
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Re: On cell specifications and battery charging characterist

Thu May 05, 2011 12:06 pm

Herm wrote:I just checked again, AESC is now actually providing discharge data for their cells, this is new, here:

http://www.eco-aesc-lb.com/en/products/ ... rmance_BEV

Look at the high energy (means high capacity) cells for BEVs.

If you integrate the area under the blue line (for 1/3C discharge) you will end up with the actual capacity of the cell at 1/3C.. normal industry practice is rating cell at 1C but lets not nitpick.. so someone should get busy, blow up that image and start counting squares that fit under the blue line. I got a headache and my eyes got blurry :)

BTW, according to the specs the cells alone weigh 153kg, or 337lbs... I believe the Nissan pack is supposed to weigh 600lbs so these MAY BE the specs for the next generation of cells to be used in the Leaf (sometime in the next 2 years).. the nickel-manganese spinel lithium-ion chemistry they have promised in the next generation Leaf, supposedly twice the capacity for the same weight. Either that or the battery case plus BMS weighs 260lbs, seems pretty heavy for a steel case.

Thanks for this heads-up Herm!

To be fair, the charts do show the curves for 1/3C, 1C, 2C, and 3C, so the nits look safe this time. :D

As for chemistry - I've high confidence that these are the current-tech cells. AESC makes clear they're LiMn variant - and their new public data shows LiMn2O4 with LiNiO2.
http://www.eco-aesc-lb.com/en/product.html

Here's a side by side with a known LiMn2O4 proven in the field to have very good performance:

AESC:
Image

e-Moli (2.9Ah - the top line is 1C)
Image
http://www.molicel.com/ca/pdf/IMR26700.pdf

User avatar
evnow
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Re: On cell specifications and battery charging characterist

Thu May 05, 2011 12:22 pm

The next gen battery is NMC, not just NM. So, yes, the specs are for current gen battery (which has some Nickle in it already).
1st Leaf : 2/28/2011 to 5/6/2013
2nd Leaf : 5/4/2013 to 3/21/2017
Volt : 3/25/2017 to ?

WiseGuy
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Re: On cell specifications and battery charging characterist

Thu May 05, 2011 12:30 pm

Sigh! AndyH: to get the true maximum installed capacity of a battery pack (in Wh), you multiply the nominal cell voltage with the max cell capacity and then the total number of cells (regardless if they are in series or parallel). For the Leaf, this gives us 24kWh.

The fact that we only use a limited voltage window to prolong life and enable power capability, means that the useable energy is LESS than the 24kWh install capacity.

I suggest you do some maths & "design" your own pack using the cell data of the Leaf (or any other cell). You'll quickly realise that 24kWh is the true installed capacity (unless there's an error on the published discharge curve ...)

AndyH
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Location: San Antonio

Re: On cell specifications and battery charging characterist

Thu May 05, 2011 1:20 pm

WiseGuy wrote:Sigh! AndyH: to get the true maximum installed capacity of a battery pack (in Wh), you multiply the nominal cell voltage with the max cell capacity and then the total number of cells (regardless if they are in series or parallel). For the Leaf, this gives us 24kWh.

The fact that we only use a limited voltage window to prolong life and enable power capability, means that the useable energy is LESS than the 24kWh install capacity.

I suggest you do some maths & "design" your own pack using the cell data of the Leaf (or any other cell). You'll quickly realise that 24kWh is the true installed capacity (unless there's an error on the published discharge curve ...)

Thank you for your advise WiseGuy.

The fact is I DO work with lithium daily, build packs, and perform road and bench testing. I do understand the math. I'm also capable of reading the display from the equipment I use to run charge and discharge tests. ;)

If one has a 1 litre container but chooses to only use 80% of the capacity, then I completely agree with your assertion that they would be only drinking 800ml of water.

However...if one starts with a 1.25l container and uses 80% of the capacity, the thirsty lab tech can drink 1l of water.

The info published by AESC show the derated, lower capacity, long-life numbers - not the ultimate capacity of the cells. We know that because we have the allowable voltage range from the car's management system - and the numbers used in the AESC graphs match the voltage limits used in the car. We also know that trying to pull 100% capacity from a lithium cell guarantees a very, very short cycle life - not good for the folks holding the warranty.

In addition, Nissan reports 24kWh usable with a total capacity that is proprietary. AESC reports 24kWh. End-users that have run the battery flat use more than 26kWh to recharge. Ingineer has pulled info on cell capacity from the car using the Nissan scan tool - this also confirms an end-user capacity in excess of 24kWh.

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