Can anything be deduced from battery pack mass v. capacity?

[donald]

I have seen some web pages reporting the [2012] Leaf battery pack as weighing 200 kg (without controllers).

The Volt battery pack also appears to weigh in around 200 kg, but is 16 kWh instead of 24 kWh.

But GM make a song and dance out of the fact that the Volt only uses 65% of its actual capacity, to protect the Li cells.

OK, so if those battery packs were otherwise equal, then does that mean both have actual all-in capacities around the 12 kWh per 100 kg, but that the Leaf is using 100% of its real capacity?

I realise there is a lot more detail on the housings and other parts to the battery pack system, and this may not be comparing like-with-like (e.g., do these other figures include battery controllers or not??), but other EV battery packs seem to come in at lower specific capacities than the Leaf's apparent 12 kWh / 100kg all-in pack mass. Are these also running like the Volt (8 kWh / 100 kg), with larger overhead margins at the top and bottom of the SOC? e.g. BMW i3 is 230 kg for 22 kWh (9.5 kWh / 100 kg), and Renault Fluence at 22 kWh in 250 kg (8.8 kWh /100 kg)

 

[TimLee]

OK, so if those battery packs were otherwise equal, then does that mean both have actual all-in capacities around the 12 kWh per 100 kg, but that the Leaf is using 100% of its real capacity?

The LEAF only allows use of ~ 21.5 kWh of the 24 kWh, a bit < 90%, as discussed extensively in numerous other threads.

 

[donald]

Just looking at specs and figures...

... could someone please explain to me what I am mis-understanding here, please?

So the 'nominal' full charge for a Li cell is 4.2V. A 66Ah pack total, in which each of 48 modules is 4 cells, two in series and two in parallel (which is, as I understand, the Leaf's spec/configuration) would mean there are 48 x 2 x 4.2V = 403V. Total 100% capacity = 26.6 kWh.

The reason I ask is to compare it with Renault's figures for their '22kWh' battery pack in the Zoe and Fluence. These are also 4 cell x 48 module arrangements, quoted as 22 kWh. Renault's specs quote this as a 65Ah battery pack with a nominal voltage of 398V ( = 26kWh?). I cannot determine if they are the same modules and or cells as the Leaf.

Leaf pack quoted mass is 200kg w/o controller (300kg with), Fluence/Zoe is 250kg (w or w/o controllers, I do not know).

Do you think the cells/modules are different? What accounts for the mass differences (I can guess at that, but seems more than 'just packaging'). Why is a nominal 100% cell voltage quoted lower than 4.2V?

Ultimately, the thing I am particularly interested in is understanding how close to 'real' min and max you are allowed to charge/discharge these batteries to. If the SOC-meter says 'X', what is that X in 'real' SOC? I just can't tell from these figures whether they are simply re-quoted inaccuracies for what are essentially the same battery packs in different form-factors, or whether they are, in fact, different.

 

[surfingslovak]

Hi Donald, please read the following thread as a primer for most of your questions. The cells Renault is using in their cars are not from AESC, although the basic chemistry might be comparable. Also, as already noted on the trickle charging thread, LMO cells reach their energy maximum at a lower potential, typically at 4.17V and are commonly not charged to 4.20V, which is commonly associated with cobalt cells. The LEAF charges the cells to 4.10V and calls it "100%" even though it's between 94 to 95% of maximum SOC. Most of these questions have been discussed at great length here and elsewhere.

http://www.mynissanleaf.com/viewtopic.php?f=27&t=7151&p=158058#p157929" onclick="window.open(this.href);return false;

Here are my best guesses for cell energy density (all in Wh/kg): 100 for SCiB (Honda), 107 for GS Yuasa (Mitsubishi), 140 for AESC (Nissan), 150 for LG Chem (GM), 160 for SB LiMotive (BMW), 240 for Panasonic (Tesla). Might warrant more research though. I didn't realize this before, but both Tesla and BMW likely benefit from using aluminum instead of steel for the battery enclosure. EDIT: Chevy Volt is reportedly going to 176 Wh/kg in 2014 MY.

 

[donald]

Thanks for the link, and I'll study it in detail.

Though primarily I was posing the thread to see if there was anything further that might be deduced or inferred from data available from other manufacturers' batteries quoted figures. Obviously, not entirely scientific and smothered in marketing stuff before we ever get to see it, but, still, some tid-bits of data into that 'press/marketing' process leak out and surface, and we have to deduce what is or is not 'real', mis-leading or BS.

For example, a press item from 3 years ago said that the Renault batteries were only to allow access to 70% of the battery capacity (which'd make it 22kWh of a 30kWh battery pack, if that were so - which'd account for the quoted 250kg Renault pack versus 200kg/24kWh Leaf pack?). So was that achieved/wrong/a future intent (but not realised, after engineering work said they needed to use more)/etc..

I think there is just about enough to discuss, albeit not get a definite answer out of it. Some guess-work will be required on these points, until the manufacturers can be relied on to give full technical details (which is, obviously, not in their interests to do so while the vehicles are still in competition with each other, and/or until they actually start trusting their potential customers as intelligent folks who will use/consider the data appropriately).

 

[surfingslovak]

Ah, I see. While that's a laudable goal, as experience has shown, press releases are not a very reliable source of technical data. More often than not, manufacturers don't release anything meaningful. The LEAF is a good example of that. When asked about the usable capacity of the vehicles, tech support representatives have responded to several owners that this was "proprietary" information. Obviously, it's not mentioned in the press kit either.

Phil (Ingineer) and several early adopters have worked tirelessly to help improve our understanding of the vehicles, and we can be reasonably certain that the usable capacity is about 92% of rated capacity in the LEAF. The observed actual capacity is somewhat lower still, most likely due to battery losses. GM has been pretty forthright with this type of information and so has been BMW. It's 10.8 kWh out of 16.5 kWh for the current generation of the Chevy Volt and 18.8 kWh out of 21.6 kWh for the BMW i3.

No manufacturer seems to be very eager to disclose energy density figures, cycle life, and most importantly, battery pack cost. All these have to be guessed and estimated based on actual performance in the field. Perhaps we will see some 3rd party labs or professional reviewers get into this level of detail, but I personally doubt that the average consumer will appreciate it. It's so much easier to do a quick drive report, and echo the data from manufacturer-supplied press kit, without ever questioning anything. Looking at the total battery pack weight, and dividing it by its rated capacity is a useful metric. Plus it's really easy to do.

For battery pack price, I would assume that the pack constitutes about 1/3 of the price of a base model trim. In the case of the LEAF, this is about $9,600. I'm not sure what it might be for the Fluence or the ZOE. Aside from that, I think most BEV manufacturers will cycle the battery deeply. At least the BEVs I looked at in the past two year all had about the same duty cycle and the usable battery capacity (including battery losses) was about 85% of the rated capacity.