Hi all . . .
If all goes well, I will be the owner of a used ’12 ES in October. The current
lessee (friend and fellow member of the local EV club here in Tucson) is handing
it back to the dealer then following his 3 year rental commitment. I actually got
to borrow this very vehicle in the fall of 2013 for a couple of weeks and became
sold on owning an i-Miev then. The price is right for the purchase of a clean used
i-MiEVs these days (affordable to even poor ol’ me) and I know this one has been
well taken care of.
So, in anticipation of all this, I’ve been catching up with my reading here as of
late and came across this thread on replacement batteries. My guess is that I’m
not going to be a candidate for such a thing for many years to come, but the
technical aspects of what is possible along these lines in the future intrigues me.
First, present day wet blanket time . . .
I’m simply not seeing how thirty 18650 cylindrical cells can fit inside a box the
size of a LEV50 (171mm X 115mm X 44mm.) But I’m also assuming that the most
desirable thing would a physical drop-replacement for a LEV50 with the same 3.7V
and an increased amperage, so I’m starting with that criteria in mind (worrying later,
of course, on how the car’s computer might electronically hiccup with a battery
module output greater than 50Ah.)
Undaunted, I spent today moving appropriately-sized shapes around in Adobe
Illustrator to see what really is possible. The best I could come up that allowed for
logical cell placement, breathing space between the cells themselves and for everything
else you would want inside each of those 88 modules (BMS board, provisions for
robust mounting of the terminals on top, allowances for wall thickness of the box
itself, etc.) was sixteen 18650 cells.
The current Tesla cell is, I think, the Panasonic NCR18650B, with the proper
3.7V for series wiring (good news there) and rated at 3.4Ah (or 3400mAh) each . . . https://www.fasttech.com/product/1141100-panasonic-ncr18650b-rechargeable-3400mah-3-7v
But sixteen of these in series would yield only 54.4Ah, which isn’t really enough
to make the venture worthwhile today. And, while I’m sure a more advantageous
pricing structure could occur when purchasing thousands in bulk instead of
a handful from a retailer like I’ve linked to, $3 per cell might always be wishful
Now, for the potential future good news . . .
It’s been widely reported that the cell Telsa is destined to make in their upcoming
gigafactory is going to be physically larger: a 20700 (20mm diameter X 70mm
length) form factor . . . http://gas2.org/2015/03/18/lighter-batteries-may-prove-tipping-point-electric-vehicles/
Assuming the chemistry is the same and allowing for a modest increase in
performance of the basic nickel/cobalt/aluminum oxide formula by production
time and in the years that immediately follow, it’s not unreasonable to assume
that this larger cell would be capable of an output between 4 to 5Ah. The physically
larger size of the cylinders is also good news in regards to fitting inside a LEV50-sized
box, as 20700s take up extra space in there over the 18650s quite nicely and it’s still
far from being cramped . . . http://i1089.photobucket.com/albums/i358/BeninTucson/LEV50_W.jpg
So, sixteen cells in the right sized box could yield between 64 to 80Ah per module.
16 X 88 modules = 1408 cells. That’s between 5232Ah (@ 4Ah per cell) and 7040Ah
(@5Ah per cell) for the whole pack. Multiply by 3.7V and that’s (respectively)
between about a 19.36kWh and 26kWh pack (someone please check my math!)
Assuming buying in lowish volume wholesale quantities is possible (by the
crateload, as opposed to by the shipping container-full,) around $5600 ($4 ea.)
to $7000 ($5 ea.) for all those cells to make what could be a 26kWh pack looks
pretty good . . . especially if a drop-in prismatic replacement for the LEV50
isn’t being made a few years from now.
We’ll also be waiting for news regarding things like lithium sulfur and (my favorite)
solid electrolyte technology to get here while all of the above is happening.
But that’s another battery dream post for another time.
Thoughts? . . .