Thanks for the link to Jack's Cobra data..... hadn't seen it yet.
Ron's Camaro is not using A123 pouches, he is using cells made by Haiyin. They are a similar cell style and shape but use a different chemistry and have a lower per-cell capacity. I think they are 5 or 6 Ah compared to the 19.6 Ah for the A123 pouches, so more cells in parallel are needed. I'm running 4 cells in parallel, and I think Ron is running 10.... and he is now thinking about increasing that.
He is trying to squeeze everything he can out of all of his power train because the one and only goal is 1/4 mile runs. I'm interested in a few drag races eventually, but my primary goal is to have some fun with the car and keep it for a long time.
The A123 is nanophosphate lithium ion, the Haiyin is lithium polymer. The lithium polymer typically does provide a higher C rate for discharging, but is also has some safety issues that the A123 cells do not. The A123 cells are still no slouches when it comes to C rate. I have tested to 19C and the cells are rated to 30C discharge. This is quite good compared to the more popular prismatic lithium cells like Thunder Sky and Calb, which can do 3 or 4 C discharge rates..... and very briefly higher. The A123 cells can discharge at the full rate until they are drained.
Ron is also trying to pull something like 4,000 amps off his pack, which needless to say, is a huge amount of current. Most of the electric drag racers expect their sag under full load to be as deep or deeper than that, up to 50% sag or so. So I am not surprised to see Ron's level of voltage sag.
In my cell testing, and at the maximum planned battery pack load of 1,400 amps, my cells only sag about 22%. But I have left headroom in the design so as to not push anything to the breaking point or premature failure.
The interconnection between cells, groups of cells and packs has to be sized properly, and our homegrown battery packs probably leave room for improvement.
One thing I am watching closely is the internal resistance of my packs. Cell set #4 in my second pack is a little higher IR than I'd like to see, especially compared to all of the other cells. So it's possible that this too is part of my range-challenged car, where the voltage might crater earlier than the other cells and in turn, the BMS reports a lower State of Charge.
Quote:
Originally Posted by RE_Farmer
ev99, Just FYI, Jack also did an EV Cobra and did a comparison test between two sets (widths/brands) of tires for the Cobra.
blog.evtv.me/2011/10/lrr-tires-worth-it
Additionally, Ron Adamowicz's "Warp Factor III", an EV "camaro" dragster, uses A123 pouch cell pack. At last month's EVCCON, Ron experienced really deep voltage sag during a 1/4 mi. run (~80V sag on a 200V pack if I recall correctly, don't quote me on that). The speculation is all the individual connections between the pouches may be causing a high "internal pack resistance" resulting in large sag during high current runs.
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