Quote:
Originally Posted by othello
Lutach i can understand your feeling that considering Teslas car price level you want the best cell powering this car. C rate of the cells used is only one criteria when it comes to define "best" cell. Of course a higher C rate allows higher power levels and an efficient cool running battery pack also translating in a higher range. On the other side Teslas rather small motor can't cope for long with high power levels. The motor if i recall it right is only air cooled and will simply overheat when pushed to hard (one of the reasons why they limited top speed to around 120mph which requires around 80KW according to their mathematical model). Pushing a street legal car to 60mph in around 4 seconds is not that bad after all ;-)
When reading articles about their battery pack and cell choice i got the impression that many other criterias (beside C rate) had to be considered: safety, cycle life, energy density are among the important ones. Don't forget that their cell tests, R&D of battery pack, passing safety tests and so on dates already a few years back and implied high costs. You can't simply switch to another cell now that the car is beeing sold and every component is optimised around a certain cell design.
Facts gathered from Teslas articles about their battery pack:
. Cells have a capacity of 2.2Ah
. 99 cells in series: around 366.3V (3.7x99)
. 69 bricks in paralel: around 151.8Ah (69x2.2)
. 151.8 x 366.3 = 55.6KwH on board (roughly comparable to 8 liters of gasoline)
Tesla states that the battery pack is able to provide up to 200KW.
. 200 / 69 = 2.89KW per paralleled string
. 2890 / 366.3V = 7.88A
If those cells were able to hold voltage at 3.7V each string must provide less then 8A to achieve a power level of 200KW (or 544A @ 366.3V).
To optimise cycle life cells are operated between 3.0V and 4.15V. To achieve 200KW with a minimum cell voltage of 3V (system pack @ 297V) one would need 673A (673 / 69 = 9.75A per cell). Amp draw per cell should never exceed 10A under those circumstances.
Lutach, i'm looking forward seeing your built.
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Yes, the motor is an AC Propulsion designed motor. They can get the same amount of energy from other better suited cells. They use lithium laptop cells which are not the best for safety. Since they are lithium laptop cells, they won't provide the life cycle many hopes for. The cells I can get are used in some very serious machines and have the capability of over 5000 cycles for the Lithium Ion and way more then that for the Lithium Iron Phosphate cells. I don't think the cells they use can handle many 9A cycles. I don't think the Energy type cells should be used in a vehicle and that's the reason there's Power type cells.
For my race car since it won't be a mass production type vehicle, things will be expensive. One of the reasons I'm looking for investors to jump in, but they don't get all this stuff and just claims it won't work.