In close partnership with Thomas Wick from ASMO Karts (www.asmokarts.com) and Thomas Dinkel, owner of the kart tracks in Winterthur (www.kart.ch), we were able to build up a standard indoor kart with the new LiFePO4 battery and test drive it on the track on Sept 20th 2008.
>>> Foto Session (PicasaWebalbum)
For the battery configuration we assembled the 32 cells of the10Ah LiFePO4 type into a completly closed and padded aluminium boxfor best protection. The box is placed on the left of the driverand compensates the weight of the motor on the right.
The handling, acceleration and top speed of the kart is impressiveand real fun to drive. The weight reduction of 60kg or 25% makesthe kart agile like a remote control model car. The voltage dropunder 100A full load is only about 7V from 54V to 47V. It'saddicting . . . there is no way back to lead acid!
1st heat: 22 laps (300m each) within about 15 Min
Battery is not empty yet, Temperature 39deg C (increase of 17degC)
Charging at 2kW / 40A (2C): 921Wh (from AC-mains) within30Min up to 58V (3.6V/cell)
Temperature 47C (increase of 8deg C)
2nd heat: 28 laps within 20 Min until voltage is down to42V limit (2.6V/cell)
Battery temperature within the case goes up top 65 deg C
Quick charge test for 1 Min at 80A: voltage goes up to 55V(3.4V/cell)
Cool down and charge 824Wh with 5A (next day)
Kart with the openbattery during charging
Voltage stability over the whole discharge time is impressive.With the 60kg weight reduction the kart is really flying. Howeverwe underestimated the temperature problem. This must be solved, sothe cell temperature will not rise above 40deg C for a 1 : 1 usageover 10hrs.
There are three ways to approach:
A) cross ventilation through the battery case: air inlet andventilators to remove the heat from the cells. However the cellsand connectors would not be protected from dust and humidity anymore.
B) increase capacity: a capacity of 30Ah or 40Ah would reduce theinternal resistance and therefore heat generation. Howeverconsiderable cost increase comes along with this.
C) cell selection of those with the least internal resistance.
A short calculation shows that (if you disregard all chemicalprocesses in the cell) a certain temperature increase can not beavoided at this high rate discharge:
Cell resistance: <5mOhms per cell gives (2p16s) a total of40mOhms, connectors between cells of about 1mOhm x 32 is 32mOhms.This adds up to 72mOhms (0.072Ohms) or at a rate of 100A a voltagedrop of 7.2V, which is comparable to our actual measurements.
However a 7V drop at 100A equals a power dissipation of 700W . . .if you put an 500W lamp into the battery box for an hour you willbe able to use the cover of the box it to fry some eggs!
With 48 cells instead of 32 we can decrease the resistance to26mOhm or about 20%. By selecting the best cells we can get downmaybe another 10%. Even if we use gold plated contacts, we stillend up with a power dissipation of 200W to 400W in the best case.If you run only one heat and charge it then for 4hrs, the batterycan store the heat and there is enough time for cooling, but withthis continuous usage and the much lower mass compared to the leadacid system, an active cooling has to be implemented.
We will add this week cooling fans and do some more mesurementswith the charge-discharge equipment.