Another
development weekend. This track presents different challenges
than West Palm Beach. There is much more braking and the turns
are more abrupt. Both of these attributes have brought out hidden
problems. The sharp turns cause the battery pack to shift minutely
which chafes through the insulation on the sensor wires. The hard
braking demands of this circuit are showing
that our friction brakes are undersized. The more time
braking has also spotlighted that we might be having some sort
of problem with 'unintended acceleration'.
Cell
#18 died in the final session. One possibility is that it was
at the same state of charge as the other cells before charging
(about 3.82 volts) and was just reading incorrectly due to the
damaged sensor wires in the cell next to it. In this case, we
held it's voltage steady while charging the other cells. 3.82
is about 65% charged. Since we used about 68% of pack capacity
during the race, it would fit that the cell reached empty and
then reversed.
The
other possibility is the cell was already damaged. The resistance
in a damaged cell tends to make the cell read higher during charge
and lower during discharge. There is some evidence
of this for #18. The day before the race we did a thirty
mile road test. Cell #18 was the lowest cell at the end of the
drive (3.749 vs. 3.816 highest) and the first to reach full voltage
during charge.
What
do we take home from this weekend? More information and another
couple of steps towards a winning car.
