The electrical team put the battery protection system through rigorous testing this weekend, and it is now calibrated correctly and fully prepared to protect our battery pack from the many variables that conspire to set it aflame. SC5 uses lithium ion batteries, which are great because they can store more energy per kilogram than any other battery chemistry. However, they are also dangerous if not treated delicately. That’s because they tend to catch fire when the voltage, temperature or current levels are too high or too low.
I’ve only been a member of NUsolar for a few weeks, so there’s a lot I don’t yet know about the team’s illustrious history. But there is one incident – one dark, terrible incident – about which I’ve heard countless whispers and rumors. Many years ago, the team’s fourth-generation car caught fire when the battery protection system failed right at the start of the 2005 North American Solar Challenge. The memory is burned (not literally) into the team’s collective memory.
But this year’s race won’t be marred by any similar catastrophes. Our battery protection system is made up of circuit boards that monitor the batteries to ensure they always stay within the safe ranges of voltage, current and temperature. The temperature sensors make sure the battery stays below 45 degrees Celsius. The differential amplifiers measure the voltage to check that it stays between 3.2 to 4.2V. And a shunt resistor measures current, which should remain below 60 amps.
The data from all these sensors is processed by a Rabbit microprocessor that was generously donated to us by Digi. If any of the measurements approach dangerous levels, the system disconnects the battery from the motor and solar array, which will avert any race-ending, heartbreaking flames. The team also monitors all these measurements during the race in the lead or chase vehicle through the telemetry system, which beams the data to the team laptop.
“The new protection system has been physically integrated with the batteries and built more robustly than before,” said senior electrical guru Johnny Sun. “Also, the electronics have been consolidated to fewer circuit boards. All of this translates to fewer, neater cables and increased system reliability.”
On Sunday, the shunt resistor was tested to make sure it is measuring current properly, and it passed. The temperature and voltage sensors beat their inspections weeks ago, so now the car can zoom along without worrying too much about overly sensitive batteries. A victory in and of itself.