In my sophomore year, I designed a compact “High Voltage!” warning indicator.

Building and servicing a battery pack is dangerous because of high currents and predominantly volatile lithium-polymer cell chemistries, so the Formula SAE rules committee have a detailed specification of safety regulations that must be followed in the design and handling of an rules-compliant accumulator (what the rules call the main battery pack).

Among these, two warning lights are required, the Tractive System Active Light (TSAL) and an accumulator voltage indicator (no acronym). Both have to be prominently visible and turn on when 60V appears across the accumulator’s main contactor terminals. The accumulator voltage indicator has to power itself from the same source its monitoring because it has to work even when the accumulator is disconnected from the car.

Before MY17, our accumulator was 84s1p with A123 pouch cells split between two modules. We had just wired a commercial panel-mount voltmeter across one of the modules. This method was acceptable because the cells were all in series and the voltage across one of the modules was within the meter’s rated voltage. However, it was totally incompatible under the rules with our new, more parallel accumulator design.

I planned a new voltage indicator that we would install inside the accumulator to preserve isolation, with an external LED mounted to the accumulator enclosure. The main design challenge was powering this low-voltage circuit from 300V. We weren’t interested in buying a Vicor DC-DC block just to drive an LED. Luckily, the Microchip LR8 linear regulator was an available alternative. I designed a cheaper driver board around the LR8, with a Zener diode blocking its input to achieve the 60V turn-on. I had to keep the footprint small while providing ample heat-sinking (I estimated it by calculating equivalent thermal circuits).

Unfortunately, going this route meant the indicator LED wasn’t isolated from the main tractive system. I ran into the problem of the LED’s current-limiting resistor delaying our accumulator’s pre-charge timing. Thankfully, the LED was still bright enough when I just swapped in a larger resistor (we also could have alternatively decreased the pre-charge resistance). The LR8 was the only compatible power IC I could find at the time. Shortly after, Analog Devices released the far more appealing, isolated LT8315 converter, which would permanently resolve the pre-charge issue if replacing the LR8 in future iterations.