One of the challenges of adding sensors/electronics to the car is the fluctuation in voltage from the starter. This is compounded by the smaller racing battery I'm using. I was seeing voltages drop to as low as 8V while starting. This is low enough that some electronics restart or freeze. I decided I wanted to fix this the right way by adding a second battery for electronics isolated from the starter battery. At the same time I wanted to fix a couple of issues with the current wiring. One was to clean up the center console wiring, and the other was to make a "real" cutoff circuit. Here's the key parts of the original circuit:
Although this has a battery cutoff switch, 1) there is no cutoff near the driver, and 2) the cutoff doesn't turn off the engine once it's running. Also the wire running from the cutoff to the electronics was insufficient - thus contributing to the voltage drop.
The first challenge was to find an isolator to allow both batteries to be charged simultaneously from the alternator, yet remain load isolated (which is the whole point). Most isolators are not very efficient & thus generate a lot of heat - which didn't strike me as a good idea. Finally I found this high-efficiency dual diode isolator:
I modified the circuit to add the isolator & to add a second cutoff switch located in the center console of the cabin. The cutoff directly disconnects the electricity for the battery driving the electronics, and it disconnects the starter battery via a relay:
I placed the batteries back-to-back in the back of the car. You can see the cutoff relay to the right of the larger battery. The yellow wire is the activation circuit for the relay:
I found a pair of in-line fuses to tie in to diode to provide additional protection:
And as I mentioned I also completely reorganized & rewired the center console. This was sorely needed. As part of this I added a 12v bus, and a sub-panel for running electronics & data loggers. This panel has a seperate power switch, so I can turn off the engine & keep the loggers/computers running:
The net effect of all of this is that power voltages to the electronics barely nudge now when starting the engine!! And I have independent control over shutting down the loggers/cameras/etc.
This is perfect timing, since I've also just mounted a 7" USB-powered *touch* display connected to the on-board computer. This will be used to augment the existing display:
I plan on using this to provide a real-time version of the 2D Lambda/EGT graphs I've shown previously. And I can set up different maps & corrections that I can quickly select by simply touching the screen.
Also, I'm playing with making an Arduino-base data logger that reads & writes sensor data directly to the CAN-bus. I have amplifiers for temp sensors, a 6D accelerometer, and even a GPS logger: