First time poster, apologize if I accidentally missed any rules.
I've built a circuit that keeps a relay closed that is attached to battery power for a specific amount of time so my Raspberry Pi has time to shutdown.
I followed this circuit as guidance Shutdown Controller for Raspberry Pi in a car.
Somewhere though something is going wrong. When the circuit is assembled on bread board and I use a bench supply it will run for approximately 30 seconds. This would be a perfect amount of time for the Pi to run through its shutdown sequence. I even upped the capacitance a bit and was able to get a full minute of time which would hopefully give the Pi time to start and shutdown if the power was inadvertently turned on for a short amount of time.
When I install the circuit in the car however things go wrong. The circuit will only last 15 seconds no matter what size capacitor I run. I've run the meter all over the vehicle trying to figure out what might be causing this and can't find anything for the life of me. Putting the meter on the gate of the MOSFET and ground I can watch the Voltage drop to 3V from 12V almost instantly whereas on the bench it takes a longer time. This is with 2x 1,000 uF capacitors in parallel. Originally for the 30 second setup I was using a 470 uF capacitor. No matter what though the relay will always lose it's field and end up open in about 15 seconds. with the 2,000 uF worth of capacitors on the bench it will last well over a minute.
At first I was thinking maybe the car has something that is cutting either battery power or ground to the car stereo wire harness where I spliced in. Some sort of fail safe other than a fuse. I kept the meter on each of the separate wires during the sequence and Batt always stays about 11V, IGN obviously falls off when IGN is turned off, and GND always keeps continuity with the chassis.
The only possible thing I can think of being different between the car's power system and the bench supply is the ground. Do I need a resistor between the C1/R1 loop and ground? What's the difference in the ground symbols in the diagram I referenced?