Why do you continue to multiply time constant by 5? This is irrelevant.
The 37% mark comes from the definition of time constant RC and is simply used as a starting point for the relay release point. The 50% mark might be a better choice. Using your graph, the 50% point occurs at 6V, 10sec. Hence the solenoid will release after 10 sec.
The 37% mark comes from the definition of time constant RC and is simply used as a starting point for the relay release point. The 50% mark might be a better choice. Using your graph, the 50% point occurs at 6V, 10sec. Hence the solenoid will release after 10 sec.