I’m using an Arduino Nano to control a MOSFET that turns on a large relay/contactor for the purpose of pre-charging some large 350F supercapacitors. When the relay is off power flows from a 12V source (1200W PC power supply) to the supercaps through a 50W 5-ohm panel resistor.
Without the resistor the supercaps will pull too much current and the power supply shuts off (in addition to the UPS it’s connected to freaking out!).
When the relay is powered it creates a parallel path to the pre-charge resistor with no resistance allowing full power to the supercaps. Once the supercaps are 80-90% charged no pre-charge resistor is needed as the amount of current they draw is naturally decreased by that point and the power supply can keep up.
I’m using two MOSFETs designed for supercap balancing. There are 5 x 2.7V supercaps in total which would allow for up to 13.5V but I’m charging to 12V. These MOSFETs keep those 5 supercaps at the same voltage but it happens kind of slowly.
Here’s the issue. I need to monitor the voltage so I know when to turn the relay on and bypass the pre-charge resistor. The first time I charge SC5 (the 5th supercap) will go up to about 500 (analogRead) which should be about 2.5V and then it’s starting to slowly drop. I’m not sure if the MOSFETs aren’t balancing correctly or if I’m measuring the voltage wrong. After a few hours it’ll drop to about 380 (1.85V??).
I think it’s most important that I just monitor the voltage of the entire stack. That’s what I thought SC1 would measure but it’s value goes up to around 1017-1023 pretty quickly while SC5 keeps changing.
I’m starting to ramble at this point and I’m getting a bit confused as to how to measure voltage here and at which points I should do it. Thoughts?