DanGT86 said:
One time I guessed at the resistor value on a precharge connection and when I connected through the resistor the controller side only showed about half the pack voltage. When I made the connection there was still a spark.
The way I understand this is that if the pre-charge resistor has too much impedance then the caps are still hungry for the rest of the voltage when you make the main connection and thus can still burn the contacts of your connector or contactor.
The resistor and the capacitors are an RC network (low pass filter), with a simple set of math to determine the time it takes to charge the caps to any particular voltage based on the voltage on the input of the resistor and the starting voltage of the caps.
https://www.electronics-tutorials.ws/rc/rc_1.html
https://www.electronics-tutorials.ws/wp-content/uploads/2013/08/rc1.gif?fit=368%2C182?fit=355,226
The charge up time to "approximately full" is in seconds about 5x the ohms of resistance times the farads of capacitance, but it is a logarithmic curve, not linear, so it takes very little time for the first 1/5th of charge, much longer for the next, and so on.
https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/rc-rc2.gif
So if you have say, 3000uF of total capacitance, and a 100ohm resistor, that is 100 x 0.003 x 5, or 1.5seconds.
A 10kohm resistor would be a hundred times as long to charge, or about 150 seconds.
A 100kohm resistor would be about 1500 seconds.
Etc.
The bigger the capacitance in the controller, or the bigger the precharge resistor, the longer it takes to charge. If a large enough resistance is used on a large enough capacitance, it will take a long time to charge--so long that it may appear to have stopped charging at some point.