The name itself says a lot, but honestly, until I went to wire up my motor controller for the first time, I had no idea I needed one, nor why.
What:
The pre-charge resistor does just that - it allows for charging of the capacitors in the motor controller prior to the closing of a of high-current switch or contactor.
How:
Simply put, the pre-charge resistor allows the motor controller capacitors to be in a circuit with the main battery pack even though the contactor is open (not completing a circuit). This by-passing of the contactor allows the motor controller capacitors to stay charged, sometimes called "fully formed". There's a catch involved here related to the importance of selecting the correct resistor for a certain system, so please read this entire post to be aware of important details.
Where:
The pre-charge resistor is wired "across" the main terminals of the contactor. That is (equivalently), one end of the resistor on the battery's positive terminal and the other end of the resistor on the controller's positive battery terminal. Being in parallel with the contactor allows for the "bypassing" (as I call it) of the contactor. Check the documentation on your own motor controller as some may already have a pre-charge resistor internally, but my Kelly KD72401 does not.
Why:
When a contactor "closes" (to complete a circuit), the amount of current which immediately tries to pass between the contacts affects the life of those contacts. In the case of EVs, uncharged capacitors in a motor controller can draw such an in-rush of current, the contacts will arc, creating pits in their surface (which reduces usable contact surface) or perhaps even welding the contacts together. Furthermore, the capacitors themselves can be damaged by such an in-rush current. On the other hand, capacitors that have been mostly pre-charged will draw a much smaller in-rush current, which limits arcing of the contacts and increases the longevity, reliability and performance of the contactor as well as the motor controller.
Don't miss the point:
It is possible to render this component useless so please still be cautious. This is exactly why I like to know what each component does and how...
Many builders put a master on/off switch (in addition to the contactor) capable of handling 300+ amps for manually opening the main battery circuit as a safety measure or convenience. If such a master switch is present, it will of course allow the motor controller capacitors to discharge over time, which is probably normal. The only dangerous part is when the system is wired such that the contactor can be closed while the master switch is off. With the capacitors uncharged, if the contactor and then the master switch is closed, then the pre-charge resistor will not have an opportunity to function. The path of least resistance will be from the batteries, through the master switch, through the closed contactor, and straight into the capacitors. Those capacitors will attempt to charge almost instantaneously and may be damaged in the process.
One fix in this situation is to install another pre-charge resistor, this time for the master switch. This will allow the motor controller capacitors to always be charged and ready - making the contactor's job (and the controller's job) less stressful.
What exactly:
Now that we know their purpose and value, lets take a practical example for application to your own project.
According to the Alltrax Lessons Learned document, the resistance of the pre-charge resistor should be selected based upon battery pack voltage. In my case, 72 volts implies a 1000 ohm, 10 W resistor. The resistor with which Kelly Controllers provided me is only a 300 ohm, 10 W resistor. To know whether or not that will suffice, I checked the following calculations:
Peak in-rush current can be determined by battery pack voltage divided by Equivalent Series Resistance (ESR) of the capacitor bank. This might be measurable on the controller with a digital ohm-meter. Assuming the bank is made of 10 capacitors, each with a resistance of .05 to .5 ohms, the bank's total ESR would be .005 to .05 ohms. Thus, without the pre-charge resistor, 72 volts / .005 ohms = 14,400 amps. The capacitors probably aren't rated for that, so even though it's just the peak and only experience for an instant, it could cause one or more capacitors to short-out internally.
With the pre-charge resistor, the peak in-rush current is 72 volts / 300.005 ohms = .24 amps. Therefore, the peak dissipation in the pre-charge resistor is 72 volts * .24 amps = 17.28 watts.
That's almost twice the 10 watt rating of the provided resistor, but it might be alright since that's just the peak dissipation which falls quickly from 17.27 to 10 watts in around .275 seconds.
Using the rule of thumb that a capacitor is 99.3% charged after 5RC seconds lets us estimate how much time the motor controller capacitors should be given to be (close to) fully charged, where R is resistance and C is capacitance. Assuming the motor controller has a total capacitance of 3300 uF, 5RC = 5 * 300 ohms * (3300*10^-6 farads) = 4.95 seconds.
One member of the diyelectriccar forum suggested selecting the pre-charge resistor based on the size of the battery pack - that is, based on its voltage. He stated a resistor with 1 ohm for every 1 volt of potential difference will result in a current of 1 amp (72 volts / 72 ohms = 1 amp). Also then, 72 volts * 1 amp = 72 watts, which sure sounds like a lot more power dissipation. I don't know much about the chemistry and makeup of resistors, but the guy stated that because the pre-charge was going to take so little time, a sand-filled 10W resistor would probably work. The time calculation checked out, as 5RC = 5*72 ohms *.003300 farads = 1.19 seconds, but I don't know for sure that the resistor I was given is sand-filled. Then again, I'm not sure about my motor controller's total capacitance either, so there are still plenty of variables.
Conclusion:
In the end, the objective is to protect your investment, as well as your own well-being. The critical issue is to make sure the controller always has an opportunity to pre-charge (at an appropriate rate and as completely as practical) to full drive power before closing that last high-current switch.
References:
Graphing / Analysis of RC Circuits
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