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View Full Version : What is a precharge circuit?



Richard230
19 January 2011, 1556
A couple of days ago, Harlan mentioned that he solved Guily's stalling problem by reconfiguring his bike's "precharge" circuit. I don't know if I have the same problem or not, but I sure would like to know what a "precharge" circuit does and how it works. Does anyone know?

DaveAK
19 January 2011, 1559
I'm not sure I can describe a pre-charge circuit very well, but I can tell you that the pre-charge is built in to the Sevcon controller, so no external circuit is required.

frodus
19 January 2011, 1602
Precharge charges up the capacitors inside the controller before the contactor closes so that the contactor contacts don't arc/spark/weld. Some controllers do this themselves via their enable wire (also pack voltage). Some require an external resistor across the contactor.

It also helps extend the life of the caps by slowly bleeding power into them, instead of slamming them with current.

Richard230
19 January 2011, 1608
Does the contractor, which is energized by the precharge circuit, turn on the power to the drive system or does it perform some other function?

frodus
19 January 2011, 1721
two seperate systems. (and its Contactor, with r between t and a)

System 1: Contactor
For systems without built in precharge, the Keyswitch turns on contactor via the contactor coils. This closes the contactor. The contactor connects the HIGH CURRENT positive terminal of the batteries to the HIGH CURRENT positive input on the controller. Thats all it does.

for systems with precharge, see below.

System 2: Precharge
For systems without an automatic precharge in the controller, the precharge is wired across the contactor terminals. It provides a low current bridge between the positive terminal of the battery and the positive terminal of the controller. Some people don't want it always on, so they run a small switch inline. The problem with a switch, is if you turn it off and then put the key switch on, the whole point of the precharge is ignored and you slam those caps and arc inside the contactor. The purpose is to charge the capacitors first so you don't get huge inrush currents, which damages contactors and caps. Look at some schematics for alltrax and curtis. It fully explains precharge.

For systems with an automatic precharge in the controller, you hook up the contactor coils to the controller. The high current contacts of the contactor get connected the same way as before. The keyswitch goes between the positive of the battery terminal and the KSI input on the controller. When the keyswitch is turned on, the controller see's voltage at its KSI input and internally connects a resistor to the capacitors and waits for the capacitor voltage to match the pack voltage. Once this is done, it closes the contactor.


I suggest a little reading and research on manufacturer websites for some circuit schematics. Almost every manual explains them, what they do and how they're wired.

Richard230
19 January 2011, 1851
Thanks Travis. That gives me a lot to think about. There may be something here that will solve my intermittent starting problem. I am going to work on it this weekend. Sorry about the miss-spelling. I have used the word "contractor" for way too many years and my fingers were typing on automatic. I realize there is a big difference between contracting something and contacting something.

frodus
20 January 2011, 0920
Just wanted to make sure you knew the difference. Glad it helped a little....

Its all a little bit of learning, every day. You'll get it, we're here to help.