openinverter forum

in this series I ask questions on behalf of all of us newbs to the sport: Why do we need a relay in the pre charge circuit? I know we need a resistor but why also a relay?

also this question comes from being a minimalist and wanting to keep things simple, so...

I recently rewatched one of Damien Maguire €1,000 BMW E36 EV Build 15 : Automatic Prechargevideos where he discusses the pre charge circuit and I noticed that he only relies on a resistor across the HV positive contactor minus the relay (though I believe the rig is a DC motor operated one, but not sure that makes a difference for those of us running AC motors these days)

I also watched Making a Pre Charge circuit for my EV RX8 video by South West EV UK where he also implements a pre charge circuit without a pre charge relay.

I have also further bastardized Robert's wiring diagram to show a simplified version of just the pre charge circuit with the relay and one without, please take a gander

It's kind of a thing to have a complete disconnect when the car is off. It isn't strictly necessary, functionality-wise. In fact in Germany cars with just one contactor (and another, smaller one for precharge) will be approved.
But say there is a isolation fault from mid-pack to chassis. Then you work on the car thinking everything is off and you can work on the battery voltage terminals. Then you touch chassis and the always connected (through the resistor) positive terminal will teach you a lesson.

on behalf of all the Newbs I thank you for a thorough and simple explanation, much appreciated

johu wrote: ↑Wed Feb 02, 2022 5:26 pm It's kind of a thing to have a complete disconnect when the car is off. It isn't strictly necessary, functionality-wise. In fact in Germany cars with just one contactor (and another, smaller one for precharge) will be approved.
But say there is a isolation fault from mid-pack to chassis. Then you work on the car thinking everything is off and you can work on the battery voltage terminals. Then you touch chassis and the always connected (through the resistor) positive terminal will teach you a lesson.

Yes well, there were incidents where front box caught on fire. Then that fire melted the cables onto the chassis which caused a general meltdown which also melted the rear box with it.
Now if you would have contactors and fusees between boxes that would allow rear pack to survive and maybe not cause total loss. Worth to think about failure modes?

in today's virtual field correspondent segment (part I of a II part series) we catch up with the now retired Mr. Fred Behning of Austin Texas and his PorschEV blog where he tells us about his Pre-Charge circuit woes Seriously, This is Getting Old

I used 470R 30W resistor with Openinverter interface. On my request Johannes programmed some 5s delay for precharge to be shutoff if required DC voltage would not be reached. I would think that was safe to use even smaller rated resistors. If 5s would not be enough to precharge that would signify something is wront in your circuit. A kind of additional layer of safety.

in this video a guy who claims to drive a 3 cylinder 1 liter Opel Corsa and calls himself Mr. Cool, Mr. EVBMW explains the pre charging system, enjoy:




high voltage junction box

I agee with @arber333. Always think about failure modes. What kind of failures should the system tolerate, and in which failure modes should a person be protected, not just the car itself.

Wires can melt, contactors can weld, there might be debris that causes a short to chassis, or short across something, the car can be on fire (even for other reasons than your system), someone might be lazy and not remove a service disconnect when working on the HV system, software can have bugs, something can be plugged in backwards, or left unplugged.

Anyway, the answer to the question is, if you leave out the precharge relay BUT you do have a contactor on the other side of the battery pack, then the contactor on the other side of the pack does double duty. If a problem occurs, there's a higher chance you'll melt or burn the precharge resistor as you have less control over it, UNLESS you program and test your software to be aware of the fact that one of the contactors also acts as the precharge relay. And if there's a short to chassis somewhere and you touch the cable coming from the precharge resistor, the resistor won't help you.