That little plastic HV disconnect switch might not be so silly after all

[louibluey]

During some recent measurements, I realized there my be some merit to regularly using the HV disconnect for those disconnecting the 12V LVB for installation (e.g. audio work), reset (cold boot), or measurements.

I think the general thought was, hey, once the 12V LVB is disconnected, hv cannot come on, which I still believe to be correct.

However, short of running an OBDII scanner of some type or a clamp on ammeter, it is very difficult to know if the DC/DC converter is running, or when it will come it. There are many times where it comes on for a short period of time.

I think the DC/DC converter is robust enough to survive an open 12V LVB terminal without damage, but I also think that in general, it would be better practice to open circuits, even 12V, with as little load/charge current present as practical / possible.

During my recent measurements, I noticed by a Fluke DC current meter that there was between about 10A and 30A flowing in the negative terminal of the LVB (charging currents at various times). (not load, like headlight load.). I don't recall the exact location of that little switch, although I have used it at times. I think it under the side cover, and the side cover needs to be pulled for its present location.

Next time I need to pull a 12V connection, I will try to remember to put the Fluke clamp on DC ammeter on the negative terminal, open the little hv plastic safety switch and hopefully observe the DC/DC converter going off, if was on.

This is only for those lifting a 12V terminal or opening that battery junction fuse box main terminal. I do not think there is any concern for connecting chargers or inverters to the jump posts "live", that should be fine.

If this revised understanding of that silly little plastic switch is correct, it might have some good use after all, not just for safety, but to turn off the DC/DC converter before opening the 12V LVB circuit, probably a good best practice. Of course if you do open it, don't forget to reset it before buttoning up all the covers!

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[Motomax]

What’s a little spark between man and machine lol.

 

[AlpaChino]

What’s a little spark between man and machine lol.

 

[Mach-Lee]

Joe, the HV disconnect cuts 12V power feed to the high voltage junction box and BECM only, which takes away the ability of any HV contactors to close. This means the HV would be isolated inside the pack unless one of the contactors were stuck or welded closed. This also means the DC/DC would lose its HV power source and would not function. The rest of the 12V system would still function and would not be disconnected, so pulling it would not power cycle anything other than the BECM.

 

[louibluey]

Joe, the HV disconnect cuts 12V power feed to the high voltage junction box and BECM only, which takes away the ability of any HV contactors to close. This means the HV would be isolated inside the pack unless one of the contactors were stuck or welded closed. This also means the DC/DC would lose its HV power source and would not function. The rest of the 12V system would still function and would not be disconnected, so pulling it would not power cycle anything other than the BECM.

My thinking is that it is less than optimal to open the battery circuit (lift the negative terminal) when the DC/DC converter is running (could be charging 30A+). The converter can probably survive the transient fine, but maybe it is better practice to be sure the DC/DC converter is off, before opening the battery circuit.

Also, I am wondering if there is any condition (little green hv interlock satisfied/closed) where one lifts the LVB 12V negative ground connection, but where the DC/DC converter was already running, so the 12V bus is already (still?) live. We always say no 12V, no HV, but does it latch on? BMS is directly across the battery terminals, so BMS is still reporting regular battery voltage, when the 12V ground is lifted. And, DC/DC converter output is directly wired by heavy cable to the 12V bus.

If the DC/DC converter is already running when the LVB is disconnected, maybe HV does not switch off? Maybe that is another reason for the little green interlock switch?

 

[Mach-Lee]

My thinking is that it is less than optimal to open the battery circuit (lift the negative terminal) when the DC/DC converter is running (could be charging 30A+). The converter can probably survive the transient fine, but maybe it is better practice to be sure the DC/DC converter is off, before opening the battery circuit.

Also, I am wondering if there is any condition (little green hv interlock satisfied/closed) where one lifts the LVB 12V negative ground connection, but where the DC/DC converter was already running, so the 12V bus is already (still?) live. We always say no 12V, no HV, but does it latch on? BMS is directly across the battery terminals, so BMS is still reporting regular battery voltage, when the 12V ground is lifted. And, DC/DC converter output is directly wired by heavy cable to the 12V bus.

If the DC/DC converter is already running when the LVB is disconnected, maybe HV does not switch off? Maybe that is another reason for the little green interlock switch?

As soon as the negative cable is lifted, that removes the return current path to the battery so everything 12V would shut down, including the BMS. I guess I wouldn't want to open the disconnect while the DC/DC was drawing HV current because could create an arc when the contactor opens. With the disconnect open or the (-) cable removed, the absence of 12V will open the contactors and shut off all 12V (they spring open with no power). The only way you would continue to have HV would be if the contactor was stuck closed (rare but possible). Proper HV procedures dictate verifying no voltage is present after the disconnect is opened just in case.

 

[Murse-In-Airy]

My thinking is that it is less than optimal to open the battery circuit (lift the negative terminal)

You good man?
Last you posted you were talking about shocking yourself. Now it’s been a month of silence. Just checking in on my Northern NY peeps.

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