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Master relay diode failed closed

ctennis

Well Known Member
Has anyone experienced the issue of your master relay protective diode falling by *shorting*? This happened while on the ground and just doing some manual work at the EFIS screen - system just went dead. Battery went into protective mode. I can replace it with a higher voltage rating diode, but I was surprised it would fail in this way and not open.

I'm more worried that if this is indeed a concerning failure mode, it would have taken out the main entire electrical system.

I confirmed the diode was properly installed to the right terminals, and has been replaced and system now works again. I'm not sure which model of diode it is offhand, something I had in my stash of parts, and what I used on my previous build - and perhaps that is part of the problem.

(EE by education, but I haven't dealt with power electronics design in a few decades)
 
Fuse or breaker on master switch circuit opened I hope. If not, you need to look into that.

My bad. I was thinking there is typically a fuse in line with the switch. I see now that is not the case.
 
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Not sure what you mean - the diode is between the activation terminal of the relay and the battery side. The battery itself went into protective mode (earthx) since it was effectively shorted while the switch was active. None of the breakers tripped as they didn't draw any excessive current (at least that I can tell).
 
Interesting -

Assuming you have the common type of single wire controlled master relay, the activation circuit is connected to ground via the master battery switch.

In the failure mode you described - If the suppression diode were to fail "closed", this would nearly instantly bypass the master relay, and cause current to flow from battery to ground via the master switch & wire until the battery BMS said "No mas!" or the diode, wire, switch, all melted and the circuit opened.

You are correct; there isn't any fuse/CB protection on the activation wire -- however, I would carefully inspect the master switch, wire between it and the relay.

And for grins -- why the diode works...
https://www.youtube.com/watch?v=LXGtE3X2k7Y&ab_channel=Afrotechmods

The diode should be in the 800V range, IIRC...
 
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Very unusual failure mode, imho. Furthermore, if you had a conventional battery in there without current limiting, the huge current draw of the essentially short circuit would have vaporized the guts of the diode in less than a second, just like a fuse.
 
A shorted diode should have conducted enough current to destroy it. Seems like the battery BMS somehow interfered with this normal failure mode progressing to the open condition as the junction is vaporized ( smoke escapes). This normally happens in a very short time. What diode were you using?
 
As Dan mentioned, The typical 1N5400 diode used is rated at 3 amps, if the PN junction shorts it will destroy itself very quickly and open acting like a fuse.
 
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The typical 1N5400 diode used is rated at 3 amps, if the PN junction shorts it will destroy itself very quickly and open basically acting like a fuse.

Thanks for confirmation, Walt. This is very important to know.
 
How does the battery notify the pilot that it has shut itself off in protective mode? Is there an indicator light?
 
I concur with what Dan & Walt said. The typical diode used for spike-suppression in a direct short to ground would have popped (literally) in a few milliseconds and you probably would not have even noticed until you looked at the solenoid (or heard it).

If your battery's control logic actually reacted faster than the diode popping, I would consider that a design flaw and might actually be really inconvenient if not dangerous. If such a minor fault causes it to completely shut down, I wonder what other relatively benign events might do the same.

I wonder if we have all the facts yet. Did you contact the battery manufacturer? Do they publish amps vs time graphs (like the ones for current limiters) that specify under what conditions the battery would do that?
 
Just to close the loop on this, the diode ended up being fine. It was the EarthX battery BMS that shut the output to the battery off, which looks like a short when testing the diode inline.

That said, the diode I was using was definitely undersized for the application and have since gotten higher voltage rated diodes into the system.

Thanks for the input everybody.
 
I understand why these types of batteries have a BMS with the required protection, but to me that makes them undesirable for this application. I'm sure I'll take some heat for this view but a heavier old technology battery is what I'll be using.
 
I understand why these types of batteries have a BMS with the required protection, but to me that makes them undesirable for this application. I'm sure I'll take some heat for this view but a heavier old technology battery is what I'll be using.

I am in the same camp. I have avoided this type of battery out of fear that a BMS issue would potentially cause more serious issues than it was trying to prevent. Best to have a heavier battery that is not subject to the failure scenarios that the Lithiums are. We each need to pick our poison.

Larry
 
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