Motor cut out, is this BMS hard cut?

So today I experienced what I believe may have been the BMS hard cut, but I’m not totally sure because it doesn’t match glyderfreaks description of the event from an earlier post… Here is the details on what happened to me today.

On Monday I flew the machine for 25 min, and ended the flight with no load battery level of 44%.

The machine sat until today, wednesday, when I decided conditions were good to fly and at about 3 pm I charged it up for 90 minutes to 97v displayed on the charger. Just shy of a full charge.

I set up to launch and the throttle display read 92% @ 96.0 .

Outside temp is about 50 degrees and the battery temp is similar.

I launched and saw a hole in the cloud layer at about 3500 ft agl and decided to try and get to it before it passed over me. In 11 min I reached the hole at 3300 agl and had consumed 2.0 kw.

Reviewing my video footage I see between 11 and 13kw on the monitor. This seems to check out because 2.0kw over 11 min comes out to about 10.8kw average consumption.

This is the hardest I have worked the motor to date. Usually I cruise around 6-8kw.

The last video clip I have at 11min 32 sec shows that I was on cruise at 10.1kw and reading 79.9v under load. 2.0kw consumed power indicated.

@12.0min into the flight the motor cuts out, the controller is beeping erratically like it does when it boots.

After about 2 min the controller comes back on and stills shows 38% @ 84.7v no load. The controller re-ams successfully and motor spins up.

I opt to take a landing in a random field vs risk returning to lz which would take me over a river. Granted I had 2800ft elevation when the controller came back on. I just didn’t want to risk crossing the river at any elevation at all if the battery could possibly disarm again.

I land in the field ( very muddy) and out of nowhere a lady comes to see if I’m ok in her van and offers me some KFC chicken.

I accept the KFC chicken because I was hungry and use the extra napkins to clean some mud off my wing.

40 min later my friend comes to pick me up in his van ( he lives in his van ) and I return home without injury or damage to any gear. I offer him some left over KFC chicken in exchange the ride.

3 hr post flight I measure the battery voltage with a fluke meter, showing 85.8v. I open up the pack and measure 3.570 to 3.578v across all cells.

Other notes of possible relevance :
Recently spliced a screw together connection into the cable for the controller. Relocated controller to right hand.

Running an extended battery cable, 8" longer than normal.

Relocated my battery behind seat ( not behind back ) and turned it sideways because there was no other way I could accomplish a safe hang point on the gooseneck bars. ( I weight 155lb ).

Mondays flight had these same mods and I flew for 25 min with no issues.

I will update this post with the video link once it uploads to YouTube ( the neighbors wifi is really slow)…


Ya sounds like it wasn’t the battery or BMS. My guess would be to check custom wire extension you did. If it was a hard BMS cut off your controller would have turned off and you would not have any motor beeping.

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Always risky to mod electronics. Everything can look good and yet, a connection is not quite made up properly and can open. Been there.

Ok, sounds like that’s not BMS hard cut which is the big thing I was worried about .

I will re-check the screw plug and extension touch points. If it happens again I guess I’ll just revert back to the original cable.

Keep eating that KFC, that will soon disappear :rofl:


If this was beeping like it does when it arms repeatedly then there is a loose control wire that goes to ESC
I have a small Rotomax motor with 300A ESC and throttle is built using servo tester
I had same problem when thin cable from servo tester to ESC got damaged by movement and had intermittent connection and it was beeping repeatedly

Last year I experienced a hard cutoff and that was that. Dead as a door knob. I rebooted the controller after landing and all was fine. I had a low battery and kind of knew it could happen, and it did when I was below 30% in the red zone. The voltage drops really quickly once you reach the 30% mark. I fly with a 1/2 hour pack. I know better now and all is good.

Sounds like there is a common occurrence with a low voltage scenario for the cutouts??

I have had experience with other circuits where if you hit it hard to increase current, the voltage can go down fooling the electronics of a low voltage scenario. Unless the electronics are programmed to ignore that signal for a sec or two, they will do what they are programmed to do, cut the power.
It is especially true if it is cool or cold out as the battery reacts to cool temps too.
I dont know what caused this shut down. It could also be a loose wire from a crimp joint or bad solder joint on the ckt board. Though a bad solder joint would show up as a pesty occasional interruption that is hard to find and makes us find new explicit words to use when it happens.
And it can happen with surface contact electronics.
Found a car radio in the dumpster. OK OK no comments on that, I get interested in electronics. When I connected it, the radio didn’t work but lit up. I found that one of the resistors had come up from the ckt board solder pad on one side. You could hardly see it. I found it by pressing down on the resistor solder joint and when they made contact the radio worked. How about that?
So in this day and age of electronics, anything can happen to cause a problem.

Yes, common in the EUC world, give a good thrust on a wheel at lower voltages and your pushing your luck. I was fortunate never to have experienced a cut out on one of my 2 wheels. A cut out on an EUC at high speed (45kph) on a road next to cars could be equated similar to a Ultralight aircraft cutout not being left with a good emergency out LZ. I know for my Flexwing trike Eouts are far less than an footer PPGer despite my good glide ratio, maybe closer to a Trike PPG.
So, it sounds based on the evidence so far it is a low voltage with high demand based issue.

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So, I investigated the problem over the weekend and found that indeed the screw type connector I installed to lengthen the cable and allow the throttle controller to reach the right hand will intermittently disconnect if pulled or tugged on.
Frustrating because this hypothetical problem which I wanted to avoid by securing the connector at the ESC and making the disconnect portion a screw / latch type connection.

I will have to select new cable ends and re-do the whole job. I may also message Paul and ask if he has a spare throttle controller cable he can part with because I hacked up my existing one :frowning:


I’d have to see what you have been working with but it sounds like you made an extention using screw terminals.
In the industry I worked in, EVERYTHING was screw terminals. For the most part, very effective and reliable.
I will assume you used connectors, crimped them on and then screws to connect the connectors together.
That should have worked. Do you have plans to reinstall that controller on the left side again???
Because we are talking about a flying control, I almost want to suggest soldering the wires, but you have to secure them carefully so the wire can see no flex at the solder joint. The solder joint becomes rigid and most wire is made of copper. If copper is allowed to flex, it becomes stiff then breaks off.
The way I used to do this with small conductor wires was to solder the wires, then from both ends make a small loop, ( how do I explain this clearly, ok hold the solder joint with your fingers, run the wire down, around and back up the other side, over the solder joint, hold the wire there and do the other side, should end up with a smallish loop about 1" diam for each wire), keeping the solder connection in the middle.
make the loop flat onto the solder joint, with about 1/2" to an inch either side of the the Solder joint and tape it down. The solder joint would be protected against most flexing. I’d stay away from the shrink tubing because it also becomes ridgid and can cause the copper wire to fail.
Cant do a lot when you are in the air and a wire fails so try to think ahead.

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It was a BMS shutdown I believe because I was below the 30% mark cruising at the minimum needed RPM to come back home a few hundred yards away and only 30 to 50’ over the beach following the shore. I remember briefly seeing 25% on the screen. My fear was that it was going to shutdown just when I was a short distance from a jetty with big boulders I had to fly over and I would have been forced to glide into it - no space to the right to turn with the water less than 20’ away from my wing tip or to the left with the cliff and same distance or less from the tip on that side. It shutdown about 200’ away from the jetty and I touched down maybe 50’ later. It was a smooth glide to the beach. I know my turf well but yet, it was a bit nerve-racking.

Sean, awesome post.

Likewise I use screw terminals for pretty much everything I design on the machines and products I work on.
Here are a few pictures of the affected areas.

The cable used was an 8 pin industrial ethernet cable. Male and female ends. The cable inside is stranded and braided, not solid wire like your typical ethernet cable. Being an industrial cable it is also well shielded and has that nylon rope braided around the jacket to prevent the wires themselves from taking any load if the cable gets pulled. I thought it was a good candidate for the application.

I did solder all the joints, and everything has heat shrink over the top. While this does make the joint more stiff, handling the throttle controler in regular use does not produce much flex or kinks near the soldered area.

I also crimped pins to the loose wire ends and soldered the female pins onto the male pins of the ESC. I used very little solder and using some solder removal braid could take everything off and clean it up.

I think I could have done a better job managing the flex at the connection to the ESC. There the cable is sort of rigid.

With all of that said, I was able to recreate the issue on the ground, it happens when I pull hard on the cable at the screw terminal, or flex back and forth the actual screw terminal connection. The tighter I get the screw terminal the less likely the interruption becomes. I wasn’t able to recreate the disconnect scenario by flexing either of my soldered areas, or by lightly pulling / tugging on the wires soldered at the ESC. Leading me to believe I just got a junk screw connector.

I guess I will re-do the whole job, and improve in the following areas.
1: The screw connector cables that I chose were used cables, I got them out of a spare wire bin at work. It’s possible someone put back a cable that was having issues into the spare wires bin, the bin is really rather disorganized.
2: Connection at the ESC. I want to use the trick you mentioned to add a wire loop near the solder area to take up all the movement and flex. Though the issue doesn’t appear to be coming from that area, I wasn’t happy with the firmness of the wire at the ESC.
3: I’m also considering to just get a new cable and abandon the mod. I really did like the screw connector and having the whole thing easily reach my right hand. I even could thread the cable through my coat sleeve if I wanted too.

If have any suggestions based on the picture I shared on what else I can do better, let me know.

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I have not had a failure of a connector like that in my memory in a long time.
Not so sure I would have used it on a flying machine where a problem would be either yes you fly or no (failure) you come down. there are a few variables to be considered.

But i will say i believe you will find your problem in a crimp joint or solder joint. Even more likely at the screw connections.
You mention screw connections and I am particularly thinking a bad connection there.

For me, this is what I consider a “complex” connector, meaning you have all those pins that must make connection female to male, then you have crimped connections that the wire must “marry” to the connection. However, in my experience, the metal used in screw connections is too hard for the wire metal to “marry” with the Screw connector so it relies on the compression of the screw. This is where problems show up.

I have been known to solder the wire to that screw terminal but be mindful that the heat of soldering can melt the connector insulation, and could affect the connections with the mating connector.

When a wire is crimped, the theory is that the force of the tool that crimps the connector to the wire is so forceful, that the wire metal actually becomes one with the connector metal.
To achieve those forces, you need to use the proper tools. Using a pliers will make a connection, but it is not truly “married” to the metal.

Screw connectors are equally a challenge. If I have to use screw terminals in a connector like this, I have stripped the wire back enough to allow me to double up or triple up with the conductor, twist it and shove it into the connector, then screw it down. The smaller the conductor, the more I may try to force into that contact for the screw to make contact with.
Depending on the connection, I have Dabbed some solder on the wire end but you really have to be sure the screw sets into that solder to prevent it from becoming an intermittant joint.

IF YOU CAN RECREATE THE FAILURE BY MOVING THE CABLES, you need to redo the connections.

Also look to see why the wire is able to move where it is secured to the connector Jacket, where it is secured by screws. That should be tight enough that the cable is unable to have any kind of movement. The fact that you can recreate the failure by moving the cable, forcefully or not indicates the cable is moving. Look into this carefully.

Even if the outside jacket is secured, your wires inside the jacket can still see some kind of movement if the cable has any flex to it.
So consider wire tying the cable in a manner that it has no movement up to the connector. This will prevent any flex which can allow the inner conductor to move to and fro. This is a concern because that very small movement of the conductors will cause flex at the solder/screw connections and can cause the wire to flex and break off at the connector where it is fastened.

Didn’t know you could have so much fun with wires and connectors, did you?

As I said before, when making connections on a device like this, remember you are on a flying machine and you can’t pull over to check the wires while in flight. TRY to build in redundancy so you can try to eliminate any simple failures caused by wires losing connections.

Note: The first thing you need to figure out is WHY your cable was seeing any kind of movement that was forceful enough to cause the intermittant problem to show up.

I’ve started another thread on this issue of BMS-induced hard cuts. The BMS is doing its job: protecting the battery. The combination of the ESC and Throttle controller need to be involved in avoiding hard cuts. Cell voltages need to be monitored such that they are not allowed to sag to the BMS log voltage limit by limiting current (power).

See BMS hard cuts - suggestion in the Throttle Controller section.