Blown 2 KT controllers in a row

I've had 2 controllers in a row blow up and melt mosfets and PCB traces and I have no idea why, but I was lucky enough that the aliexpress sellers refunded me.

My bike is running a custom made 14s7p Samsung INR18650-2 in a soft bag, with a Bafang Swx02 (I think?). at least I know that it's a geared bafang motor and rated at 250w at 36v.

The first controller has a Kunteng 9 Mosfet controller rated up to 500w aparently for 36/48v. I had it for about a week and it made a pop and smoke came out of the controller and it and the motor and really hot to the touch until I struggled to unplug my XT60.

Got a refund for that, and thanks to China shipping 1 month without a bike and my controller finally arrived.

This time it was a Kunteng 18 mosfet 48v controller, aparently rated to 2000w.

This one lasted a bit longer, maybe an entire month of riding perfectly normal.

So yesterday I was taking my bike out for another ride and was out about 15km from home, when this controller made a pop and started smoking as well, strangely similar to the other controller I had before this.
The same thing happened, the motor and controller never get over slightly warm to the touch, and when I got home and checked around half of the 18 mosfets were completely melted and the entire board was charred and tiny SMD devices melted everywhere.

So I contacted the seller and they sent me a refund and I ordered another.

BUT before I brutally murder another $100AU controller, does anyone know what I'm doing wrong here. All I do is plug a controller which should handle 2000w continuous(?) onto a 250w rated motor and they keep committing suicide left and right. I honestly have no idea what I'm supposed to do to keep them from blowing.

Is it my 14s battery on a 48v controller?
Is it my phase wires shorting ?
Is it just bad luck to get 2 faulty devices from 2 different sellers in a row?

Someone please help because I'm going to set this bike on fire if it dies on me again

Can you change any settings to cutoff current at something reasonable? Looks like you cells are capable of 20 amps continuous. So with 7P, that 140 amps continuous (7kw). If there's no cutoff, then you're the cutoff, so you need to keep the power draw below the controller's rating.

I've very limited knowledge, but that 250W is fiction, can pull lots more, find out how much and get a controller that can handle at least 20% more.

But then you can burn the motor out.

Do you have big hills?

A Grin Cycle Analyst v3 will let you set a power limit.

E-HP said:

Can you change any settings to cutoff current at something reasonable? Looks like you cells are capable of 20 amps continuous. So with 7P, that 140 amps continuous (7kw). If there's no cutoff, then you're the cutoff, so you need to keep the power draw below the controller's rating.

Click to expand...

What does the batteries rating has to do with it? It is the controller that sets the limits.
I cant help you much though, I have a bike with a 18 fet kt, It has been fine so far. But I am only using 13s, I suspect that 14s may very well be the problem. Can you see what mosfets there are in the controller? I dont think it is a good idea to run 14s if the mosfets are for 60v

j bjork said:

E-HP said:

Can you change any settings to cutoff current at something reasonable? Looks like you cells are capable of 20 amps continuous. So with 7P, that 140 amps continuous (7kw). If there's no cutoff, then you're the cutoff, so you need to keep the power draw below the controller's rating.

Click to expand...

What does the batteries rating has to do with it? It is the controller that sets the limits.
I cant help you much though, I have a bike with a 18 fet kt, It has been fine so far. But I am only using 13s, I suspect that 14s may very well be the problem. Can you see what mosfets there are in the controller? I dont think it is a good idea to run 14s if the mosfets are for 60v

Click to expand...

I'm asking if his controller has the ability to set the limit. Are you saying all controllers have something to limit current, other than their physical ratings. My understanding is that they have voltage limits, but you can exceed the rated current and fry the components, if the battery can provide the current to do so.

My 18 FET is 1000W rated (20A continuous @ 48v), but I pull in excess of 2000W+ all the time (40A @ 52v), since my battery can provide it, and I have the current unlimited in the settings. I’d fry the controller if I rode continuous at that level.

It is possible for the batteries to be the limiting factor.

But ideally the controller (or CA3) enforces a programmed failsafe limit, which the human purposefully refrains from approaching.

I'd go through the wiring with a fine tooth comb. Especially the phase and hall wires. Sounds like an intermittent short to me, so metering alone might not necessarily show it. Most common place is probably chaffing where the wires enter the motor but it could be anywhere, up to and including inside the motor itself.

Yeah I'd also be looking for intermittent phase shorts, either in the wiring harness or possibly inside the motor.

Either that or you have very bad luck with controllers.

But good luck with AliExpress refunds.

What is your BMS?

E-HP said:

Can you change any settings to cutoff current at something reasonable? Looks like you cells are capable of 20 amps continuous. So with 7P, that 140 amps continuous (7kw). If there's no cutoff, then you're the cutoff, so you need to keep the power draw below the controller's rating.

Click to expand...

Yeah I have the KT LCD3 and there's something in there about setting current limit but I never messed with anything except for speed sensor settings.

j bjork said:

E-HP said:

Can you change any settings to cutoff current at something reasonable? Looks like you cells are capable of 20 amps continuous. So with 7P, that 140 amps continuous (7kw). If there's no cutoff, then you're the cutoff, so you need to keep the power draw below the controller's rating.

Click to expand...

What does the batteries rating has to do with it? It is the controller that sets the limits.
I cant help you much though, I have a bike with a 18 fet kt, It has been fine so far. But I am only using 13s, I suspect that 14s may very well be the problem. Can you see what mosfets there are in the controller? I dont think it is a good idea to run 14s if the mosfets are for 60v

Click to expand...

They're K150E09NE and the genuine ones are made by Toshiba.

I'm not sure exactly what i'm looking for but it says Drain source voltage rates at 85v and drain current rated at 120-150 depending on temps so it should easily handle 14s

Alan B said:

What is your BMS?

Click to expand...

It's a 14s 45a Bms off ebay.

Found it:

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com.au%2Fulk%2Fitm%2F113359717469

E-HP said:

Are you saying all controllers have something to limit current, other than their physical ratings. My understanding is that they have voltage limits, but you can exceed the rated current and fry the components, if the battery can provide the current to do so.

Click to expand...

Most controllers have a current limit that is set to whatever the label says, and that's the max continuous current they will draw from the battery.

They may *peak* higher for a moment, but if you keep the load at that point they'll begin limiting to whatever the limit is.

Most aren't programmable, so the labelled limit is the limit. If you have one that's programmable, then it's limit is whatever the max programmable limit is.

But there is always a limit--something in hardware to protect the controller from damage.


If you have one that's programmable, and you've set it to "unlimited", then it's now limited to that hardware limit (which may not be labelled anywhere, and may actually still be in the firmware, but isn't alterable without altering hardware like changing shunt resistance, etc),


It's still possible to damage a controller that isn't actually designed to handle it's max current (especially at it's max voltage), but if it's designed correctly (most seem to be) then it won't be damaged by running at that continuously.


A battery can't limit it's output, other than by shutting off it's output completely (if the BMS does current sensing and has a cutoff limit), to zero voltage, zero current. It would have to lower it's average voltage output via PWM in order to limit current, just like the controller does, or a charger, etc., and that would require the other hardware that's in a contorller, charger, etc. But BMSes don't do that.

bjeromin said:

I've had 2 controllers in a row blow up and melt mosfets and PCB traces and I have no idea why,

Click to expand...

It's possible to just be bad luck.

But it's more likley to be overheated melted phase wires inside the axle or cabling, or even the windings inside the motor, since that motor is not designed for the amount of current your stuff is capable of allowing thru it. That typically leads to overheating of the wires, whcih can melt their insulation and allow phase wires to short, whcih often fries FETs, which themsleves then provide an even better short circuit, which then can burn traces, blow up other FETs, gate drivers, etc. Even damage BMS FETs in a battery in rare cases.

A short is usually constant--but sometimes it isn't, when it's *almost* shorted when cool, but then as it heats up metals expand and eventually touch, causing the short...which then goes away as it cools off so you can't find it and it'll operate normally most of the time.

This happens more often with windings than phase wires, but it can happen with either one.


Can also just be damaged phase wires, usually right at the exit of the axle's wire channel, where the sharp metal can cut into the insulation, and vibration eventually allows the short to happen--then it goes away from further vibration or movement, etc., so again it is hard to find, and it'll operate normally most of the time.


But a visual inspection in good direct sunlight wiht a magnifying glass should show that problem, and will also show melted phase wires if they're directly visible (but possibly not thru the outer jacket of a cable). Can't see inside the axle or the motor without disassembly, though.

bjeromin said:

All I do is plug a controller which should handle 2000w continuous(?) onto a 250w rated motor

Click to expand...

Remember that the rating of a motor doesn't determine how much power it pulls.

If you have a controller capable of putting out more power than the motor can handle, *and* you put enough of a load on the motor to do that, then the motor will draw more power than it can handle, and can be damaged if this goes on long enough. Usually it'll get very hot under these conditions (like what happened the first time around).

So you need to use a controller that can only output the amount of power the motor is designed to handle, or you need to use a motor that is designed to handle the amount of power a controller can output.

For a 250w rated motor, that might be able to handle 350w for a while, or 500w for very short periods, not often, then at 36v you want a controller with a 7-8A current limit. 10A at the most.


If you want to see how all this works, go to http://ebikes.ca/simulator , read the entire page so you know what everything does and how it works, then play with different setups to see what happens.

amberwolf said:

bjeromin said:

I've had 2 controllers in a row blow up and melt mosfets and PCB traces and I have no idea why,

Click to expand...

It's possible to just be bad luck.

But it's more likley to be overheated melted phase wires inside the axle or cabling, or even the windings inside the motor, since that motor is not designed for the amount of current your stuff is capable of allowing thru it. That typically leads to overheating of the wires, whcih can melt their insulation and allow phase wires to short, whcih often fries FETs, which themsleves then provide an even better short circuit, which then can burn traces, blow up other FETs, gate drivers, etc. Even damage BMS FETs in a battery in rare cases.

A short is usually constant--but sometimes it isn't, when it's *almost* shorted when cool, but then as it heats up metals expand and eventually touch, causing the short...which then goes away as it cools off so you can't find it and it'll operate normally most of the time.

This happens more often with windings than phase wires, but it can happen with either one.


Can also just be damaged phase wires, usually right at the exit of the axle's wire channel, where the sharp metal can cut into the insulation, and vibration eventually allows the short to happen--then it goes away from further vibration or movement, etc., so again it is hard to find, and it'll operate normally most of the time.


But a visual inspection in good direct sunlight wiht a magnifying glass should show that problem, and will also show melted phase wires if they're directly visible (but possibly not thru the outer jacket of a cable). Can't see inside the axle or the motor without disassembly, though.

Click to expand...

I actually replaced all the wires going through the axle so it could just be my terrible soldering onthe PCB in the motor or on the plug shorting.

I'll check it out when I get home and see if I find anything and fix it up before my new controller arrives.

amberwolf said:

bjeromin said:

All I do is plug a controller which should handle 2000w continuous(?) onto a 250w rated motor

Click to expand...

Remember that the rating of a motor doesn't determine how much power it pulls.

If you have a controller capable of putting out more power than the motor can handle, *and* you put enough of a load on the motor to do that, then the motor will draw more power than it can handle, and can be damaged if this goes on long enough. Usually it'll get very hot under these conditions (like what happened the first time around).

So you need to use a controller that can only output the amount of power the motor is designed to handle, or you need to use a motor that is designed to handle the amount of power a controller can output.

For a 250w rated motor, that might be able to handle 350w for a while, or 500w for very short periods, not often, then at 36v you want a controller with a 7-8A current limit. 10A at the most.


If you want to see how all this works, go to http://ebikes.ca/simulator , read the entire page so you know what everything does and how it works, then play with different setups to see what happens.

Click to expand...

The motor wasn't getting overly warm even while I was running 500w continuous through it, but sometimes it got to over 1kw on acceleration. If the motor doesn't get hot, does that mean its fine to run? I was having heaps of fun riding at 40kmh until my controller self destructed and the motor still seemed only warm, but never hot.

E-HP said:

j bjork said:

E-HP said:

Can you change any settings to cutoff current at something reasonable? Looks like you cells are capable of 20 amps continuous. So with 7P, that 140 amps continuous (7kw). If there's no cutoff, then you're the cutoff, so you need to keep the power draw below the controller's rating.

Click to expand...

What does the batteries rating has to do with it? It is the controller that sets the limits.
I cant help you much though, I have a bike with a 18 fet kt, It has been fine so far. But I am only using 13s, I suspect that 14s may very well be the problem. Can you see what mosfets there are in the controller? I dont think it is a good idea to run 14s if the mosfets are for 60v

Click to expand...

I'm asking if his controller has the ability to set the limit. Are you saying all controllers have something to limit current, other than their physical ratings. My understanding is that they have voltage limits, but you can exceed the rated current and fry the components, if the battery can provide the current to do so.

My 18 FET is 1000W rated (20A continuous @ 48v), but I pull in excess of 2000W+ all the time (40A @ 52v), since my battery can provide it, and I have the current unlimited in the settings. I’d fry the controller if I rode continuous at that level.

Click to expand...

Your controller pulls 40A because (as you said yourself) you have progammed it to do so. The only thing your battery can do about it is sag and get hot.
If you have a low volt cutoff in the controller, that would kick in if it sags too much.
If you have a bms it can cut off if the amp draw is more than what it is designed for, or if the voltage gets to low.

bjeromin said:

The motor wasn't getting overly warm even while I was running 500w continuous through it, but sometimes it got to over 1kw on acceleration. If the motor doesn't get hot, does that mean its fine to run? I

Click to expand...

If the motor is only designed for 250w, it's likely a very small geared hubmotor, no bigger around than the gear cluster for the chain on it's right side. These don't pass heat thru quickly from the inside to the outside unlike DD hubmotors. So they may not seem hot for some time, while inside things are severely overheating or even melting.

You can't know without opening up the motor and examning the coils/windings/gears/etc., (or posting good clear well-lit pics of them for us to check if you don't know what you're looking for). Usually it's pretty obvious when stuff has overheated that badly.

However...most very small geared hubmotors aren't going to run very long at two to four times their power rating without damage, because they are just too physically small to shed the waste heat generated at those power levels.

Most of the bigger DD hubmotors can run twice their rated power levels easily enough continously, with bursts of a few times their rated power. Depends on how the seller or manufacturer happened to rate the particular motor, because some of them will underrate them (like QSmotors, and some will overrate them (like MXUS). It's also easy (easier than geared hubs) to modify them to cool better so they can run the higher power longer, or even higher power in bursts.

bjeromin said:

I actually replaced all the wires going through the axle so it could just be my terrible soldering onthe PCB in the motor or on the plug shorting.

Click to expand...

Why were the wires replaced?

For the controller to blow the FETs the voltage or current ratings must be exceeded. If the controller is rated at 48V you are pushing the voltage ratings of the FETs. make shure the wiring from the battery to the controller is short and solid. Length from battery to controller is inductance that adds spikes to the FETs. You can have long wires on the motor but short wires from battery to controller. Any intermittent connections here also cause high voltage transients that can damage FETs. The power switch, relay or circuit breaker or fuse could be a problem. They all need to be solidly connected, properly rated and close together. If you get a controller rated at higher voltage you will have more margin here.

If the controller and motor aren't getting hot then the average current and power is not the problem.

An occasional very high peak current will blow the FETs. A voltage transient causes the FETs to short which then causes a high current failure. A short downstream of the controller (the motor side) will also cause failure inducing currents.

The wiring, connectors, solder and crimp joints must be solid and clean. No whiskers of wire or solder floating about. Just a tiny short for just a moment and the FETs are toast.

amberwolf said:

bjeromin said:

I actually replaced all the wires going through the axle so it could just be my terrible soldering onthe PCB in the motor or on the plug shorting.

Click to expand...

Why were the wires replaced?

Click to expand...

They were ripped off where they went into the axle so I took them all out and replaced them with some spare wires I had lying around that looked the same or bigger than the original ones.

Alan B said:

For the controller to blow the FETs the voltage or current ratings must be exceeded. If the controller is rated at 48V you are pushing the voltage ratings of the FETs. make shure the wiring from the battery to the controller is short and solid. Length from battery to controller is inductance that adds spikes to the FETs. You can have long wires on the motor but short wires from battery to controller. Any intermittent connections here also cause high voltage transients that can damage FETs. The power switch, relay or circuit breaker or fuse could be a problem. They all need to be solidly connected, properly rated and close together. If you get a controller rated at higher voltage you will have more margin here.

If the controller and motor aren't getting hot then the average current and power is not the problem.

An occasional very high peak current will blow the FETs. A voltage transient causes the FETs to short which then causes a high current failure. A short downstream of the controller (the motor side) will also cause failure inducing currents.

The wiring, connectors, solder and crimp joints must be solid and clean. No whiskers of wire or solder floating about. Just a tiny short for just a moment and the FETs are toast.

Click to expand...

The controller had some small metal strings/pieces floating around inside when I opened it up, I was suspicious that they could've shorted something and caused it to fail but I'm gonna open my next controller and make sure it's clean inside and make sure my wiring is all OK before killing a 3rd controller

So, you have blown one controller in less than a month, and the first one lasted an even shorter time.

Both happened AFTER the wires were all replaced and re-soldered into the motor. Describe the events surrounding this incident. Powered on when the wires were ripped, perhaps?

HOW LONG DID THE CONTROLLER LAST *BEFORE* THE WIRES WERE REPLACED????

Also, take the controller out of the bag and get it into cooling airflow. Might not solve the problem, but it sure as hell won't hurt.

I would not recommend regularly running 2000 watts into a 250W rated motor. A winding short which may be the cause of the controller problem is a near-certain result. The outside of the motor is nowhere near as hot as the motor internals. 58V into such a motor is at or beyond the accepted operating limit for most such motors.

You are attempting to tow a boat with a VW Beetle. It might work, but not for long.

j bjork said:

If you have a bms it can cut off if the amp draw is more than what it is designed for

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I have never seen a cheap BMS with that functionality, usually the amps rating needs to be higher than the current it will ever see.

Some types, the current does not run through the BMS, so not amps limited, they just trigger large external contactors.

> or if the voltage gets to low

Yes that is universal, but usually set way too low, only good as a failsafe to prevent bank destruction, say 2.8Vpc, rather than a healthier LVC like 3.2Vpc.