Bafang Max Error 21 at low gear

[Sideview07]

Getting error 21 when riding a ejoysport Bafang Max 35V250W on the lowest gear (36T x 42T largest cog) when climbing uphill. It gets jerky like power cut in and out. Eventually within secs screen reboot. Changed the chainring to 32T and now even the 2nd largest cog 36T would have the same problem. What could possibly cause this and what config that i can go try to change ?

 

[AHicks]

You are very likely overheating SOMETHING. Could be the motor, the controller, or the battery BMS. The simplest cure is to avoid long hills, or possibly to approach this hill with a lot of momentum to help carry you up the majority of it.

Geared motors, at wide open throttle, will quickly overheat. Like within just a couple of minutes, or maybe less.

 

[billmeek]

Check the speed sensor alignment and spacing.

 

[PedalUma]

@AHicks is on it. One test is to try turning down the power level and doing more pedaling. Hub motors are single speed. To the motor it doesn't matter what gear you are in. It has no way of knowing. It is just protecting itself from overload and or heat on that hill.

 

[Sideview07]

Could be the motor, the controller, or the battery BMS. The simplest cure is to avoid long hills, or possibly to approach this hill with a lot of momentum to help carry you up the majority of it.

Geared motors, at wide open throttle, will quickly overheat. Like within just a couple of minutes

conceptually if i switch to the largest cassette cog , it should be less burden on the motor .. no ?
eg. I am able to cycle up a hill with Power L4 (out of L5 max) with 36T cog quite comfortably but when i switch it to 42T cog with rest being the same, immediately the error occurs. I can repeat this pattern.
something to do with high rpm vs low torque/low speed configuration ??

 

[Sideview07]

Check the speed sensor alignment and spacing.

first thing that i did after i research initially. misaligning it and realigning it.. etc . works like normal at all gears as expected
even change the sensor meter signal 1 to 2 (type of sensor) using SPEEED connection & USB cable, also same problem

 

[PedalUma]

This is inherent for hub-drives. Back off of the power level for hills. Or install a mid which runs through the chain and gears.

 

[AHicks]

conceptually if i switch to the largest cassette cog , it should be less burden on the motor .. no ?
eg. I am able to cycle up a hill with Power L4 (out of L5 max) with 36T cog quite comfortably but when i switch it to 42T cog with rest being the same, immediately the error occurs. I can repeat this pattern.
something to do with high rpm vs low torque/low speed configuration ??

Conceptually, if we were talking about an internal combustion engine, sure! Electric motors play by a slightly different set of rules - as you're finding.

Because I know nothing about Bafang 250w motors, I'm going to take a wild guess that we're talking about a mid drive? That's why you are getting different results with the different rear/cassette gears?

Regardless, electric motors are strongest/make the most torque at low rpm's. They don't have to get up on to the power band, nor are they required to run within a narrow range of rpm's (power band) like an internal cumbustion (IC) engine requires.

It sounds to me like you are are on REALLY marginal power considering the task at hand, and the fact you've found a way to get up this hill at all is a good thing. What happens when you slow the motor even more in the next smaller gear?

There's also a question about exceeding the max amperage capabilities of the battery, or a setting in the controller?

 

[Sideview07]

@AHicks - u meant that motor torque probably reach it limit with me (90kg+) + bike (20kg) on it ? ha ha .. its torque only 80Nm. There are some parts of hills that really challenging 20% gradient, probably need to push coz i could hear the motor grinding.

Amp wise - it was set to max 20A per motor spec and i tune it down to 18A but still happens. Maybe i try to set it lower to avoid controller trip.

 

[Sideview07]

BTW this the mid drive that i have at the moment on my bike with the spec at page 11

 

[AHicks]

@AHicks - u meant that motor torque probably reach it limit with me (90kg+) + bike (20kg) on it ? ha ha .. its torque only 80Nm. There are some parts of hills that really challenging 20% gradient, probably need to push coz i could hear the motor grinding.

Amp wise - it was set to max 20A per motor spec and i tune it down to 18A but still happens. Maybe i try to set it lower to avoid controller trip.

Given your local conditions, I'm betting lowering that amp setting will make your motor/controller last MUCH longer than the way it's currently set....

 

[Artie13]

Getting error 21 when riding a ejoysport Bafang Max 35V250W on the lowest gear (36T x 42T largest cog) when climbing uphill. It gets jerky like power cut in and out. Eventually within secs screen reboot. Changed the chainring to 32T and now even the 2nd largest cog 36T would have the same problem. What could possibly cause this and what config that i can go try to change ?

I also have this problem, I changed my rear cassette, 38t and rear 37t fine no problems, but 38t and 46t i get same problem, did you solve the problem.. cheers

 

[Surly]

error 21 is a speed sensor error, check the alignment and rotation of the magnet on your spokes, maybe check connection too. It's not the motor or anything to do with the cassette.

 

[PedalUma]

These are cadence sensor motors. It likes cadence. When this happens, how fast are you pedaling? If you are pounding it will spike the controller. These motors also have lag and surge. At extra low rotational rates of the wheel magnet it can't predict your speed from RPM with the speed sensor.