E-Bike stopped giving full power.

General Discussion about electric bicycles.
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bulfinb   1 µW

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E-Bike stopped giving full power.

Post by bulfinb » Oct 07 2018 2:54am

Hi everyone, I am having trouble with a converted e-bike, and I thought someone here might be able to help me out.

The set up:
The kit was bought from Leoncycles and has LG Das-kit written on the Display which goes on the dash.
Motor: Bafang 250 W 36V, BFSWX02
Magnetic peddle sensor plus controller.
I did not connect the trottle or break sensors.
Controller has a 30V low voltage cut off.

The display shows the speed, which it must get from the motor revolutions. From the display you can select the amount of assist with six different levels.

Problem:
The bike no longer gives full power, unless the speed is very low, around or less 8 km/h.
The display also does not always show the right speed. Often it shows 0, but this doesn't affect the amount of power. The power seems seems to still be determined by the actual speed. his problem with the speed display started long before I had any issues with power, so I just ignored it.

Edit:
I put a very similiar set up on two bikes. So I was able to use the second bike to trouble shoot the first. By connecting different parts of both bikes together I narrowed down two issues.

1. The speed measurement which is often wrong is from of the control box, or one of it's connections.
2. The low power loss at lower speed than usual, is from the motor, or possibly the connection to the motor.


Some more info:
The kit was very easy to fit, and worked great for over 1500 km or so. Then I had a problem where it would cut out over pumps, which I guessed was due to a bad connection to the battery. The connectors from the controller didn't fit well to those on the battery. So i got rid of them and solder the wires instead. This fixed the cutting out problem, but now I have the low power problem. Could it still be a problem with connection to the battery?

I put the bike under load and wobbled all of the main connections, none of the seemed to have an effect. Could it be the motor itself. I think those are usually very reliable so I reluctant to take it off and open it up.

One other point. In the display you can choose a wheel size. I have 28 " wheels, but I set them to 20 ", as this seemed like a simple way to turbo charge it. Then it assisted a little at around 30 km/h max which was great.

Thanks in advance for any advice.
Last edited by bulfinb on Oct 07 2018 7:22am, edited 1 time in total.

StuRat   100 W

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Re: E-Bike stopped giving full power.

Post by StuRat » Oct 07 2018 3:48am

Sounds like the magnetic speed sensor on the wheel??

bulfinb   1 µW

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Re: E-Bike stopped giving full power.

Post by bulfinb » Oct 07 2018 3:55am

StuRat wrote:
Oct 07 2018 3:48am
Sounds like the magnetic speed sensor on the wheel??
Hey, thanks for the reply.
There is no magnetic speed sensor on the wheel. The controller determines the speed from the motor itself, and the wheel sizes. Maybe it's the sensor in the motor though , yeah.

StuRat   100 W

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Re: E-Bike stopped giving full power.

Post by StuRat » Oct 07 2018 2:49pm

The motor speed depends on what gear you're in. It can't generate a road speed calculation.

There should be a magnet on a spoke, and a sensor that counts wheel revolutions, which then calculates road speed using the wheel diameter that you set in the display.

Without that magnet and sensor my bike (bbshd) has that behaviour you described.

Voltron   1 MW

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Re: E-Bike stopped giving full power.

Post by Voltron » Oct 07 2018 3:15pm

Many hub motors directly read the rpms from the motor itself, not from an external sensor, and calibrate the speed from the wheel size setting on the display. It's on crank drive motors the the speed depends on the gearing, so you need a wheel speed sensor.

Voltron   1 MW

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Re: E-Bike stopped giving full power.

Post by Voltron » Oct 07 2018 5:57pm

Can you run it with the display disconnected to rule that out?

oldskoolhead   100 mW

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Re: E-Bike stopped giving full power.

Post by oldskoolhead » Oct 07 2018 6:48pm

if the motor reads speed internally then could not this be a hall sensor fault?

oldskoolhead   100 mW

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Re: E-Bike stopped giving full power.

Post by oldskoolhead » Oct 07 2018 6:51pm

could also be just hall sensor connections are corroded or something?

bulfinb   1 µW

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Re: E-Bike stopped giving full power.

Post by bulfinb » Oct 08 2018 1:20pm

oldskoolhead wrote:
Oct 07 2018 6:51pm
could also be just hall sensor connections are corroded or something?
I think this might well be the problem.
It was under high current a few days ago for quite some time, and I noticed the cable to the motor even got a bit warm.

I opened up the motor and got a multimetre to check the connections. The hall sensor cables, HA, has a poor intermittent connection.
Would this cause the motor to operate at lower speed than usual?

Also there are 6 magnets inside the hub which are rotating with the wheel and which pass by a sensor, and that must be how it measures the speed. One of them fell off, which probably caused the incorrect speed readings.

bulfinb   1 µW

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Re: E-Bike stopped giving full power.

Post by bulfinb » Oct 08 2018 1:28pm

Voltron wrote:
Oct 07 2018 5:57pm
Can you run it with the display disconnected to rule that out?
Yes, I had a spare display, so I connected that, and the behaviour was eaxctly the same. Also, I appreciate the help.

Voltron   1 MW

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Re: E-Bike stopped giving full power.

Post by Voltron » Oct 08 2018 2:51pm

It's great when you find a nice obvious reason like that!

oldskoolhead   100 mW

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Re: E-Bike stopped giving full power.

Post by oldskoolhead » Oct 09 2018 5:10pm

yes they are often on tiny pins on a plug with other periferals such as the throttle brakes keyswitch etc using a flat ribbon style cable connector and these can corrode quite quickly, much better are the round water proof types found on others such as bafangs etc

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