Unable to spin motor - Need help determining cause

[rickzor]

Hey Endless Sphere, I've been lurking these forums pretty heavily for the past three months. Finally making my first post, my new build has stopped working properly, and I'm pretty lost in fixing it. Spend all day trying to figure this one out, browsing old posts, Googling everything, but I can't find much. Maybe I'm just not looking for the right things, but I'm gonna try my luck here to see if anyone out there can give me more direct advice.

Some background:
Almost two months ago I bought a little kit from GreenBikeKit to throw on my Trek 3-Series mountain bike. I'm a college senior, live on a big hill and totaled my old car on Valentine's day. I've ridden electric bikes before, and thought it would be a great investment for my situation. I wanted one mostly to pedal along with and get to campus quickly without having to buy a parking pass. I didn't need or want much power, so I went with the Bafang SWXK 36V 250W front driving e bike conversion kit with a 36V8.8Ah bottle battery + charger. I knew from reading on these forums that the bottle battery isn't the most powerful or reliable, but I'm pretty good with electronics and thought I'd just replace the cells once they die with something more reliable, the bottle battery case was exactly what I needed, the cells that came with it were more so a bonus for me.

kit: http://www.greenbikekit.com/electric-bike-kit-1/front/bafang-swxk-36v-250w-front-driving-e-bike-kit.html
battery: http://www.greenbikekit.com/lithium-battery/li-ion/bottle-battery-36v.html

GreenBikeKit were absolutely awesome in answering questions, speedy at shipping, and even upgraded both my battery and bike kit shipping to express at no cost. They were pretty great.

Two weeks ago the kit arrived, I assembled it and it was AMAZING. Exactly the power, capacity and weight that I needed. I've charged it immediately after each ride, taking my battery into class/work with me and charging it before biking home, then charging it at home. I've been very careful not to overload the motor, always pedaling when going up hills or starting from a stop. Seriously love this thing. I plan to upgrade my aluminum fork (I've been reading about broken aluminum forks for weeks) to a steel rigid fork, but for now I've just got toque arms on it.

The problem:
This morning, I got up to go to class and got on my bike. The motor barely spun, jerking a little on its own when I pull the throttle and dying. I thought it was a dead battery, so I charged the battery. Charging looked completely normal, so when it was fully charged I put it on again. Same thing - pull the throttle, quick "whirr", and immediately the motor cuts out. I can't seem to get any movement if I put weight on the motor, but without weight it lurches a little and gives an initial spin, and dies. It's like the motor cuts out whenever it gets triggered. but I can flick the throttle and keep it spinning, just so long as it doesn't have weight on it.

I checked all my wiring, took the motor off, the battery off, unplugged and plugged everything back in, inspected all the cabling, and put everything back together again. No luck. I made sure nothing was getting hot when I applied the throttle (it's not) and that nothing gets hot when it charges (it doesn't). So now I'm stuck.

There was no indication anything was going wrong before this, all weekend I was riding my bike all over the place with no problems at all. Didn't get it wet (I rode through a snow storm on Wednesday, but it worked fine the many times I rode it after that) and It hasn't been dropped or hit or damaged in any way.

From reading other posts, it seems like it could be an issue with the BMS, the battery itself, possibly the controller, or the motor, so basically it could be anything. I don't have any other batteries, controllers or motors to test with, and I don't think there are any local shops that could help me out. I'm perfectly comfortable going the DIY route on this, I love building and fixing things, I just have no idea where to start.

Here's a video I just took of the problem: https://www.youtube.com/watch?v=zhE3lqWQxO0
I pull the throttle back, the motor lurches, dies and spins a little, then I let go of the throttle, and repeat. If I keep holding down on the throttle, nothing happens after the initial 'jerk', just slowly spins to a stop.
apologies for the portrait-style video embedded in a widescreen format. Blame the cell phone

Photo of controller:

Photo of the battery:

photo of the bike right after I built it

any help would be greatly appreciated, I do miss riding this thing.

 

[wesnewell]

Sounds like the controller is hitting LVC. Main cause of this is going to be the battery power. I can duplicate what you have by not connecting the main connection to my battery pack, with it only being connected through my precharge resistor. The controller hits LVC as soon as I give it throttle. So my guess is that either your battery pack isn't really charged, or the BMS is cutting it off under load. If you have a voltmeter, you need to test the voltages while applying throttle. If you see it sag way down, fix your battery/connections.

 

[rickzor]

Thanks for your response!

Just noticed another thing - when I plug the battery in, the charger immediately goes to green (charged) which it never, ever does, even on a full charge. So it definitely seems to be the battery, or BMS, causing the issues.

Next step - how do I figure out if it's a battery problem, or a BMS problem? Should I open it up and test each cell? Or is there a faster way?

 

[dogman dan]

One thing the green light can mean is the charger is unplugged, like you have a broken wire in the plug. That may mean you never got a charge.

 

[atom1025]

The charger lights can stay on even with a blown fuse.

Use a voltmeter to assure that the chargers output leads are live.

 

[dogman dan]

Yeah, a voltmeter is needed to figure out what has power and what doesn't.

I suspect an uncharged battery or a very badly unbalanced battery. This kind of bms equipped battery needs overnight charging from time to time. So it could have just gotten way out of balance if the charger is never left running long enough to balance it.

 

[rickzor]

This kind of bms equipped battery needs overnight charging from time to time. So it could have just gotten way out of balance if the charger is never left running long enough to balance it.

The last overnight charge I gave it was the night before, but that's still definitely something I'll look into. It seems pretty likely to cause the symptoms I'm having.
I know I don't have a lifepo battery, but are the steps at the 'troubleshooting' part of http://www.electricbike.com/lifepo4-battery/ a good way to test this battery? It seems like it's a good visual guide to troubleshooting lithium batteries in general.

From that article, I found this:

The undercharged cells are causing the BMS to shut off the power sooner than usual. Plug the BMS back in, and put the battery on the charger. Leave it charging for a long time, overnight for sure, but possibly for a week or more! If the battery fails to balance, you may have to resort to charging your battery one cell at a time to restore balance.

Even if the charger goes 'green' immediately, would it still be working towards balancing the battery? That bit got me a little confused. Also - Is there any way to see if it's a faulty BMS? That seems like a hard thing to test for without a working, spare BMS. Pretty sure the BMS is tightly integrated into the bottle case, I can't seem to find much about it online, not even a model number.

I'll do some voltimeter testing today or tomorrow and report back. Thank you everyone for your help, and answering my many questions! I'm very determined to get this running again.

 

[dnmun]

not hard to test a BMS but it is not likely to have failed if you have not done anything to it yet. usually they fail when people decide to do something with it and short them out to the battery.

but you need to verify that the battery is getting charge like they said. it could be blown fuse or loose or broken contact or connection.

 

[shov6]

I have the same issue, the charger is cutting and charging with only a few seconds of charging before dropping out. The wheel on the other hand is stuck, when cycling the other day without battery mounted the motor seems to be with load. i have to paddle with good effort to get anywhere and the cassette is almost stuck.

I opened the battery pack with the charger plugged in and could not find any loose wiring or obvious signs of malfunction.
My guess is the bms is broken but have no clue how to find a diagnose.

My battery is mostly charged over night and 90% of the times 100%.

The last ride with battery power was weird. It seemed like it lost power for a fraction of a second and the moment when it started up again, it only gave power to 2 phases. i only use pas sensor and had to stop paddling for a second and it could work for 2-10 seconds.

i REALLY hope someone could tell me why my motor is so hard to turn and if anyone know of different BMS compatible with the bottle pack.

 

[Racer_X]

shov6, where do you live? if you lived near me, I could take a look at it and tell you what is wrong.