Cowboy C3 battery teardown / disassembly and rear light fix

A bit over a week ago the rear light of my Cowboy v3 / C3 broke down. Everything else was fine.

Update on January 4th 2024: In the end I managed to fix the light. Check the bottom of the post for the conclusion. Thanks to reddit user icannotfindagoodname for the help in finding the issue!

After chatting with Cowboy support they offered me a new battery for 490 euros and that they would also send an offer with a discount, which appeared a couple days later. The discounted price was 367 euros. A bit much for a broken LED module! The battery was still totally fine.

So I'll attempt to fix it myself and I'll show you how to disassemble the battery here.

WARNING - DO NOT DO THIS AT HOME

Now I need to warn you to not try this at home. I have been working with electronics for a long long time and I know what to do and what not to do, and most importantly, know what I should not even try. Battery packs are not a joke and could lead to serious fire and health problems if not handled correctly. Real professionals will probably even look at my photos and say that I'm an idiot for doing this in my living room. I do not assume any responsibility for anything you do in your life, including opening up dangerous lithium ion battery packs.

WARNING - DO NOT DO THIS AT HOME

We start with the 4 screws at the bottom.

Great success.

Then remove these 4 screws here.

Here it gets complicated. There's only a tiny hole through which to operate.

I also pried open the top thinking that there would be easy access to the PCB connectors, but no. It's just the cylinder lock. The two compartments are totally isolated. No need to open this up unless there's any problem with your lock. Coincidentally the tiny hole that looks like a "insert paperclip to reset CMOS"  hole is just a water(?) drain. There's no reset button behind it.

I also removed the long screw, which you DO NOT NEED TO DO. It's just for the structural integrity of the battery pack and is not attached to the frame.

After a lot of prying through the tiny hole I managed to disconnect the connectors and remove the outside interface. This was the hardest part of the disassembly! It's really difficult to operate through a tiny hole and still be careful because you're near the main output terminals of the battery pack.

Now only the light is still attached. If you push REAL HARD through the hole you can eject the rear light which is glued down. Then gently remove the connector for the light. I didn't figure out how it worked before so I  slightly damaged some plastic of the lights. All in the name of science I guess.

Ok now how do you get the battery pack out of the enclosure? The fit is very tight and there's no surface to apply pressure to. You can't push from the inside and you definitely DO NOT want to stick metal tools anywhere near the cells. The side with the PCB has the most clearance from the edge of the frame. I figured that by slamming the frame down onto a hard edge (like the side of a workbench), the momentum of the cells will do the work and slowly slide out. And after 4 or 5 "gentle" slams the pack was indeed ejected.

Here are some shots of the pack and the PCB.

The white "paste" covers the leads that go to the batteries. I'm not replacing any cells so I'm not touching it.

There's a small push button here. Not sure what it's for.

Also an on/off switch for the entire pack.

Here's the pack.

This is where it's attached to the rear lights.

And here next to the enclosure.

Ok now we get to what we came here for... 

The rear light module in all its (broken) glory.

It's easily pried open.

Alright so here we just have 8 LEDs. It seems the voltage is about 1.5V tot 2V per LED.

Now I tested my LEDs and they all still work. Hm.. Could be the rear light module or the signal from from the bike's controller.

The red and black terminals of the LED module are hooked directly to the main battery terminals, so there's some power modulation from 36V down to a usable level for the LEDs. The green wire is controlled from the main PCB in the frame of the bike, the connection goes through one of the small terminals of the battery connector and then directly to the led module. The signal is a 3V PWM at 1khz. So it's probably the LED module that's broken. This is actually the first time I'm seriously using my oscilloscope and it was awesome!

The LEDs are wired in serial. When lighting up the first or last 7 of them with 12V it works, but when connecting all 8 of the LEDs I got a short circuit, which means that there's something wrong with my LED PCB. I suspect it's the power inductor that overheated and will try to fix it.

Here are some close-ups of the LED PCB.

Update: after some helpful comments on the reddit thread and chatting to user icannotfindagoodname, I desoldered some components and tested them in isolation. The diodes and resistors all seemed fine but the fuse (F1) showed weird  behavior. As I was seeing a short if I put a voltage across all of the LEDs I traced the circuit to see where those points connect. This turned out to be the capacitor. I shorted the fuse and put in a beefier 10 uF electrolytic capacitor I had lying around, I applied 32V and a PWM signal and voila, there was light!

I ordered new capacitors, a resettable fuse and a new 100uH inductor (the specs of the ZXLD1362 LED driver suggested a bigger one than the 68uH that was used. The battery voltage can apparently reach 42V.)

Here is what I ordered at Farnell:

It arrived within 2 days, pretty cool. Now I put everything in place and re-assembled the battery.

I put in the LED module last, so that I could reach the connectors through the hole. Then I glued it shut using hot glue. Then I noticed some small cracks in the black plastic enclosure for the LED module. This probably happened because of the force I applied when removing it. As another reddit user pointed out that his battery died in the end because some water got in that way, I put super glue on the cracks to hopefully prevent the same fate. On close inspection the glue is visible and it doesn't look as clean as before, but better than the entire battery becoming e-waste.

After the whole ordeal the light seems a bit more dim than before, though I might just be imagining that. It could also be because of using a slightly different resistor at R4 because I damaged it while desoldering and testing. I really need to buy some SMD soldering equipment.