[Rant] VanMoof S3 Battery Drain / Dead Bike issue

It's British winter, it's wet and cold. 

Don't suppose anyone sane would be riding their bike out during this weather. 

Needless to say, I stored my bike on the wall and didn't touch it for a long while. 

I expected my VanMoof S3 electric assisted bicycle to be drained of battery when I take it back out.

No biggie, I thought to myself, just plug it in, charge, and off we go again. 

Nope. No no no. Nope, it doesn't work like that with the pretty face S3. 

If you were unlucky like me and allowed your VanMoof battery to fully drain, your bike might never charge and becomes a super dumb single gear heavy two wheeled paper weight. 

This is the second time my bike died of this cause, last time I sent it back to VanMoof to be fixed.

They applied some firmeware update which was supposed to have fixed this issue. 

Yet it reoccurred! So I did some digging to see what is actually going on.

Based on my diagnosis, this is what happened:

1. VanMoof S3 has 2 internal batteries, one main big battery that powers the front wheel while cycling, another small battery for the electronics (they call it the Smart Cartridge, for reasons I'll cover below it's more suitable to be called dumb cartridge, but I digress). 

2. During normal operation, the bike electronics boot up and charges both batteries. The state of charge of the big battery does not play a role in the boot up process.

3. The bike electronics boot up process relies heavily on the small battery having sufficient charge - this is quite common in modern electronics as the battery acts as a capacitor to supply enough power during power surge (e.g., during boot up). 

4. For whatever design reasons unknown to anyone, the bike electronics do NOT check or care about the state of charge of the small battery when booting up, which results in boot -> crash -> boot -> crash loop. 

5. The electronics does pump a very small amount of charge into the small battery in between the crashes, so if the small battery is only marginally discharged below the required capacity, you might get away with leaving the bike plugged in over night, and let it trickle charge.

The small battery (3.7v 3.89Wh) inside the VanMoof S3 "Smart" cartridge.

6. From my testing, the battery voltage goes up and down with every boot / crash loop, but over time it goes up 0.001v every 60 seconds or so. Generous, I know. 

At the end of the day, this is a very bad electronics design by VanMoof. 
This could simply be prevented if the electronics wait for enough charge in the small battery before attempting a boot, as is the case with many electronics! Phones, laptops, you name it!

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Day 1 update

My small battery is currently sitting at 2.750v which is 0.95v away from the rated 3.70v. 

Assuming this trickle charge rate is constant, I hope the small battery gets enough charge to hopefully power back on in ~16 hours... fingers crossed. 

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Day 2 update

It has been around 12 hours, the Dumb Cartridge is still doing the boot / crash loop. 

This does not look promising.

Measuring the battery voltage it is now sitting at around 2.850V, waaaay lower than what I had expected, which means the cartridge isn't really pumping much energy into the battery. 

#shitdesign, fuck VanMoof.

I'm now resorted to something I didn't want to do - manually charging the Lithium cell. 

I don't have a proper charger, and at the same time I didn't fancy paying £40 or so for a cell charger, so I improvised with an existing buck/boost converter. 

(Yes I know this is really far from ideal and it is a massive fire hazard, this is a one off use case that I hope never to have to repeat in the future.)

I set my buck/boost to mimic a 0.2C charge programme: 

Voltage limit: 4.2V

Current limit: 0.2A (I started with 0.1A but gradually increased to 0.2A)

Since I didn't fancy having a fire in my house if things went wrong, this is happening in the garden right now... 

Warning: don't do this at home! Do this in the garden instead.

I hope a 20% charge will be adequate to let the Dumb Cartridge to boot, so I will be unplugging once the battery stabilises at just over 3.70V. Reporting back soon...