Electric scooters have long been a hacker’s friend, Xiaomi ones in particular – starting with M365, the Xiaomi scooter family has expanded a fair bit. They do have a weak spot, like many other devices – the battery, something you expect to wear out.
Let’s say, one day the scooter’s diagnostics app shows one section of the battery going way below 3 volts. Was it a sudden failure of one of the cells that brought the whole stage down? Or perhaps, water damage after a hastily assembled scooter? Now, what if you measure the stages with a multimeter and it turns out they are perfectly fine?
Turns out, it might just be a single capacitor’s fault. In a YouTube video, [darieee] tells us all about debugging a Xiaomi M365 battery with such a fault – a BQ76930 controller being responsible for measuring battery voltages. The BMS (Battery Management System) board has capacitors in parallel with the cells, and it appears that some of these capacitors can go faulty.
Are you experiencing this particular fault? It’s easy to check – measure the battery stages and see if the information checks out with the readings in your scooter monitoring app of choice. Could this be a mechanical failure mode for this poor MLCC? Or maybe, a bad batch of capacitors? One thing is clear, this case is worth learning from, adding this kind of failure to your collection of fun LiIon pack tidbits. This pack seems pretty hacker-friendly – other packs lock up when anything is amiss, like the Ryobi batteries do, overdue for someone to really spill their secrets!
I wonder if these same kind of faults crop up in EV Car batteries? Be a shame to have to pay tens of thousands of dollars to replace a 2 cent cap. (Of course I’m sure Tesla wouldn’t be overly upset if you did.)
Possibly and good garages will take the time to fix the cause rather than replace the pack. Remember there is a much longer warranty on EV packs, 8 years is I think a legal requirement across the EU, so they tend to be designed to outlive that period.
I thought the dealership would just tell you they have to replace the battery pack, and if you take it to an independent shop your warranty is void. This is the 21st century after all.
I guess it could happen, but I would hope they’d be using proper automotive grade components, which I don’t think Xiaomi is using on their scooters. these caps do fail now and then but it doesn’t seem to be such a common issue on cars, from my limited experience
Hey, for anyone doing this:
Those caps are part of the balancing circuit, that keeps the cells balanced. This is important for keeping the battery from overcharging any particular cell. The capacitor is used to shunt ac power between cells.
What likely happened is the capacitor failed short (ceramic caps often fail this way) making the voltage on that cell apear much lower than it was.
Removing it will fix that issue, but if you don’t replace the cap you won’t have any balancing between cells below that cap, and cells above it.
This will likely take a little bit of time to show up as an issue, but will eventually reduce the lifetime of the pack significantly.
If you fix electronics, don’t just remove caps without checking what they do first. Better to replace them.
not exactly sure, what you mean with “shunt ac power” but I assume you mean the RC-delay of this input filter that has to settle within 12.5ms. The proper explanation what these capacitors (Cc) do, is explained in the application note here: https://www.ti.com/lit/pdf/slua749 … just removing them is still a bad idea for sure as explained above as the reading of the BMS will get noisy at best.
No. This is not a capacitive balancer. The caps are there for noise filtering. The balancing is done with resistors and is wasted as heat. The caps are there “for good measure” but the citcuit can work without them.
Mechanical failure would seem unlikely, those look like 0603 sized caps or maybe even 0402, you don’t really get mechanical failures in multilayered ceramics until the cap’s area is quite big (1206 or bigger) and therefore really notices it when the board flexes or vibrates. The smaller ones (0402…) do ofcourse have lower voltage ratings at any given capacitance than bigger ones, so maybe it is an occasional voltage spike too many which brings these to their demise. As vibrations for bigger MLCCs go though, I’d be interested to know if vibrations of the whole board at once (all parts of PCB uniformly accelerating, like with a board mounted well at multiple screw points and without any heavy components to give inertia away from the mounting holes) ever cause cracking of the ceramic layers, or if such failures only ever happen when the vibrations are such as to cause a flexing action in the PCB.
Untrue, capacitor short-circuit is one of the most failure mode in mobile devices (smartphone, etc). And those capacitors are usually 0402 or smaller
I have seen several caps fail that are that close to the edge of a board. Potentially it’s the problem here as well? I tend to keep some margin where I can.
moisture would also likely creep in from the edges. Maybe it is not the cap itself.