Battery thingy

[PlainName]

My Amazfit Bip battery bulged enough to pop the front off, so I got a replacement and fitted that. But now it doesn't work - I presume I shorted something. Well, there is something shorted and I guess it was me, but that's all I know. The result is a component that comprises mostly charcoal.

The circuit below shows the relevant part. The battery is a Li-ion but there is a control or management board attached to the terminals. That supplies a connector which, for simplicity, I've shown as the battery. There is a cap, the ex-component and then another cap before the supply goes off around the watch.

My question is, what should the ex-component be? I am guessing a resistor or inductor, and I can't see that a resistor would make sense here so it is probably an inductor. If so, one would be used either for EMC or smoothing, and since this is all internal it's probably smoothing. Since it's 0201 (or was), would a bit of wire likely prove to be OK?

[shakalnokturn]

I'd check upstream if there is anything there for current limiting if not, most reasonable would be to fit a fuse or polyswitch where the gone component was.

[PlainName]

Isn't 0201 a bit small for a polyswitch or fuse?

[shakalnokturn]

Sorry I overlooked the footprint size...
Definitely too small for polyswitch, fuse may fit but I doubt that would have charred.
Ferrite SMD maybe...

[PlainName]

No probs. I guess it turned out to be fuse-like, albeit unintentionally

[Shock]

Did you put the battery on backwards? Could have been a diode for reverse current protection.

As it looks to be like a short has occurred you want to check if there is any lowish ohms measurement or short before a wire is added (check across both caps with battery D/C). If it was circuit protection that blew just adding a wire (or fuse) that handles more current will likely cause something else to die (until the fault is remedied that is). The "Macgyver method" works better on TV than in real life.

It's probably not crucial to the operation of the circuit a wire is there but sometimes you can glean what a blown component was previously by what is next in circuit on the positive rail. There are also images and hand drawn schematics of many pcbs these days so look for others repairs for an image.

[PlainName]

It's not really possible to put the battery on backwards. And the package size is a bit small for a diode. I can't see a reason for having circuit protection between two caps (I assume they are caps) - I would expect it to be before or after but not in the middle. A ferrite, OTOH, would be between two caps as in this circuit.

I also assume I shorted something on removing the old battery or fitting the new one. However, in clearing all that up I couldn't get rid of a short in that area. I think the PCB is burnt through to an underlying ground plane. Still fixable though, hopefully.