Sony WF-C500 Teardown by 52Audio
52Audio has published their Sony WF-C500 teardown and with this model being a lower end device, there are some differences versus the other WF headsets from past years. Check the full teardown on 52Audio.
First looking at the exterior, there appears to be some extra plastic (yellow circle) on the edge of the earbuds in the teardown. I am not sure if this is an isolated incident or if the build quality is generally like this for the WF-C500 and other Sony devices made in Vietnam. The factory is an OEM company, and not a plant directly owned by Sony, so perhaps quality control is not as high as in Malaysia.
An interesting design feature is the hole for the microphone, which has been designed to minimize wind noise. For anyone who owns a pair, how is the wind noise versus other Sony earbuds and other brands? Does the new design work? I wonder if we will see something similar on the WF-L900 once it releases and other future WF headsets.
I forgot to mention the IPX4 rating (thanks for the reminder in the comments), but it does look like there is something on the earbud seam, could be glue but not sure if this is enough. There is no rubber or seal around the charging pins, but they are directed towards the face and should avoid direct exterior water but perhaps not sweat. Personally I would not subject these to too much water.
Looking at the inside of the charging case, it does not look like there is much extra space. but a wireless charging coil would fit, but it would need to connect directly to the board like in the WF-1000XM4. I do not think it would be possible to just add a charging coil to the board since it would be lacking the necessary components, but one could attach a wireless charging pad via the USB type-C port and cover the unit in a silicone case.
A more drastic option would be to somehow try and connect the wireless charging pad USB connection to the PCB board on the unit. But I am not sure that is possible and I have seen reports of some Chinese made Qi charging adapters catching fire. So I would not recommend this route.
Anyways, the battery is a Li-ion 621733 type. The numbers represent the dimensions, 6.2mm x 17mm x 33mm. The capacity is 300mAh/1.11Wh, rated at 3.7V. Googling for this battery type does show different battery capacities, with some even offering more than 300mAh, but these are on alibaba or aliexpress and I would bet that some of these larger capacity batteries are fake. But assuming you could get your hands on an authentic 621733 type in larger capacity, say 350 or 400mAh, then swapping it in should give you more recharges. But I am not sure if the battery protection board would have to be swapped over, or the one attached to the new battery would suffice. The only downside would be longer time to recharge the bigger battery.
