This has never happened with my appliances (I have 3!). However, I don't use them every day as I still have a robot. Actually, you should just recharge them as normal. The manufacturer probably knows whether you can replace the battery.

I don't have this problem... my device works perfectly.
It's best to claim the warranty.

Unfortunately, I can't help you there either. You could undo the screws and then probably open the device. But to be honest, I don't think the battery is replaceable at this price. A replacement battery usually costs more. It's just a cheap device, so you can't expect much. Of course, it's not ecological to replace the device just because of the battery. And even if the manufacturer could replace the battery, it would probably cost more than the price of the device.