Question

Why my smartphone stop working when m battery reduces to 30%?

Answer 1

It sounds like you have a bad battery to be honest. I don't know how old your phone is but you might want to get a battery replacement. In the mean time when your battery becomes low, try to disable non essential features on your phone like wireless, bluetooth, lower the brightness, close non essential apps.

Answer 2

Some phones system are made in such a way that requires a minimum battery percentage for some feature to operate properly. Especially the system's in-built applications. For instance the camera's  flash light wouldn't  work at a percentage lower than 30% but your calls and messages would be working properly. So it would now depend on which features or applications that are not working properly. In some cases you would need to shut down running processes and reduce the background light intensity.