I have a Lumia 535 with Windows 10 Mobile. When my battery is at 30%, the system shows a low battery warning and switches off automatically. How can I fix this problem? I have searched Google but haven't found a solution.
You need to replace the battery with a new one and everything will work fine. Also, you'll be able to use the phone for a full day without charging again.
I have the exact same problem with my Lumia 930 with Windows 10 Mobile.
The problem is the battery, that has been used for a long time. The longer you use your battery (we're talking months/years) the more times you've charged it, consuming its charging cycles. Todays standard batteries can only be charged a limited number of times, meaning that you have to replace the battery every other year.
Replacing the battery in newer phone is harder than in older phones, you probably need tools to do it (and maybe even a hairdryer) but there are many videos out there showing how to do it for every model. You can also visit a mobile repair shop and have the people there do the job for you.
Batteries can be found cheaply online and are often a bit expensive in Store (that would be if they happen to have the battery you're looking for as there are thousands of different models).