How can I know the cause

Hi,

There may be a lot of reasons why the battery discharges more quickly than you think it should.

Is the WiFi or Bluetooth left permanently on if you’re not using it? If so they are constantly searching for new networks and devices and this uses more power. To check if they’re enabled or not, go to Settings > Connections > WiFi and Settings > Connections > Bluetooth

Do you leave the mobile data connection switched on? Unless you need constant access to the internet or send/receive numerous MMS messages (messages with pictures or videos) turn it off and only turn it on when required. Apps may be downloading data in the background, again using extra power and also affects your data allowance.

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Try starting and then using the phone in safe mode and check if battery performance improves. If it does then a downloaded app may be the cause of the problem. The trick is to find out which one. You will need to uninstall each downloaded app, one by one and test the phone in normal mode to see if it works better. When it does then the last downloaded app that was uninstalled was the cause of the problem. A downloaded app is one that you installed on the phone and didn’t come pre-installed on the phone.

It could also simply be that the battery is failing and needs to be replaced. Even rechargeable batteries have a lifespan. Here’s the ifixit 三星 Galaxy A70 电池更换 guide that shows how to replace the battery. Replacement batteries are available online. Just search for EB-BA705ABU to get results for suppliers that suit you.