How can I backup WhatsApp files to free some space? I have an old 2.3 gingerbread phone and I'd like to free some memory from time to time. What's the recommended procedure to backup files from WhatsApp and free space?
(extra: Better if I wouldn't have to go again through the whole send-SMS-registrate process)
WhatsApp automatically makes backups every day at 4 AM and stores them in the WhatsApp folder of your Android phone. This folder is either located in your device's internal memory or external microSD card.

If you have a external sd-card, then you can back up your WhatsApp chat history on to sd-card.


  Go to WhatsApp > Menu Button > Settings > Chat settings > Back up
  chats.


For phones that have an internal memory / internal SD card (like most Samsung devices), you will need to transfer the WhatsApp folder from your phone. There are 2 possible ways for this:
1. You can use File Explorer app to do this or  
2. You can copy files to to your storage card in Windows. Please find details of how to do so in this Link.

This way you are freeing the device's internal memory and it is one of the recommended ways of backing up the chat as per FAQ of WhatsApp.