WhatsApp lists only 2 ways to restore previously backed up WhatsApp data.
- Google Drive
- Local Backup
The following is an excerpt from a chat with WhatsApp customer support team with regards to chat restoration.
You may only restore your local backup by using the same mobile telephone number as stored in your previous backup. If you are trying to restore from Google Drive, please make sure you are using the same mobile telephone number and Google account used to create the backup.
Please make sure your WhatsApp backup files are stored in WhatsApp/Databases folder. The location of WhatsApp folder is determined by your device. It is normally located in your “Internal Storage”, or if you have an older device it may be located in your “SD card”. Please note, when changing to a newer handset, you may have to move the folder from your SD card to your internal storage.
We are unable to merge your backup files, you may only restore from one file at a time.
Backup files are only readable by restoring them in WhatsApp.
If your device is plugged into your computer at night, low on storage space or is set to sleep with no background data; WhatsApp will not be able to backup your chat history.
If your restoration is successful but you are missing some chats, it is possible that your database backup was corrupted and these are the only messages we can restore for you.
If you cannot uninstall WhatsApp on your device, you may go into your phone's [Settings] > [Applications] > [WhatsApp] > [Clear Data]. Once you open WhatsApp again, it will prompt you to set up as if it were a fresh install.
Please note, if you are unable to restore your chats, we can not restore your chats, contacts, call history, or multimedia files for you.
Note If your restoration is successful but you are missing some chats, it is possible that your database backup was corrupted and these are the only messages we can restore for you.
If your backup was successful but not all images were restored then likely your phone did not fully backup your images or the backup itself was corrupted.There's really no reprieve for this. I'd advice you to store your internal backup in an external storage device when you're about to format your phone.