Whatsapp chats only partially restored in new phone
Unfortunately, if your old phone is wrecked, I'm afraid you can't.

You were able to recover the data that was backed up.

It happened to me when I changed my phone - In my case, my old phone was still working, so all I had to do was turn it on, connect to internet and backup everything I wanted.

On Whatsapp, you go to Config->Conversations (or messages, not sure in English, since my app is in another language)->Backup. There you can check when it was your latest backup, and you can to force a backup.