The Nokia 7380 is a fashion cell phone that is not very practical. The Battery is not removable by normal mean and it's very difficult to find in the US. You can try eBay or search web sites in Europe.
- An old credit card with one corner sharpened
- Some tacky tape to protect the surfaces while you work on the phone.
- Power down the phone.
- Remove SIM card.
- Cover all shiny parts of the phone with some tacky clear Scott tape to protect it while you work on it.
- Find an old credit card and sharpen one corner a little.
To the left is the 7380.
The back cover part (B-Cover) is only snapped in.
It can be popped out using the sharpened corner of an old credit card. Use it to get between the seam running along the B-Cover. A good point to start at is the Nokia logo tag. Push the credit card corner into seam of B-Cover part then run it up and down to separate it from the hooks inside. Be careful not to put too much pressure on B-Part or you will break it.
Here is the back cover partially opened.
Note the little metal hooks on the "C-Cover" part. These are the places to work the credit card corner in to pop the B-Cover out. Note: The bottom where the charger plug in has no hook but some spring snap points so try to work this part out first.
B-Cover part removed.
Note the hook receptacle and snap-in points that help secure it to the body.
Inside the B-Cover
With B-Cover removed, the battery can be gently pulled out. Watch out for the Flash part.
Here is the battery removed.
Replace with a new battery and assemble in reversed order.
Reference: where to find repair manual and parts.