To import a battery from HOMER 2:
1) Create a .hmr file with the battery in it. You can add other components to the model if you like, but you don't have to. You can move more than one component at a time this way.
2) Open the .hmr file in HOMER Pro. In the "Open HOMER File" dialog, you'll need to change the file type from *.homer to *.hmr with the small dropdown menu at the bottom of the window.
3) Go to the component menu for the battery, and click on the button "Copy to library". The item is now saved in your HOMER Pro library.
The process is similar for most of the types of components in the library.
To model a new battery or modify an existing battery in HOMER Pro 3.1:
Go to the library, then under the "components" menu, open "battery". You can't modify built-in library items, but you can select the item you want and click the "copy" button. The procedure is the same if you want to create a brand new battery: start by copying an existing battery and go from there. The battery designer here has a lot in common with the one in HOMER 2.