diff --git a/src/plexosdb/db.py b/src/plexosdb/db.py
index 7052793..aa363d7 100644
--- a/src/plexosdb/db.py
+++ b/src/plexosdb/db.py
@@ -352,7 +352,7 @@ def add_category(self, class_enum: ClassEnum, /, name: str) -> int:
             return self.get_category_id(class_enum, name)
 
         class_id = self.get_class_id(class_enum)
-        rank = self.get_category_max_id(class_enum) or 1  # Default to max rank of 1 if no category exists
+        rank = self.get_category_max_id(class_enum) + 1 or 1  # Default to max rank of 1 if no category exists
         params = (class_id, name, rank)
         placeholders = ", ".join("?" * len(params))
         query = f"INSERT INTO {Schema.Categories.name}(class_id, name, rank) VALUES({placeholders})"
diff --git a/src/plexosdb/enums.py b/src/plexosdb/enums.py
index 308d878..d4042a4 100644
--- a/src/plexosdb/enums.py
+++ b/src/plexosdb/enums.py
@@ -145,9 +145,14 @@ def str2enum(string: str, schema_enum: type[Enum] = Schema) -> Schema | None:
 
 def get_default_collection(class_enum: ClassEnum) -> CollectionEnum:
     """Return default collection for class."""
-    # Special case for Data File
-    if class_enum == ClassEnum.DataFile:
-        return CollectionEnum.DataFiles
+    # Special cases for Data File and Battery objects
+    special_cases = {
+        ClassEnum.DataFile: CollectionEnum.DataFiles,
+        ClassEnum.Battery: CollectionEnum.Batteries,
+    }
+
+    if class_enum in special_cases:
+        return special_cases[class_enum]
 
     collection_name = f"{class_enum}s"
     if collection_name not in CollectionEnum.__members__: