diff --git a/docs/FSD.md b/docs/FSD.md
index 82dc15b..162d239 100644
--- a/docs/FSD.md
+++ b/docs/FSD.md
@@ -54,6 +54,7 @@ Each brand-type-chemistry combination can have batteries in three states:
 - Can select from predefined chemistries (NiMH, Li-ion, etc.) or add custom
 - Can enter brand name
 - Can set initial quantity and status
+- If the same brand-type-chemistry combination already exists, the quantity is added to the existing group (upsert behavior)
 
 ### US-02: View Inventory
 > As a user, I want to see all my batteries at a glance so I know what's available.
@@ -87,6 +88,14 @@ Each brand-type-chemistry combination can have batteries in three states:
 - Mark charging complete (moves to available)
 - Optional: track charging start time
 
+### US-06: Add Batteries to Existing Group
+> As a user, I want to add more batteries to an existing group when I purchase more or find additional batteries.
+
+**Acceptance Criteria:**
+- When adding batteries with same brand-type-chemistry as existing group, quantity is added to that group
+- User receives feedback indicating batteries were added to existing group vs new group created
+- Works seamlessly whether user intentionally or accidentally selects existing combination
+
 ## 5. Data Model (Conceptual)
 
 ```
diff --git a/docs/TSD.md b/docs/TSD.md
index 05450bb..346b9d9 100644
--- a/docs/TSD.md
+++ b/docs/TSD.md
@@ -193,7 +193,7 @@ POST   /api/brands             # Create brand
 
 # Battery Groups
 GET    /api/batteries          # List all battery groups (with brand/type/chemistry info)
-POST   /api/batteries          # Create battery group
+POST   /api/batteries          # Create or add to battery group (upsert)
 GET    /api/batteries/:id      # Get single battery group
 PATCH  /api/batteries/:id      # Update battery group
 DELETE /api/batteries/:id      # Delete battery group
@@ -309,6 +309,49 @@ async function assignToDevice(groupId: number, deviceId: number, quantity: numbe
 }
 ```
 
+### 6.4 Add Batteries (Upsert Behavior)
+
+When adding batteries, the system checks if a group with the same brand-type-chemistry combination exists:
+- **If exists**: Adds the quantity to the existing group's available count
+- **If not exists**: Creates a new battery group
+
+```typescript
+// POST /api/batteries - Upsert behavior
+async function addBatteries(brandId: number, typeId: number, chemistryId: number, quantity: number) {
+  // Check for existing group
+  const existing = await db
+    .select()
+    .from(batteryGroups)
+    .where(and(
+      eq(batteryGroups.brandId, brandId),
+      eq(batteryGroups.typeId, typeId),
+      eq(batteryGroups.chemistryId, chemistryId)
+    ))
+    .get();
+
+  if (existing) {
+    // Add to existing group
+    await db
+      .update(batteryGroups)
+      .set({ 
+        availableCount: sql`available_count + ${quantity}`,
+        updatedAt: new Date()
+      })
+      .where(eq(batteryGroups.id, existing.id));
+    return { ...existing, isNew: false };
+  } else {
+    // Create new group
+    const result = await db
+      .insert(batteryGroups)
+      .values({ brandId, typeId, chemistryId, availableCount: quantity })
+      .returning();
+    return { ...result[0], isNew: true };
+  }
+}
+```
+
+Response includes `isNew` flag to allow UI to show appropriate feedback message.
+
 ## 7. UI/UX Design Details
 
 ### Color Scheme
diff --git a/src/app/api/batteries/route.ts b/src/app/api/batteries/route.ts
index 72f25b5..8995641 100644
--- a/src/app/api/batteries/route.ts
+++ b/src/app/api/batteries/route.ts
@@ -1,6 +1,6 @@
 import { NextResponse } from 'next/server';
 import { db, schema } from '@/lib/db';
-import { eq, sql } from 'drizzle-orm';
+import { eq, sql, and } from 'drizzle-orm';
 
 export async function GET() {
   try {
@@ -56,19 +56,58 @@ export async function POST(request: Request) {
       return NextResponse.json({ error: 'Brand, type, and chemistry are required' }, { status: 400 });
     }
 
-    const result = await db
-      .insert(schema.batteryGroups)
-      .values({
-        brandId,
-        typeId,
-        chemistryId,
-        availableCount: availableCount || 0,
-        chargingCount: chargingCount || 0,
-        notes,
-      })
-      .returning();
+    // Check if a group with the same brand-type-chemistry already exists
+    const existing = await db
+      .select()
+      .from(schema.batteryGroups)
+      .where(
+        and(
+          eq(schema.batteryGroups.brandId, brandId),
+          eq(schema.batteryGroups.typeId, typeId),
+          eq(schema.batteryGroups.chemistryId, chemistryId)
+        )
+      )
+      .get();
 
-    return NextResponse.json(result[0], { status: 201 });
+    if (existing) {
+      // Add to existing group
+      const quantityToAdd = availableCount || 0;
+      const chargingToAdd = chargingCount || 0;
+      
+      await db
+        .update(schema.batteryGroups)
+        .set({
+          availableCount: sql`${schema.batteryGroups.availableCount} + ${quantityToAdd}`,
+          chargingCount: sql`${schema.batteryGroups.chargingCount} + ${chargingToAdd}`,
+          notes: notes || existing.notes,
+          updatedAt: sql`(datetime('now'))`,
+        })
+        .where(eq(schema.batteryGroups.id, existing.id));
+
+      // Fetch updated record
+      const updated = await db
+        .select()
+        .from(schema.batteryGroups)
+        .where(eq(schema.batteryGroups.id, existing.id))
+        .get();
+
+      return NextResponse.json({ ...updated, isNew: false }, { status: 200 });
+    } else {
+      // Create new group
+      const result = await db
+        .insert(schema.batteryGroups)
+        .values({
+          brandId,
+          typeId,
+          chemistryId,
+          availableCount: availableCount || 0,
+          chargingCount: chargingCount || 0,
+          notes,
+        })
+        .returning();
+
+      return NextResponse.json({ ...result[0], isNew: true }, { status: 201 });
+    }
   } catch (error) {
     console.error('Failed to create battery group:', error);
     return NextResponse.json({ error: 'Failed to create battery group' }, { status: 500 });
diff --git a/src/components/battery/AddBatteryModal.tsx b/src/components/battery/AddBatteryModal.tsx
index 20550fe..b6ed7c0 100644
--- a/src/components/battery/AddBatteryModal.tsx
+++ b/src/components/battery/AddBatteryModal.tsx
@@ -93,7 +93,15 @@ export function AddBatteryModal({ isOpen, onClose, types, brands, chemistries }:
         throw new Error(error.error || 'Failed to add battery');
       }
 
-      showToast('success', 'Battery added successfully');
+      const result = await res.json();
+      const quantity = (parseInt(formData.availableCount) || 0) + (parseInt(formData.chargingCount) || 0);
+      
+      if (result.isNew) {
+        showToast('success', `New battery group created with ${quantity} batteries`);
+      } else {
+        showToast('success', `${quantity} batteries added to existing group`);
+      }
+      
       onClose();
       router.refresh();