The original battery for the HP Pro x2 612 G1 tablet PC is the HP KK04XL, a 2-cell lithium-polymer battery designed specifically for this model. 
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HP KK04XL Battery – Key Specifications
• Voltage: 7.4V
• Capacity: 29Wh (approximately 3820mAh)
• Battery Type: 2-cell Li-Polymer
• Color: Black
• Weight: Approximately 296.7 grams
• Dimensions: 245.70 x 90.64 x 5.50 mm
• Battery Life: Up to 6–8 hours, depending on usage and system configuration