What battery does a Miller welder use?

This depends entirely on which Miller PAPR (powered air purifying respirator) system you have — Miller’s welding-PAPR batteries are not interchangeable across generations, which is a well-documented trap in the Miller ecosystem:

• Gen 1 PAPR (used with the T94i-R and Miller hard-hat headtops) takes battery 244131, rated roughly 8 hours runtime and ~3 hours to charge, using charger 244132.
• Gen 2 / PAPR II takes battery 287178, also roughly 8 hours runtime, using charger 282077. This does NOT fit Gen 1 units.
• The standalone Face Shield PAPR (a face-shield-based system rather than a flip-front helmet) uses its own Li-ion battery, rechargeable in about 3 hours and rated for roughly 500 charge cycles, paired with its own charger.

These are manufacturer-listed pairings, each confirmed against Miller’s own owner’s manuals — always identify which generation and system you have (check the label/part number on your existing battery or blower assembly) before ordering a replacement, since visually similar batteries across Miller’s PAPR lines are not cross-compatible.