The is the Gentex Pureflo PR05806SP spare battery pack for the ESM+ and ESM series of PAPR's. It is a 5 cell Nickel Metal Hydride battery delivering 6V and 2.2Ah.
They are sold by the each but Pureflo Helmets require (2) each battery packs to operate.
This was was formerly p/n PR02030SP.
This is also the battery for the discontinued PR02000 series PAPR's. The old part number was PR01270SP.