Extended capacity battery works for Polycom / SpectraLink 8400, 8440, 8450, and 8452 phones. Replaces OEM battery 1520-37215-001 and RS658. The 2350 mAh capacity rating is 31% higher than the1520-37215-001 and RS658 1800 mAh extended battery packs, and is an unheard of 104% higher than the 1520-37214-001 and RS657 standard battery packs.