Amazon.com: Synergy Digital Camera Battery, Works with Pentax Optio 550 Digital Camera, (li-ion, 3.7V, 1800 mAh) Ultra Hi-Capacity, Compatible with Fuji NP-120 Battery : Electronics
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3.7V battery for PENTAX Optio MX4, Optio 550 Li-ion NEW | eBay
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Pentax Rechargeable Lithium-ion Battery D-L17 39117 for Optio 550 / 555 / 450 / MX Digital Cameras 39117 — CampSaver
D LI7 Battery Pack for PENTAX Optio 450, 550, 555, 750, 750Z, Optio MX, MX4 Digital Camera|Digital Batteries| - AliExpress
Pentax Optio 550 Battery and Charger - Optio 550 Digital Camera and chargers