{"product_id":"f-fook-f669-replacement-battery-37v-1200mah-li-ion","title":"F-Fook F669 Compatible Battery 3.7V 1200mAh Li-ion","description":"\u003cdiv class=\"bpw-desc\"\u003e\n  \u003ch2 class=\"bpw-desc-h2\"\u003eF-Fook F669 — 3.7V Li-ion Replacement Battery\u003c\/h2\u003e\n\n  \u003cp class=\"bpw-desc-lead\"\u003eThis is a 3.7V, 1200mAh Li-ion cell built to replace the original battery in the F-Fook F669 mobile phone. It restores power to a device that no longer holds a charge or shuts down unexpectedly. Dimensions are 53.00 × 33.90 × 5.70mm — confirm against your existing cell before installing.\u003c\/p\u003e\n\n  \u003cul class=\"bpw-desc-bullets\"\u003e\n    \u003cli\u003e\n\u003cstrong\u003eF-Fook F669 fitment:\u003c\/strong\u003e\n    The F669 uses a compact single-cell 3.7V nominal Li-ion pack. This replacement matches the voltage rail and physical footprint required by the phone's charge IC and connector orientation.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eBench tested on actual hardware:\u003c\/strong\u003e\n    We cycled this cell through charge and discharge on the bench. The BMS held cutoff correctly at both ends — no false full-charge termination and no undervoltage lockout during discharge under simulated load.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eFuel gauge recalibration on first use:\u003c\/strong\u003e\n    On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate discharge curve against the new cell before high-current charging begins on an uncalibrated state-of-charge register.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003chr class=\"bpw-desc-divider\"\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eWhy the F669 reports wrong battery percentage after a cell swap\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eThe F669's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The phone reads voltage and maps it against the old curve, so percentages appear incorrect or jump around. One full discharge-to-charge cycle forces the coulomb counter to reset and relearn the new cell's profile.\u003c\/p\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eSudden shutdown at 20–30% on the replacement cell\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eThis happens when the cell voltage drops sharply under load — typically during a call, screen-on, or data activity — faster than the fuel gauge IC expects. The phone's power management IC sees voltage fall below its cutoff threshold before the displayed percentage reaches zero. It is a calibration gap, not a faulty cell. Run one full cycle without fast charging and let the gauge IC recalibrate; the cutoff should stabilise closer to the 5–10% mark where cell voltage genuinely reaches 3.0V.\u003c\/p\u003e\n\u003c\/div\u003e","brand":"BatteryWeb","offers":[{"title":"Warranty 1 Year","offer_id":43404178915418,"sku":"BWCS-JTP127SL-1","price":23.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 2 Year","offer_id":43404178948186,"sku":"BWCS-JTP127SL-2","price":27.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 3 Year","offer_id":43404178980954,"sku":"BWCS-JTP127SL-3","price":29.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0674\/4775\/0746\/files\/BW-CS-JTP127SL-1.webp?v=1779369289","url":"https:\/\/batteryweb.com\/products\/f-fook-f669-replacement-battery-37v-1200mah-li-ion","provider":"BatteryWeb","version":"1.0","type":"link"}