{"product_id":"bbk-vivo-x21-replacement-battery-385v-3200mah-li-polymer","title":"VIVO X21 Replacement Battery B-D7 3.85V 3200mAh Li-Polymer","description":"\u003cdiv class=\"bpw-desc\"\u003e\n  \u003ch2 class=\"bpw-desc-h2\"\u003eBBK Vivo X21 \/ X21 UD Series — 3.85V Li-Polymer Replacement Battery (B-D7)\u003c\/h2\u003e\n\n  \u003cp class=\"bpw-desc-lead\"\u003eThis is a 3.85V, 3200mAh (12.32Wh) Li-Polymer cell sourced to match the OEM part number B-D7 for the Vivo X21 and X21 UD series. It fits the standard X21, X21 UD, X21 UD Dual SIM, and X21 UD Dual SIM TD-LTE variants. The physical dimensions — 76.88 x 61.60 x 3.82mm — match the original chassis cavity without modification.\u003c\/p\u003e\n\n  \u003cul class=\"bpw-desc-bullets\"\u003e\n    \u003cli\u003e\n\u003cstrong\u003eX21 and X21 UD platform fit:\u003c\/strong\u003e\n    The standard and UD (under-display fingerprint) variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol, so one cell covers the full model range including Dual SIM and TD-LTE configurations.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eBench tested on actual hardware:\u003c\/strong\u003e\n    We ran this cell through full charge and discharge cycles on the X21 platform. The BMS accepted the cell without triggering fault flags, and the charge IC stepped through trickle, constant-current, and constant-voltage phases as expected.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eFuel gauge recalibration on first cycle:\u003c\/strong\u003e\n    On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This allows the fuel gauge IC to recalibrate its coulomb counter against the new cell's discharge curve before high-current charging is applied to an uncalibrated cell.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003chr class=\"bpw-desc-divider\"\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eSudden shutdown at 20–30% on the Vivo X21 after a cell swap\u003c\/h3\u003e\n\n  \u003cp class=\"bpw-desc-p\"\u003eThis is a voltage cliff failure. The fuel gauge IC still holds a charge map calibrated to the old, degraded cell. When the new cell's actual voltage under load drops faster than the IC expects at a given percentage, the phone sees a voltage it associates with near-empty and shuts down. The modem radio and AMOLED display are the two biggest draw spikes — both can pull enough current to trigger this. One full discharge down to automatic shutdown, followed by a full charge without interruption, resets the coulomb counter and resolves it.\u003c\/p\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eUSB fast charge not working on the first cycle after replacement\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eVivo's proprietary fast charge protocol negotiates between the charge IC and the adapter on the first handshake after a new cell is detected. Some BMS units ship in a conservative state and default to standard 5V charging until the first cycle completes. If fast charge is not activating, let the phone complete one full standard charge from below 10% to 100%. After that cycle, the BMS handshake flags clear and the charge IC will accept the higher voltage negotiation from the adapter on the next connection.\u003c\/p\u003e\n\u003c\/div\u003e","brand":"BatteryWeb","offers":[{"title":"Warranty 1 Year","offer_id":43392021397594,"sku":"BWCS-BKX210SL-1","price":24.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 2 Year","offer_id":43392021430362,"sku":"BWCS-BKX210SL-2","price":27.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 3 Year","offer_id":43392021463130,"sku":"BWCS-BKX210SL-3","price":30.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0674\/4775\/0746\/files\/BW-CS-BKX210SL-1.webp?v=1779142943","url":"https:\/\/batteryweb.com\/products\/bbk-vivo-x21-replacement-battery-385v-3200mah-li-polymer","provider":"BatteryWeb","version":"1.0","type":"link"}