{"product_id":"xiaomi-hongmi-4-replacement-battery-385v-4000mah-li-polymer","title":"Xiaomi Redmi 4 BN42 Replacement Battery 3.85V 4000mAh","description":"\u003cdiv class=\"bpw-desc\"\u003e\n  \u003ch2 class=\"bpw-desc-h2\"\u003eXiaomi Redmi 4 — 3.85V Li-Polymer Replacement Battery (BN42)\u003c\/h2\u003e\n\n  \u003cp class=\"bpw-desc-lead\"\u003eThis is a 3.85V, 4000mAh Li-Polymer replacement cell for the Xiaomi Redmi 4 (Hongmi 4), including the Standard Edition, Standard Edition Dual SIM, and Standard Edition Dual SIM TD-LTE variants. It replaces the original BN42 cell when capacity fade causes shortened screen-on time or forces overnight charging just to reach a full day. Rated at 15.4Wh, it matches OEM voltage and capacity specifications exactly.\u003c\/p\u003e\n\n  \u003cul class=\"bpw-desc-bullets\"\u003e\n    \u003cli\u003e\n\u003cstrong\u003eRedmi 4 Standard Edition compatibility:\u003c\/strong\u003e\n    All variants listed — Standard, Dual SIM, and TD-LTE — use the same BN42 cell with the same connector pinout, BMS handshake protocol, and physical footprint. One battery covers the entire Redmi 4 Standard Edition line without modification.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eBench tested on actual hardware:\u003c\/strong\u003e\n    We ran full charge-discharge cycles through a Redmi 4 unit and confirmed the BMS accepted the cell without triggering fault flags. The fuel gauge IC completed its first read pass without error codes and charge termination occurred at the correct cutoff voltage.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eFuel gauge recalibration on first use:\u003c\/strong\u003e\n    On first installation, disable fast charging and run one complete discharge down to automatic shutdown, then charge to 100% on standard 5V\/1A input. This lets the coulomb counter recalibrate against the new cell's actual discharge curve before high-current charging is introduced.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003chr class=\"bpw-desc-divider\"\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eWhy the Redmi 4 reports wrong battery percentage after a cell swap\u003c\/h3\u003e\n\n  \u003cp class=\"bpw-desc-p\"\u003eThe Redmi 4 uses a coulomb counter in the PMIC to estimate state-of-charge. That counter was calibrated against the original BN42 cell's discharge curve over hundreds of cycles. A fresh replacement cell has a different internal resistance profile, so the existing calibration data no longer maps accurately to real voltage. The phone may display 40% while the cell is already below 3.5V under load. One full discharge-charge cycle on standard input resets the learning registers and restores accurate percentage reporting.\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 modem, display, or both draw a current spike the new cell cannot sustain at that state-of-charge without dropping below the BMS low-voltage cutoff threshold — typically 3.0V per cell. The fuel gauge still shows 20–30%, but the cell voltage collapses under load faster than the counter updates. It is not a faulty battery. Run the recalibration cycle first: discharge to auto-shutdown, then charge uninterrupted to 100% at 5V\/1A before returning to normal use.\u003c\/p\u003e\n\u003c\/div\u003e","brand":"BatteryWeb","offers":[{"title":"Warranty 1 Year","offer_id":43392024346714,"sku":"BWCS-MUM414XL-1","price":25.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 2 Year","offer_id":43392024379482,"sku":"BWCS-MUM414XL-2","price":28.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 3 Year","offer_id":43392024412250,"sku":"BWCS-MUM414XL-3","price":31.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0674\/4775\/0746\/files\/BW-CS-MUM414XL-1.webp?v=1779142944","url":"https:\/\/batteryweb.com\/products\/xiaomi-hongmi-4-replacement-battery-385v-4000mah-li-polymer","provider":"BatteryWeb","version":"1.0","type":"link"}