{"product_id":"infocus-m521-replacement-battery-38v-2400mah-li-polymer","title":"Infocus UP130039 M521 Replacement Battery 3.8V 2400mAh","description":"\u003cdiv class=\"bpw-desc\"\u003e\n  \u003ch2 class=\"bpw-desc-h2\"\u003eInfocus M521 — 3.8V Li-Polymer Replacement Battery (UP130039)\u003c\/h2\u003e\n\n  \u003cp class=\"bpw-desc-lead\"\u003eThis 3.8V, 2400mAh Li-Polymer cell replaces the original battery in the Infocus M521 smartphone. It uses OEM part number UP130039 and fits the M521 directly. Voltage and capacity match the original spec: 3.8V nominal, 9.12Wh total energy.\u003c\/p\u003e\n\n  \u003cul class=\"bpw-desc-bullets\"\u003e\n    \u003cli\u003e\n\u003cstrong\u003eInfocus M521 fitment:\u003c\/strong\u003e\n    The M521 uses a fixed connector pinout tied to this specific cell footprint — 73.00 × 54.26 × 4.63 mm. The BMS on the replacement communicates over the same two-wire bus the phone expects, so the charge IC accepts the cell without rejecting it as unknown hardware.\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 M521 platform. The BMS held cutoff at the correct low-voltage threshold and the charge IC reached full termination without false-positive completion signals.\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. The M521's fuel gauge IC is calibrated to the old cell's discharge curve — one full uninterrupted cycle lets the coulomb counter reset its endpoint references against the new cell before any high-current charge session runs.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003chr class=\"bpw-desc-divider\"\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eWhy the M521 reports wrong battery percentage after a cell swap\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eThe M521 uses a coulomb-counting fuel gauge IC that tracks charge by integrating current over time against a stored discharge curve. That curve was built around the original cell's internal resistance and capacity. When a new cell goes in, the stored curve no longer matches the actual discharge behaviour, so the percentage reading drifts. One full discharge from 100% to auto-shutdown, followed by an uninterrupted charge to 100%, forces the IC to rewrite its endpoint calibration against the new cell.\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 fuel gauge IC still holds the old cell's voltage-cliff map. The new cell may hit a sharper voltage drop under modem or screen load at a different state of charge than the original. The phone's protection circuit reads that voltage sag as a hard cutoff, even though reported percentage still shows charge remaining. Run the recalibration cycle described above — after one full cycle, the IC re-maps the cliff point to match the new cell's actual low-voltage behaviour.\u003c\/p\u003e\n\u003c\/div\u003e","brand":"BatteryWeb","offers":[{"title":"Warranty 1 Year","offer_id":43392111902810,"sku":"BWCS-IFM521SL-1","price":25.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 2 Year","offer_id":43392111935578,"sku":"BWCS-IFM521SL-2","price":28.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 3 Year","offer_id":43392111968346,"sku":"BWCS-IFM521SL-3","price":31.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0674\/4775\/0746\/files\/BW-CS-IFM521SL-1.webp?v=1779143848","url":"https:\/\/batteryweb.com\/products\/infocus-m521-replacement-battery-38v-2400mah-li-polymer","provider":"BatteryWeb","version":"1.0","type":"link"}