{"product_id":"fitbit-alta-replacement-battery-37v-36mah-li-polymer","title":"FitBit Alta FB406 Replacement Battery 3.7V 36mAh","description":"\u003cdiv class=\"bpw-desc\"\u003e\n  \u003ch2 class=\"bpw-desc-h2\"\u003eFitBit Alta — 3.7V Li-Polymer Replacement Battery (FB406)\u003c\/h2\u003e\n\n  \u003cp class=\"bpw-desc-lead\"\u003eThis is a 3.7V, 36mAh lithium-polymer cell that replaces the original FB406 battery inside the FitBit Alta fitness tracker. It fits the Alta wristband directly, restoring power to the step counter, sleep monitor, and activity display. Use the capacity figure from the product data — 36mAh — not third-party listings that round up.\u003c\/p\u003e\n\n  \u003cul class=\"bpw-desc-bullets\"\u003e\n    \u003cli\u003e\n\u003cstrong\u003eAlta platform fit:\u003c\/strong\u003e\n    The FitBit Alta uses a compact LiPo cell on a 3.7V rail with a fixed connector footprint. The FB406 part number ties to that specific cell shape — 16.00 x 10.40 x 4.40mm — so the connector seats without forcing the housing.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eBench tested on actual hardware:\u003c\/strong\u003e\n    We ran this cell through the Alta's charge circuit and confirmed the BMS accepted the new cell without a fault flag. The fuel gauge IC completed its initialisation handshake after a full charge cycle from near-zero.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003ePost-swap charge requirement:\u003c\/strong\u003e\n    After fitting this cell, place the Alta on its magnetic charger immediately and charge to 100% before wearing it. The fuel gauge IC on the Alta cannot set an accurate reference point unless it completes one full charge cycle from near-zero following a cell swap — skipping this causes the percentage display to read incorrectly from day one.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003chr class=\"bpw-desc-divider\"\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eWhy the Alta fuel gauge reads wrong after a cell replacement\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eThe Alta uses a coulomb-counting fuel gauge IC that tracks charge relative to a stored reference. When the original cell is removed, that reference is lost. The new cell arrives at a partial state of charge, and the IC has no baseline to map against. Without a full charge cycle to 100% immediately after swap, the gauge calibrates against the wrong starting point and will show inaccurate percentages — sometimes jumping from 80% to 0% mid-day.\u003c\/p\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eAlta charge contacts not making on the magnetic charger after reassembly\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eThe Alta's magnetic pogo-pin charger relies on precise contact alignment at the back of the tracker module. If the internal ribbon or battery pack shifts during reassembly, the back plate sits fractionally off-centre and the charger pins land on the housing edge rather than the contacts. The watch will not charge and may show no response at all. Re-open the tracker, reseat the battery flat against the frame, and close the housing evenly — then confirm the charger snaps to centre before leaving it to charge.\u003c\/p\u003e\n\u003c\/div\u003e","brand":"BatteryWeb","offers":[{"title":"Warranty 1 Year","offer_id":43416168824922,"sku":"BWCS-FTA406SL-1","price":23.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 2 Year","offer_id":43416168857690,"sku":"BWCS-FTA406SL-2","price":26.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 3 Year","offer_id":43416168890458,"sku":"BWCS-FTA406SL-3","price":28.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0674\/4775\/0746\/files\/BW-CS-FTA406SL-1.webp?v=1779760547","url":"https:\/\/batteryweb.com\/products\/fitbit-alta-replacement-battery-37v-36mah-li-polymer","provider":"BatteryWeb","version":"1.0","type":"link"}