Motorola FL40 Moto X 3a Replacement Battery 3.8V 3500mAh

Motorola Moto X 3a / Droid Maxx 2 — 3.8V Li-Polymer Replacement Battery (FL40 / SNN5963B)

This is a 3.8V, 3500mAh lithium-polymer cell that replaces the original FL40 battery in the Moto X 3a, Moto X 3a Dual, Droid Maxx 2, and Moto X Play. It fits the same connector and physical footprint as the factory cell. Capacity is 13.3Wh — matching OEM spec.

• Moto X Play and Droid Maxx 2 shared platform: These models run the same MSM8939 power rail and use an identical BMS handshake on the FL40 connector. One cell revision covers all of them without firmware conflicts.
• Bench tested on actual hardware: We ran this cell on a Moto X 3a through three full discharge-charge cycles. The BMS accepted the cell on first contact, charge termination triggered correctly at 4.35V, and the coulomb counter tracked state-of-charge without dropout.
• Fuel gauge recalibration after install: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate its discharge curve against the new cell before high-current charging pushes current into an uncalibrated state.

Sudden shutdown at 20–30% on the Moto X 3a after a cell swap

This is a voltage cliff problem, not a capacity fault. Under heavy modem or screen load, the new cell momentarily drops voltage below the threshold the OS uses to trigger an emergency shutdown — even though the coulomb counter still shows charge remaining. The fuel gauge IC is still using the discharge curve from the old degraded cell, so its percentage estimate is off. Run one full charge and drain cycle with the screen on and mobile data active to force a full recalibration.

Phone not powering on after the replacement cell sat in storage

Lithium-polymer cells self-discharge during storage. If the cell dropped below 2.5V, the BMS enters lockout mode and the phone will not respond to the power button or a standard charger. Connect the phone to a wall adapter — not a PC port — and leave it for 20–30 minutes without pressing anything. The charge IC will trickle current into the cell until it clears the BMS lockout threshold, typically around 2.9V, after which normal charging resumes.