About this replacement
NUU Mobile F4L 4G LTE / S2801L — 3.8V Li-ion Replacement Battery (NUBS2801)
This is a 3.8V, 1200mAh Li-ion replacement battery for the NUU Mobile F4L 4G LTE and S2801L smartphones. It slots directly into devices where the original NUBS2801 cell has degraded or failed. Capacity figures come from the product data — 4.56Wh total energy at nominal voltage.
- F4L and S2801L compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — the NUBS2801 part number covers both. No adapter or modification is needed.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on an F4L unit. The BMS accepted the cell, charge termination triggered correctly at 4.35V, and protection cutoff engaged at the expected low-voltage threshold without dropping into lockout.
- Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one full discharge-to-charge cycle before using the phone normally. The F4L's fuel gauge IC holds the old cell's discharge curve in memory — one unconstrained cycle overwrites it against the new cell's actual capacity curve before high-current charging begins.
Sudden shutdown at 20–30% on the NUU F4L after a cell swap
A new Li-ion cell has a slightly different internal resistance profile than the worn cell the fuel gauge was calibrated against. When the modem fires a high-current transmission burst or the screen brightness spikes, the phone's load draws more current than the fuel gauge expects at that state-of-charge. Voltage sags below the protection threshold even though the displayed percentage reads 20–30%, and the phone shuts down. One full discharge-charge cycle — draining to automatic shutdown, then charging uninterrupted to 100% — lets the coulomb counter re-anchor its curve to the new cell and eliminates most of these mid-charge shutdowns.
Phone won't power on after the replacement battery sat in storage
Li-ion cells self-discharge during storage, and if the NUBS2801 cell dropped below approximately 2.5V per cell before installation, the BMS enters a deep-discharge lockout state and will not pass current to the phone. The fix is to apply a trickle charge via a known-good USB cable and charger for 15–30 minutes before attempting to power on — this brings cell voltage back above the BMS reactivation threshold, typically around 2.7–3.0V. Once the BMS releases lockout, the phone will boot and charge normally. If the device still won't respond after 30 minutes on charge, reseat the battery connector and try a second charger before drawing any other conclusions.