About this replacement
LG Q70 / K50S Series — 3.85V Li-Polymer Replacement Battery (BL-T45)
This is a 3.85V, 3900mAh Li-Polymer replacement cell for the LG Q70, K50S 2019, X540EMW, LMX540HM, and over 36 compatible variants. It matches OEM part numbers BL-T45 and EAC64578501. If the original cell is swelling, dropping to empty by midday, or shutting off unexpectedly, this is the direct swap.
- Q70 / K50S platform fit: These models share the same battery bay geometry, connector pinout, and BMS handshake protocol. The fuel gauge IC reads cell state-of-charge over the same communication line, so the replacement cell integrates without firmware conflicts across the listed variants.
- Bench tested on actual hardware: We cycled this cell on a Q70 unit through charge and discharge under screen-on, modem-active, and idle load conditions. The BMS held cutoff correctly at both ends and did not trigger false overvoltage protection during USB charging at 5V/2A.
- First-cycle fast charge protocol: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate its coulomb counter against the new cell's actual discharge curve before high-current charging pushes into an uncalibrated state.
Sudden shutdown at 20–30% on the LG Q70 after a cell swap
This happens when the fuel gauge IC is still mapped to the old cell's voltage curve. The new cell hits a voltage cliff under modem or display load before the reported percentage reaches zero. The phone reads 25% but the cell voltage has already dropped below the minimum sustainable load threshold. One full slow discharge-to-cutoff followed by a full charge corrects the coulomb counter and eliminates premature shutdowns.
USB-PD or fast charge not engaging on the first cycle after replacement
Some LG Q70 units will only negotiate fast charge once the BMS has completed one standard charge cycle on a new cell. The charge IC is conservative — it defaults to 5V/0.5A until it confirms cell impedance is within expected range. Charge once at standard rate using the stock cable and charger. On the second cycle, fast charge resumes at the normal handshake voltage, typically confirmed when the charge LED changes colour or the notification shows "Charging rapidly."