About this replacement
ZTE Q519C / Yuanhang 2 — 3.8V Li-Polymer Replacement Battery (515978)
This 3.8V, 4000mAh Li-Polymer cell replaces the original ZTE 515978 battery in the Q519C, Q519E, Q519, and Yuanhang 2 smartphones. It fits the same footprint at 80.80 × 58.94 × 5.16 mm and connects to the stock charge IC without modification. Capacity is rated at 15.2Wh, matching the original cell specification.
- Q519 and Yuanhang 2 platform fit: These models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell covers the full Q519 line and the Yuanhang 2 variant without adapter or rewiring.
- Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on Q519C hardware. The BMS accepted the charge IC handshake cleanly and held voltage above the low-battery cutoff threshold across modem-active and screen-on load states.
- First-cycle fuel gauge reset: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current charging begins on an uncalibrated cell.
Sudden shutdown at 20–30% on the Q519C replacement cell
This happens when the fuel gauge IC is still calibrated to the old, degraded cell's voltage curve. The replacement cell has a steeper voltage cliff under modem and display load — the phone reads 25% but the actual cell voltage drops below the shutdown threshold when the screen or LTE radio pulls current. The fix is one full discharge cycle without fast charging, which forces the coulomb counter to relearn the new cell's capacity endpoints. After that cycle, the percentage readout stabilises and early shutdowns stop.
Fast charging not working after a cell swap
On the first cycle after fitting a new cell, the charge IC may reject the fast-charge protocol because the BMS presents higher impedance than the calibrated threshold expects. This is a calibration state, not a fault. Run one standard-speed charge from near-empty to 100%, then reconnect the fast charger. The charge IC will re-negotiate the protocol once it has logged a complete cycle against the new cell — verify by checking that the phone reports fast charging at above 10W on the second charge session.