MyPhone MP-S-K 8870 Replacement Battery 3.7V 1050mAh

MyPhone 8870 — 3.7V Li-ion Replacement Battery (MP-S-K)

This is a 3.7V, 1050mAh Li-ion replacement battery for the MyPhone 8870 smartphone. Part number MP-S-K matches the original cell specification. It fits directly into the 8870 battery bay at 59.90 x 39.90 x 4.50mm.

• MyPhone 8870 fit: The 8870 uses a slim single-cell Li-ion pack at 3.7V nominal. This replacement matches that voltage rail and physical footprint, so the connector seats correctly and the charge IC receives the expected cell parameters.
• Bench tested on actual hardware: We cycled this cell through charge and discharge under load typical of smartphone modem and display draw. The BMS held cutoff correctly at both ends of the voltage window without triggering false low-battery shutdowns.
• Fuel gauge recalibration on first use: On first install, disable any fast-charge option and run one full discharge-to-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state.

Why the MyPhone 8870 reports wrong battery percentage after a cell swap

The 8870's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual cell chemistry and internal resistance. The gauge reads voltage and estimates percentage against old data, so it can show 40% while the cell is actually near depletion. One full discharge down to automatic shutdown followed by an uninterrupted charge to 100% forces the coulomb counter to rebuild its reference points against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under the combined load of the modem radio, backlight, and active apps — a voltage cliff the fuel gauge did not predict. The phone's protection circuit cuts power before the percentage display reaches zero. A fresh cell with an uncalibrated gauge is most vulnerable in the first few cycles. Run the first discharge under light use, let the device shut off naturally, then charge fully to 4.2V — the gauge recalibrates and the cliff effect typically disappears by the third cycle.