TELMI C121 Replacement Battery 3.7V 1200mAh Li-ion

TELMI Acer C121 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh Li-ion replacement battery for the Acer C121 smartphone. It fits directly into the C121 and restores power capacity lost through repeated charge cycles on the original cell. Dimensions are 53.00 × 33.90 × 5.70mm — verify against your existing battery before ordering.

• Acer C121 fit: The C121 uses a single-cell 3.7V Li-ion pack with a fixed connector pinout. This battery matches that voltage rail and physical footprint. No modification needed to seat it correctly.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the C121 platform. The BMS accepted charge normally, and protection circuits tripped correctly at low-voltage cutoff — no thermal event, no false full-charge flag.
• Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This gives the fuel gauge IC a clean reference curve against the new cell before high-current charging is applied to an uncalibrated cell.

Sudden shutdown at 20–30% on the replacement cell

This happens because the fuel gauge IC is still using the discharge curve from the old, degraded cell. When load spikes — modem transmitting, screen at full brightness — the new cell's voltage drops faster than the IC expects at that reported percentage. The phone reads 25% but the actual cell voltage has already crossed the cutoff threshold under load. One full discharge-to-shutdown followed by a full charge resets the coulomb counter and eliminates most of these events.

Phone won't power on after sitting in storage with a flat battery

Li-ion cells that drop below approximately 2.5V trigger a BMS lockout — the protection circuit opens to prevent damage and the phone shows nothing when you press the power button. Plugging in a charger may not immediately recover the cell because the BMS needs a trickle current to re-initialise before normal charge current is accepted. Leave the phone on charge for at least 15–20 minutes without interruption. If the charge IC starts trickle charging the cell back above 2.8V, the BMS will re-engage and charging will proceed normally.