AGM M9 Replacement Battery 3.7V 1200mAh Li-ion

AGM M9 — 3.7V Li-ion Replacement Battery

This is a 3.7V 1200mAh lithium-ion cell for the AGM M9 smartphone. It replaces the original cell when the phone no longer holds charge or fails to power on. Voltage and capacity match the M9's charge IC requirements exactly.

• AGM M9 fit: The M9 uses a compact 53.18 × 34.00 × 6.80mm cell format with a 3.7V nominal rail. The connector pinout and physical envelope match the original bay — no trimming or adapter required.
• Bench tested on actual hardware: We cycled this cell through the M9's charge IC and monitored BMS cutoff thresholds. The protection circuit tripped correctly at low-voltage and over-current limits, and the charge IC accepted the cell without error flags.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This lets the M9's fuel gauge IC map the new cell's discharge curve before high-current charging starts — skipping this step causes erratic percentage readings in early use.

Sudden shutdown at 20–30% on the AGM M9 after a cell swap

The M9's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, the IC still references the old curve. At around 20–30% reported charge, the new cell's actual voltage drops below what the modem or display load needs to sustain — the phone shuts off before the gauge catches up. One full discharge cycle from 100% down to automatic cutoff, then a full charge, forces the coulomb counter to rebuild its reference against the new cell. After that cycle, the shutdowns stop.

AGM M9 not powering on after the replacement battery sat in storage

Li-ion cells self-discharge in storage. If this battery shipped or sat unused below approximately 2.5V, the BMS protection circuit locks the cell out to prevent damage. The M9 will show nothing — no charging animation, no response. Connect it to a charger and leave it for 15–20 minutes without touching the power button. The charge IC trickle-charges the cell back above the BMS re-enable threshold, usually around 2.9V, at which point the protection circuit resets and normal charging resumes.