About this replacement
FRITZ!Fon MT-F Series — 3.7V Li-ion Replacement Battery (312BAT006)
This 3.7V, 700mAh Li-ion cell replaces part number 312BAT006 in the FRITZ!Fon MT-F, C4, and AVM 2000 2446 cordless DECT handsets. It fits the same footprint as the factory cell at 47.06 × 35.52 × 4.55mm. Install it when the original battery no longer holds a usable charge between base sessions.
- MT-F and 2446 series fit: These handsets share the same battery bay dimensions, connector pinout, and 3.7V nominal charge rail — one cell covers the full listed model range without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the FRITZ!Fon platform. The BMS accepted full charge termination cleanly and held voltage within the handset's operating window across repeated sessions.
- First-charge protocol for Li-ion in DECT handsets: Place the handset in the base immediately after fitting the new cell and leave it for a full uninterrupted charge before making any calls. FRITZ!Fon handsets top-balance the Li-ion cell through the base station's charge circuit — interrupting this first cycle can leave the reported charge level miscalibrated for several days.
Why the MT-F handset loses range after a battery swap
DECT transmit power draws short, sharp current spikes during RF bursts. A Li-ion cell that shipped with a partial state of charge can sag below the handset's transmit threshold under that load, causing the radio to step down power or drop sync with the base. This shows up as reduced range or choppy audio — not a pairing fault. A full charge cycle from the base station restores the cell to its working voltage and the range issue clears without any re-pairing.
Handset not pairing with base after battery replacement
Removing the battery cuts all power to the handset, which erases the active DECT pairing on some MT-F firmware versions. The base station retains its end of the registration, but the handset comes up blank. To fix this, press and hold the registration button on the base for at least three seconds, then navigate to the handset's menu and run a new DECT registration. The process takes under two minutes and restores the pairing without a full factory reset.