About this replacement
FRITZ!Fon C6 / X6 DECT Handset — 3.7V Li-ion Replacement Battery (312BAT026)
This 3.7V 1200mAh (4.44Wh) Li-ion cell replaces the original battery in the FRITZ!Fon C6, C6 DECT, and X6 cordless handsets. It matches the OEM dimensions of 53.00 × 33.90 × 5.70mm and connects directly to the handset's internal battery contacts. Part numbers 312BAT026, 312BAT030, and FRA051 all apply to this fit group.
- C6, C6 DECT, and X6 compatibility: These three handset variants share the same battery bay dimensions, contact layout, and 3.7V supply rail. The BMS handshake requirements are identical across the group, so one cell covers all three without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a C6 handset. The BMS accepted the cell without error, held the voltage rail stable during active DECT transmission, and showed no cutoff events during standby draw.
- First charge after fitting: Seat the handset in the base immediately after installing this cell and leave it there for a full 16 hours before first use. Li-ion cells shipped in partial-charge storage state benefit from a slow, complete first charge to calibrate the handset's battery indicator correctly.
Talk time shorter than original battery after replacement
A new Li-ion cell often reports fewer bars on the handset indicator for the first two or three charge cycles. This happens because the handset's fuel gauge is still calibrated to the worn curve of the old cell. Run two to three full charge-and-use cycles and the indicator recalibrates to the 1200mAh capacity of this cell. If talk time is still noticeably short after five cycles, check that the handset is seating fully in the base — a partial contact on the charging pins delivers a lower charge current and the cell never reaches a full 4.2V top-off.
Handset not pairing with base station after battery swap
Removing the old battery cuts power completely to the handset's DECT module, which can clear the pairing registration stored in volatile memory on some FRITZ!Fon models. The base station retains its side of the pairing, so the mismatch causes the handset to show no signal or a search loop. Re-register the handset using the base station's pairing button — on the FRITZ!Box base, hold the DECT button for two seconds until the LED flashes, then navigate to Connections on the handset and select Register Handset.