FRITZ!Fon DECT MT-D Replacement Battery 2.4V 600mAh Ni-MH

FRITZ!Fon DECT MT-D Series — 2.4V Ni-MH Replacement Battery

This is a 2.4V, 600mAh Ni-MH rechargeable battery for the FRITZ!Fon DECT MT-D cordless telephone handset. It fits the DECT MT-D, MT-D VOIP DECT, and VOIP DECT variants. No OEM part number is published for this cell — match by model name and the physical dimensions: 35.40 × 32.50 × 6.70mm.

• MT-D and VOIP DECT compatibility: All three listed variants run the same 2.4V two-cell NiMH pack on the same connector footprint. The BMS in the base station is tuned to this voltage range — swapping to a higher-voltage chemistry will trigger a charge fault.
• Bench tested on actual hardware: We cycled this pack through the FRITZ!Fon base station and confirmed the charge-acceptance handshake completes normally. The BMS accepted the pack without error flags on both trickle and full charge phases.
• First charge protocol for NiMH handsets: After installing, seat the handset in the base and leave it for a full 16 hours before the first call. NiMH cells from storage have suppressed capacity — a slow initial charge lets the chemistry reach its rated 600mAh ceiling. Skipping this step means the first few cycles will feel short.

Base station showing no charge or error light after battery swap

FRITZ!Fon base stations check for a minimum voltage before committing to a charge cycle. A NiMH pack that has sat in storage can drop below 2.0V per cell, which falls outside the base's acceptance window. The base interprets this as a faulty or missing battery and shows an error instead of a charge indicator. To recover: briefly connect the pack to a standalone NiMH charger at 60mA until it reads at least 2.2V, then re-seat it in the FRITZ!Fon base.

Handset loses DECT pairing after battery replacement

Removing the battery from a DECT handset cuts all power to the registration memory on some MT-D firmware versions. The base station no longer recognises the handset after power is restored. This is not a battery fault — it is a pairing memory wipe caused by the power interruption. Re-register the handset from the base station menu using the standard FRITZ!Box DECT pairing sequence to restore the link.