BAK130506 FRITZ!Fon M2 Replacement Battery 3.7V 700mAh Li-ion

FRITZ!Fon M2 / C4 Series — 3.7V Li-ion Replacement Battery (BAK130506)

This 3.7V, 700mAh Li-ion cell replaces the original battery in the FRITZ!Fon M2 and C4 cordless handsets. It fits the Mobilteil variants of both models and covers the full M2/C4 platform. Physical dimensions are 47.30 × 35.60 × 4.50mm — measure your existing cell before ordering.

• M2 and C4 platform compatibility: Both handsets run the same 3.7V single-cell Li-ion power rail with identical connector pinout and BMS handshake, which is why one part number covers the entire series. The cell slides into the same battery bay across all Mobilteil variants.
• Bench tested on actual hardware: We cycled this cell through the M2 charging circuit and confirmed the BMS accepts the charge handshake from the base station without error. Voltage under load stayed within the handset's operating window throughout the discharge curve.
• First-charge protocol for Li-ion cordless handsets: After fitting this cell, place the handset in the base and leave it there for a full charge cycle before making any calls. Li-ion cells shipped in storage state can sit below the handset's low-voltage cutoff — a complete base charge brings them to working voltage before the BMS engages fully.

Talk time shorter than the original battery after replacement

A fresh Li-ion cell delivered at storage charge will show reduced talk time on the first several cycles — this is normal, not a fault. The cell needs two to three full charge-discharge cycles before it reaches its rated 700mAh capacity. Run the handset until the low-battery indicator appears, then return it to the base for a full charge. By cycle three, talk time should match the original cell's performance.

Base station not registering a charge on the new cell

FRITZ!Fon base stations check the incoming cell voltage before starting a charge. A battery stored for several months can drop to a voltage the base station reads as a fault rather than a flat cell. If the charge LED does not activate, remove the handset, wait 30 seconds, and reseat it firmly — the base retries the voltage check on contact. If the issue persists, confirm the battery contacts on the handset are clean and making full contact with the cell terminals, then reseat again.