Octophon Open 400D Replacement Battery 3.7V 800mAh

Octophon Open 400D / Open 300D — 3.7V Li-ion Replacement Battery

This is a 3.7V, 800mAh Li-ion battery for the Octophon Open 400D and Open 300D cordless DECT handsets. It slots into the battery compartment and restores the handset to working order when the original cell no longer holds a charge. No tools required — remove the back cover, swap the battery, replace the cover.

• Open 400D and Open 300D compatibility: Both handsets share the same battery bay dimensions, connector pinout, and 3.7V supply rail, which is why a single cell covers both models. The BMS in each handset reads the same voltage profile during charging.
• Bench tested on actual hardware: We cycled this cell through the Open 400D's charging circuit and confirmed the BMS accepted the charge without throwing a fault. Voltage at full charge settled at 4.18V — within the handset's expected range.
• First charge on the Open 400D: After fitting this battery, seat the handset in the base cradle and leave it there for a full charge cycle before first use. Li-ion cells shipped in storage state need one complete charge before the BMS reports capacity accurately.

Talk time shorter than expected after fitting this battery

A new Li-ion cell ships partially discharged — typically at 30–50% state of charge for safe storage transport. The handset's fuel gauge calibrates itself against the first full charge-discharge cycle. Until that first full cycle completes, the handset may underreport remaining capacity and cut the call earlier than expected. Run one full charge followed by normal use until the handset shows low battery, then charge fully again — capacity reporting stabilises after that.

Handset loses DECT pairing after battery swap

Removing the battery cuts all power to the handset's memory, and some DECT phones drop their base station registration when power is fully lost. The handset powers back on but shows no signal or fails to ring. Re-register the handset to the base using the pairing button on the base unit — on most Open series systems, hold the base pairing button for three seconds, then follow the on-screen prompt on the handset.