Vonage DECT 6.0 Cordless Phone Compatible Battery 3.6V 700mAh

Vonage MD6401 / MD6400 — 3.6V Ni-MH Replacement Battery

This 3.6V, 700mAh Ni-MH rechargerate battery replaces the original pack in Vonage DECT 6.0 cordless handsets, including the MD6401 and MD6400. It restores standby capacity and talk time to a handset that has started dying faster than it charges. Dimensions are 46.00 × 31.00 × 10.50mm — same footprint as the factory cell.

• MD6401 and MD6400 compatibility: Both models run the same 3.6V single-cell Ni-MH pack on the same connector pinout. The base station charges at a fixed trickle rate matched to this voltage and chemistry — swapping to a different voltage or chemistry breaks the charge cycle.
• Bench tested on actual hardware: We cycled this pack in the MD6401 base cradle and confirmed the charge circuit accepted it without error. The Ni-MH voltage curve stayed within the base station's detection window across full charge and discharge.
• First-charge protocol for Ni-MH cordless packs: After installing, seat the handset in the base for a full 16-hour charge before first use. Ni-MH cells shipped from storage arrive partially discharged — a slow first charge allows each cell to reach rated capacity. Skipping this step is the most common reason a replacement battery appears to underperform.

Base station charge light staying off after fitting a new Ni-MH pack

Vonage base stations use a voltage threshold to confirm a battery is present before starting the charge cycle. A pack that has sat in storage for months can drop below that threshold — typically under 3.0V — and the base reads it as no battery. The fix is a brief external boost charge to bring the pack above 3.0V, or placing the handset in the base and waiting 20–30 minutes for the trickle circuit to prime it. Once voltage clears the detection window, normal charging resumes.

Range dropping mid-call after a battery swap

DECT handsets draw a burst of current every time they transmit — the radio pulse pulls more than standby draw. If the Ni-MH pack is still in its first few cycles, internal resistance is higher than rated, and voltage sags under that RF load. The handset reads the sag as low battery and reduces transmit power, cutting range. This resolves after three to five full charge-discharge cycles as the cells condition and internal resistance drops to spec.