About this replacement
Avaya FC4 / IH4 / D4 Series — 3.7V Li-ion Replacement Battery (5010808000)
This is a 3.7V, 700mAh Li-ion replacement battery for Avaya DECT cordless phone handsets. It fits the FC4, IH4, D4, and D4 Office models, along with two additional handset variants sharing the same battery format. The OEM part number is 5010808000.
- FC4, IH4, D4, and D4 Office compatibility: These handsets share the same 3.7V Li-ion cell format, connector pinout, and BMS handshake requirements. One battery SKU covers the full range because the base station charging circuit and handset BMS treat all four models identically.
- Bench tested on actual hardware: We ran this battery through charge and discharge cycles on DECT handset hardware. The BMS held the correct 3.7V nominal rail, accepted charge from the base station without error flags, and maintained stable voltage through standby and active call states.
- First installation charge on DECT handsets: After fitting this battery, seat the handset in the base station cradle and leave it for a full charge cycle before use. Li-ion cells shipped in partial-charge storage states can read low on the handset indicator until the BMS completes its first full calibration charge.
Why the FC4 handset shows a low battery icon immediately after a new battery is fitted
Li-ion cells leave storage at around 40–60% charge to protect cell chemistry during transit. The handset BMS reads this as a low state of charge and displays the low battery indicator straight away. This is normal and not a fault with the replacement cell. Place the handset in the base cradle and allow the BMS to complete a full charge cycle — the indicator will clear once the cell reaches 4.2V.
Talk time noticeably shorter than the original battery after replacement
A new Li-ion cell can take two to three full charge and discharge cycles before the BMS accurately tracks capacity and the cell reaches its rated 700mAh output. If talk time is short after the first or second use, this is a calibration issue rather than a defective cell. Run the handset until the low battery warning appears, then return it to the base for a full charge. Repeat this two to three times and talk time will normalise.