Ascom 51 Advanced Cordless Phone Compatible Battery 3.7V 700mAh

Ascom 51 Advanced / A51 / A71 — 3.7V Li-ion Replacement Battery (660089/1B)

This 3.7V 700mAh Li-ion battery replaces the original power cell in the Ascom 51 Advanced, A51, A71, and AMIE-AABAB cordless DECT handsets. It matches the original voltage and physical footprint, so the handset seats and charges in the base as normal. Capacity is rated at 700mAh (2.59Wh) — identical to the factory specification.

• Ascom DECT handset family: The 51 Advanced, A51, and A71 share the same battery bay dimensions and 3.7V Li-ion voltage rail. The connector orientation and BMS handshake protocol are common across this platform, so one cell fits the full model range listed.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Ascom base station. The BMS accepted charge without fault and held voltage within the expected range across multiple cycles. No thermal events or cutoff triggers were recorded.
• First charge after installation: Li-ion cells in DECT handsets are shipped in a partial-charge state for transport safety. After fitting, place the handset in the base and run a full, uninterrupted charge before first use — the Ascom base needs to complete a full charge cycle to calibrate its charge termination correctly with the new cell.

Talk time shorter than expected after fitting the new cell

Li-ion cells can underperform on the first one to three cycles if they were stored at a low state of charge before shipping. The electrochemical capacity stabilises after a few full charge and discharge cycles through normal use. If talk time is noticeably short after the first cycle, run the handset down to the low-battery indicator, then place it back in the base for a complete charge. After two to three full cycles, capacity should settle at the rated 700mAh.

Handset not pairing with base after battery replacement

Removing the battery cuts all power to the handset, which can erase the stored DECT pairing on some Ascom models. The base and handset need to re-register to each other before calls work again. To re-pair, hold the registration button on the base unit until the LED flashes, then follow the handset menu to search for and register to the base. Once paired, the handset should show signal bars at 3.7V cell voltage under idle standby.