Topcon BT-77Q Cygnus KS-102 Replacement Battery 7.4V

Topcon Cygnus KS-102 Total Station — 7.4V Li-ion Replacement Battery (BT-77Q)

This 7.4V 2800mAh Li-ion pack replaces the OEM BT-77Q battery in the Topcon Cygnus KS-102 total station. It fits directly into the KS-102 battery compartment and delivers 20.72Wh of capacity for field surveying and construction layout work. Voltage and connector match the original specification.

• KS-102 platform fit: The BT-77Q form factor is specific to the Cygnus KS-102 body. The cell arrangement, connector polarity, and BMS communication lines match what the KS-102 instrument bus expects. No adapters or firmware changes are needed.
• Bench tested on actual hardware: We ran this pack through full charge and discharge cycles on the KS-102 bench rig. The BMS held within operating voltage thresholds across motor drive initialisation, EDM module power-up, and sustained angle encoder load simultaneously.
• First-install calibration cycle: After fitting the new pack, run a full calibration cycle through the KS-102 instrument menu before taking it to site. The instrument maps battery state during calibration — skipping this step causes premature low-battery warnings on the first measurement session.

KS-102 shutting down at EDM or prism lock initialisation

The KS-102's EDM module draws a short current spike the moment it fires to acquire a prism. If the battery's BMS has a low instantaneous-current threshold — common in aged or deeply discharged cells — it trips the protection circuit at exactly this moment. The instrument powers off cleanly, with no error code, which makes the battery the last thing operators suspect. A fresh cell with a healthy BMS handles the spike without tripping. If the shutdown repeats after fitting a new pack, check that the battery contacts in the compartment are clean and making full contact before suspecting the instrument.

KS-102 shows "battery full" then drops to critical within minutes of starting a job

This happens when a new pack is installed without a calibration cycle — or when a stored pack is put into service with its BMS in a partially reset state. The KS-102 reads terminal voltage at power-on to set the indicator, but that voltage hasn't stabilised under load yet. Once the EDM and data logging start drawing current, the resting voltage figure collapses and the display catches up. Fix this by running the full on-instrument calibration sequence after every new battery install, which forces the instrument to sample voltage under representative load. After calibration, the percentage display should track accurately down to approximately 7.0V before a low-battery alert appears.