Topcon SX-1 BT-1A Replacement Battery 7.4V 3400mAh

Topcon SX-1 / GP-SX1 Total Station — 7.4V Li-ion Replacement Battery (BT-1A)

This 7.4V 3400mAh Li-ion battery replaces the Topcon BT-1A and CGR-B/201LC cells used in the SX-1 and GP-SX1 total stations. It fits the battery bay directly and connects to the instrument's onboard BMS without modification. Capacity is 3400mAh (25.16Wh) — matched to the original specification.

• SX-1 and GP-SX1 compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The 7.4V rail feeds the angle encoder, EDM module, and display from a single regulated source — any deviation in cell voltage triggers an immediate shutdown.
• Bench tested on actual hardware: We cycled this pack through EDM ranging bursts and continuous angle logging. The BMS held the 7.4V rail steady under combined sensor and display load, and the protection circuit tripped correctly at low-voltage cutoff without locking the pack.
• First-use calibration cycle: After installing a new pack, run a full calibration cycle through the instrument menu before field deployment. The SX-1 maps battery state during calibration — skipping this step causes premature low-battery warnings on the first measurement session, even with a fully charged cell.

Why the SX-1 shuts down at instrument power-up despite a charged battery

At startup, the SX-1 initialises the EDM module, angle encoders, and display simultaneously. That combined inrush current can spike well above steady-state draw and trip a BMS with a conservative overcurrent threshold. A pack that has been stored partially discharged is especially vulnerable — cell internal resistance rises during storage, amplifying the voltage sag during that startup surge. If the BMS trips, the instrument cuts power before the boot sequence completes, which looks like a dead battery even when the pack shows charge. Letting the pack charge fully before the first power-on reduces internal resistance and lowers the risk of a false BMS cutoff.

SX-1 displaying inconsistent battery percentage after swapping to a new pack

The SX-1 estimates state-of-charge by comparing cell voltage against a stored threshold table calibrated to the original pack's discharge curve. A new cell with slightly different resting voltage will read out of step with that table until the instrument recalibrates. This shows up as a percentage that jumps — often reading 100% then dropping sharply after the first measurement session. Run one full discharge-to-cutoff cycle followed by a complete charge, then run the instrument calibration routine. After that cycle, the percentage display stabilises and tracks accurately.