Topcon SX-1 Replacement Battery BT-1A 7.4V 2200mAh

Topcon SX-1 / GM52 Series — 7.4V Li-ion Replacement Battery (BT-1A)

This 7.4V 2200mAh Li-ion battery replaces the Topcon BT-1A, CGR-B/201LC, and BDC71 packs. It fits the SX-1, GP-SX1, and GM52 total stations — survey-grade instruments used for distance, angle, and coordinate measurement in the field. Voltage and capacity match OEM spec exactly.

• SX-1 / GP-SX1 / GM52 platform fit: These three models share the same battery bay geometry, 7.4V supply rail, and BMS handshake protocol. One pack covers all three without modification or adapter.
• Bench tested on actual hardware: We cycled this pack through a simulated survey load — EDM firing, tilt sensor polling, and backlit display active simultaneously. The BMS held voltage without spurious cutoff across the full discharge curve.
• First-use calibration on the SX-1: After installing a new pack, run a full calibration cycle through the SX-1 instrument menu before field deployment. The instrument maps battery state during calibration — skipping this step causes premature low-battery warnings during your first measurement session.

BMS lockout after the SX-1 sat unused in a carry case for months

Li-ion cells self-discharge at roughly 2–3% per month. After several months in a carry case, a pack can drop below 2.5V per cell — the threshold where most BMS circuits lock the output rail to prevent further discharge damage. At that point the instrument shows nothing when powered on, and the charger may also refuse to begin a charge cycle. To recover the pack, use a charger with a trickle or recovery mode that applies a low current until cell voltage climbs back above 3.0V per cell. Once recovery completes, charge fully to 8.4V before reinstalling.

SX-1 shuts down mid-measurement with the battery indicator still showing charge

This happens when the EDM fires a distance measurement while the tilt sensor and display are drawing current simultaneously — the combined load pulls voltage below the BMS trip threshold for a fraction of a second. An aged or partially discharged cell cannot sustain that peak draw, so the BMS cuts the rail even though the resting voltage looked acceptable. It is not a firmware fault or a connection issue. Charge the pack fully to 8.4V and retest — if the shutdowns continue at full charge, the cell capacity has degraded below usable range and the pack needs replacement.