Topcon BT-31QB 111C Replacement Battery 7.2V 1400mAh

Topcon 111C / 101C Series — 7.2V Ni-Cd Replacement Battery (BT-31QB)

This is a 7.2V, 1400mAh Ni-Cd replacement battery for the Topcon 111C, 101C, DL-100, and DL-101C surveying instruments. It fits the same bay as the original BT-31QB and BT-31Q packs. Voltage and capacity match OEM spec exactly — no modifications needed to the instrument.

• 111C and DL-100 series fit: These models share the same 7.2V battery bay, connector pinout, and BMS handshake protocol. One pack covers all listed models without adapter or firmware change.
• Bench tested on actual hardware: We ran this pack through charge and discharge cycles on a Topcon instrument. The BMS accepted the cell chemistry without fault codes, and the low-battery threshold triggered at the correct voltage level.
• Post-install calibration on first field use: After fitting this pack, run a full calibration cycle through the instrument menu before your first measurement session. The instrument maps battery state during calibration — skipping this step causes premature low-battery warnings that interrupt field measurements even when the pack still has capacity.

BMS lockout after a Topcon instrument sat unused in its carry case for months

Ni-Cd cells self-discharge at roughly 1–2% per day at room temperature. A pack stored for three months inside a case can drop below 5.0V total — the point where the BMS enters sleep mode and refuses a standard charge cycle. The instrument will show no response on power-up, and the charger may not register the pack at all. To recover, apply a trickle charge at 100mA until cell voltage climbs above 6.0V, then resume normal charging.

Instrument shuts off mid-measurement when angle or distance logging is active

Sustained sensor load during active angle measurement draws more current than standby, and aged or partially discharged Ni-Cd cells develop voltage sag under that load. If the pack voltage dips below the instrument's cutoff threshold — typically around 5.4V under load — the instrument powers off to protect its electronics, even if the display showed adequate charge moments before. This is a cell condition issue, not a firmware fault. Charge the pack fully and retest; if the cutoff repeats at the same point in the measurement cycle, the cells are no longer holding voltage under load and the pack needs replacement.