About this replacement
M-Tronic SDT-2000/U — 8.4V Ni-MH Replacement Battery (91507401)
This is a 8.4V, 1200mAh Ni-MH replacement battery for the M-Tronic SDT-2000/U surveying instrument. It replaces OEM part number 91507401. It fits the SDT-2000/U directly and restores field operation when the original pack has degraded past usable capacity.
- SDT-2000/U platform fit: The SDT-2000/U uses a fixed 8.4V Ni-MH pack with a specific connector and cell arrangement. This replacement matches that voltage rail and cell count so the instrument's internal power circuit operates within its specified input range.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on the bench and monitored the BMS response at probe initialisation. The pack held voltage through the current spike at sensor power-up without triggering a protective cutoff.
- Calibration cycle before first field use: After fitting this pack, run a full calibration cycle through the SDT-2000/U instrument menu before deploying it to a job. The instrument maps battery state during that sequence — skipping it causes premature low-battery warnings in the first measurement session even when the pack has substantial charge remaining.
BMS lockout after the SDT-2000/U sat unused in its carry case for months
Ni-MH cells self-discharge at roughly 1–2% per day at room temperature. A pack stored for several months can drop below the BMS recovery threshold, putting the protection circuit into a locked state that the charger alone cannot wake. The SDT-2000/U charger checks for a minimum voltage at the battery terminals before enabling the charge current — if the pack is below approximately 5.5V it reads the circuit as a fault, not a flat battery. To recover it, apply a slow trickle charge externally at 100mA until terminal voltage climbs above 6V, then return it to the instrument charger to complete a full charge cycle.
Readings drifting or logging session resetting mid-measurement
Sustained sensor load during a logging session draws more current than standby operation, and an aged or partially charged Ni-MH pack experiences a steeper voltage sag under that continuous draw. When the pack voltage dips below the instrument's operating threshold — even briefly — the SDT-2000/U can reset its active session to protect data integrity. This is not a firmware fault; it is a voltage dropout at the supply rail. Start a logging session with a fully charged pack, and confirm terminal voltage reads at or above 8.0V before beginning a long measurement run.