NUM 1020 Survey Instrument Li-SOCl2 Replacement Battery 3.6V 5400mAh

NUM 1020 / 1040 / 1060 — 3.6V Li-SOCl2 Replacement Battery

This is a 3.6V lithium-thionyl chloride cell rated at 5400mAh (19.44Wh), built to the same form factor as the original pack in the NUM 1020, 1040, and 1060 surveying instruments. Li-SOCl2 chemistry delivers a flat discharge curve across the cell's full capacity range — critical for instruments that rely on stable voltage for sensor excitation and accurate logging. Dimensions are 51.00 × 31.40 × 14.85mm, matching the original cell bay on all three models.

• NUM 1020, 1040, and 1060 compatibility: All three instruments share the same battery bay dimensions and voltage rail. The 3.6V Li-SOCl2 chemistry matches the supply rail these models require for probe power-up and continuous sensor load — switching chemistry or voltage here will cause the BMS to trip at initialisation.
• Bench tested on actual hardware: We ran this cell through a full instrument power-up sequence, including probe initialisation and a sustained logging cycle. The BMS held steady through the inrush current at sensor start-up without tripping, and cell voltage remained within spec under continuous measurement load.
• First-use calibration cycle: After fitting this cell, run a complete calibration cycle through the instrument menu before field deployment. The NUM instruments map battery state during calibration — skipping this step causes the instrument to report premature low-battery warnings on the first real measurement session.

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

Li-SOCl2 cells enter a passive state when stored for extended periods — the internal protection circuit drops below its recovery threshold and the BMS locks out, preventing the instrument from powering on at all. This is not a dead cell. The cell needs a brief reconnection cycle to wake the BMS. Connect the instrument to its charger for 10–15 minutes before attempting to power on, even if the original cell appeared fully charged when stored. Once the BMS recovers, the cell voltage should read above 3.5V on the instrument's diagnostic screen.

Readings drift or reset mid-session during sustained sensor logging

Sustained sensor load draws continuous current across the cell, and if the cell voltage sags even briefly below the instrument's measurement threshold, the logging session resets or produces erratic output. This is not a firmware fault — it is a voltage dropout event at the cell terminals under load. Check that the cell contacts in the battery bay are clean and making solid contact; oxidation on the contact pins increases resistance and amplifies voltage sag. If dropout persists, confirm terminal voltage under load stays above 3.2V using a multimeter with the instrument running a live measurement.