Testo 175-H1 Data Logger Compatible Battery 3.6V 1200mAh

Testo 175-H1 / 175-H2 / 175-S1 / 175-S2 — 3.6V Li-SOCl2 Replacement Battery (175-SL750)

This is a 3.6V 1200mAh lithium thionyl chloride cell for Testo 175-series data loggers. It fits the 175-H1, 175-H2, 175-S1, and 175-S2 instruments used in HVAC surveys, refrigeration monitoring, and building audits. Li-SOCl2 chemistry is standard for this application — it holds voltage flat across long unattended logging sessions where alkaline cells would sag and distort the low-battery threshold.

• 175-series platform fit: The H1, H2, S1, and S2 share the same battery bay dimensions, connector orientation, and voltage rail. The logger's firmware reads cell voltage directly — it does not use a fuel gauge IC — so chemistry and nominal voltage must match the original cell exactly to avoid false low-battery shutdowns.
• Bench tested on actual hardware: We ran this cell under continuous sensor polling load typical of the 175-series and confirmed the BMS did not trip on probe initialisation. Voltage held within 3% of nominal through the first 48-hour logging cycle.
• Post-installation calibration step: After fitting this cell, run a full calibration cycle through the instrument menu before deploying the logger in the field. The 175-series maps battery state during calibration — skipping this step causes premature low-battery warnings to appear during the first active measurement session, even with a full cell installed.

Why the 175-series logger cuts out when the probe initialises

At power-up, the 175-H1 and 175-S1 send a short current burst to the probe or sensor module to initialise it. A cell with any internal resistance increase — from age, deep discharge, or long storage — sees its terminal voltage dip sharply during that burst. If the dip crosses the logger's undervoltage threshold, the instrument interprets it as a depleted cell and shuts down immediately. A fresh Li-SOCl2 cell has low enough internal resistance to absorb that spike without the voltage dropping below cutoff. If the logger still cuts out on probe init after fitting a new cell, check that the probe connector is fully seated — a partial connection increases load impedance and worsens the spike.

Logger display shows erratic battery percentage after a cell swap

The 175-series estimates remaining capacity by tracking voltage against a fixed discharge curve — there is no coulomb counter recalibrating in the background. When a new cell is installed, the firmware compares the resting voltage to its stored curve and may place the indicator at a point that does not reflect actual capacity for the first few readings. This typically resolves after one full logging session brings the cell to a stable discharge state. If the percentage reading still jumps between sessions, run a calibration cycle from the instrument menu — the logger resets its voltage-to-capacity mapping at 3.6V nominal during that routine.