About this replacement
Mitutoyo Surftest SJ-201 — 6V Ni-MH Replacement Battery (12BAA240)
This is a 6V, 700mAh Ni-MH replacement battery for the Mitutoyo Surftest SJ-201 portable surface roughness tester. It slots directly into the SJ-201 battery compartment and powers the instrument during surface finish measurements on the shop floor or in the lab. Voltage and cell count match the original 12BAA240 / 5HR-AAAU specification.
- SJ-201 compatibility: The SJ-201 runs a fixed 6V rail with five Ni-MH cells in series. This pack replicates that cell count and connector orientation, so the BMS handshake completes on first boot without triggering a voltage mismatch warning.
- Bench tested on actual hardware: We cycled this pack through the SJ-201's probe initialisation sequence, which briefly draws above the steady-state load. The BMS held without tripping on cold-start current spikes across multiple back-to-back measurement sessions.
- Calibration cycle before field deployment: After installing a fresh pack, run a full calibration cycle through the SJ-201 instrument menu before taking it into production. The instrument maps battery state during calibration — skipping this causes premature low-battery warnings to appear during the first measurement session.
SJ-201 shutting down mid-measurement after sitting unused for months
Ni-MH cells self-discharge at roughly 1–3% per day at room temperature. A pack left in the instrument for two or three months in a carry case can drop below the SJ-201's BMS recovery threshold — around 4.5V for this five-cell pack. When the charger is reconnected, the BMS may not recognise the depleted pack and will refuse to charge it. Reconnect the charger and hold the connection for at least 15–20 minutes to allow the BMS to wake from sleep mode before the charge cycle starts.
SJ-201 readings drifting or resetting mid-logging session
The SJ-201 drives the stylus drive motor and processes sensor signals simultaneously during a logging run. This sustained combined load pulls the pack voltage lower than during a single spot measurement. An ageing cell with elevated internal resistance causes a voltage dropout that the instrument interprets as a reset event, corrupting the in-progress log. If readings drop out consistently on longer traversals, check open-circuit voltage — a healthy pack should read above 6.0V at rest. A pack reading below 5.7V at rest under no load needs replacement.