About this replacement
Senter ST327 / ST103 — 3.8V Li-ion Replacement Battery (ST655518PL)
This 3.8V 7200mAh (27.36Wh) lithium-ion battery replaces the OEM pack in the Senter ST327 and ST103 surveying instruments. It slots into the same battery bay and connects through the original BMS handshake. Voltage and capacity match the factory specification exactly.
- ST327 and ST103 platform fit: Both instruments share the same battery bay geometry, connector pinout, and BMS communication protocol — that is why one pack covers both models. The voltage rail is identical across the series, so the instrument firmware reads state-of-charge the same way on either unit.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on the ST327 platform and monitored BMS handshake confirmation, cutoff thresholds, and cell balance across all stages. The BMS reported no fault codes and held cutoff voltage within spec throughout.
- First-use calibration on the ST327: After installing this pack, run a full calibration cycle through the instrument menu before heading into the field. The ST327 maps battery state during calibration — skipping this step causes premature low-battery warnings during your first measurement session, even with a full charge.
BMS lockout after the ST327 sat unused in a carry case for months
Lithium-ion cells self-discharge slowly in storage. If the ST327 has been shelved for several months with the battery installed, the cell voltage can drop below the BMS recovery threshold — typically around 2.5V per cell. At that point, the BMS enters a protection lockout and the instrument will not power on or accept a charge through the standard port. To recover the pack, connect it to a charger that supports a trickle or recovery charge mode and hold it there until voltage climbs above 3.0V per cell. Once the BMS exits lockout, a normal charge cycle will complete without issue.
ST327 shuts down mid-measurement with no low-battery warning beforehand
This happens when sustained sensor load pulls the cell voltage down faster than the instrument's state-of-charge indicator updates. The display can show a healthy charge level right up until the BMS hits its undervoltage cutoff and drops the instrument. An aged or partially degraded cell is the usual cause — internal resistance rises, so voltage sags sharply under load even though resting voltage looks fine. Replace the pack, run the calibration cycle from the instrument menu, then verify the battery indicator tracks correctly through a full logging session before the next field deployment.