BP-5S Topcon STONEX Unistrong Compatible Battery 7.4V 3400mAh

Topcon STONEX Unistrong — 7.4V Li-ion Replacement Battery (BP-5S)

This is a 7.4V 3400mAh Li-ion replacement for the BP-5S battery pack used in the STONEX Unistrong GNSS receiver. The Unistrong is a portable positioning and surveying instrument used in construction layout, land boundary work, and geospatial data collection. Voltage and capacity match the OEM specification exactly.

• STONEX Unistrong compatibility: The Unistrong GNSS receiver uses a dedicated battery bay with a four-contact interface that handles both power delivery and BMS communication. This pack meets that voltage rail and handshake requirement so the receiver recognises the battery and reports state correctly.
• Bench tested on actual hardware: We ran this pack through the Unistrong's power-on sequence, satellite acquisition, and sustained logging. The BMS held regulation across the load transitions without tripping cutoff during GNSS signal lock.
• Post-install calibration cycle: After fitting this pack, run a full calibration cycle through the instrument's system menu before your first field session. The Unistrong maps battery state during calibration — skipping it causes premature low-battery warnings during your opening measurements even with a full charge.

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

Li-ion cells self-discharge slowly in storage. If a pack sits long enough, cell voltage can drop below the BMS recovery threshold — typically around 2.5V per cell for this chemistry. At that point the BMS enters a protective sleep state and refuses to accept a charge through the normal dock. To recover, place the pack in the charger for a full 30 to 60 minutes before the BMS wakes and allows current to flow. If the charger shows no activity after that window, the cells have likely dropped below safe recovery voltage and the pack should not be force-charged.

Unistrong shuts down mid-logging session with no low-battery warning

This is a voltage dropout failure, not a capacity failure. Under sustained GNSS logging with active satellite tracking, the combined sensor and processor draw can cause a momentary voltage sag that the BMS reads as an undervoltage fault — cutting power before the display shows a low-battery indicator. It happens most often when cell capacity has degraded and internal resistance is elevated. Check that the battery contacts in the bay are clean and making full contact, then verify the pack rests at or above 7.2V under a light load before your session.