Topcon FC-236 Replacement Battery 3.7V 6800mAh

Topcon FC-236 / FC-336 — 3.7V Li-ion Replacement Battery (60991)

This 3.7V 6800mAh Li-ion battery replaces the OEM 60991 pack in the Topcon FC-236 and FC-336 field controllers. These are ruggedised handheld data collectors used in survey and construction work, often running full shifts away from any power source. The cell matches the original voltage rail and connector so the device powers on without configuration changes.

• FC-236 and FC-336 compatibility: Both controllers share the same battery bay dimensions, connector pinout, and BMS handshake protocol — which is why a single 60991-spec pack covers both models. The voltage rail sits at 3.7V nominal across the platform.
• Bench tested on actual hardware: We ran this pack through charge and discharge cycles on the FC-236 bench unit. The BMS communicated state-of-charge correctly to the OS, and the charge controller accepted the pack without fault codes on first insertion.
• Field swap tip for survey use: If you are mid-job with active data files open, save and close your project before pulling the old pack. The FC-236 holds a brief capacitor reserve, but an unexpected drop during a write operation can corrupt the active job file.

Why the FC-236 reports incorrect battery percentage after a pack swap

The FC-236 tracks state-of-charge through a fuel gauge IC that learns discharge curves over time. When a new pack goes in, the gauge has no calibration history for that cell, so it interpolates from the last known data — which belongs to the old battery. This causes the percentage readout to jump or read incorrectly for the first one to two full cycles. Run the new pack from a full charge down to the automatic low-battery cutoff, then charge it fully again. After one complete cycle the fuel gauge recalibrates and the readout stabilises.

FC-236 shutting down mid-session despite battery showing charge

This usually points to a BMS under-voltage trip rather than actual cell depletion. The FC-236 draws a surge current when writing data to storage or refreshing the GPS fix, and if the cell voltage dips below the BMS cutoff threshold under that load, the pack disconnects even though the resting voltage reads fine. A fresh pack that has not been through a full charge cycle has lower electrolyte conductivity and is more prone to this on first use. Charge the replacement pack to 100% before the first field session — confirmed full charge on the FC-236 is indicated by a solid green charge LED, not just the OS percentage reaching 100.