About this replacement
Geo-Fennel Multi-Liner FL 50 plus — 3.6V Ni-MH Replacement Battery (500000-13)
This 3.6V 1800mAh Ni-MH battery replaces OEM part 500000-13 in the Geo-Fennel Multi-Liner FL 50 plus and FL 50 laser levels. It restores power to the self-levelling and line projection functions used in construction layout and site measurement. Dimensions are 49.50 × 42.50 × 16.50mm — confirm these match your battery bay before ordering.
- FL 50 and FL 50 plus fitment: Both models share the same battery bay dimensions, connector orientation, and 3.6V supply rail, so this single pack covers the full FL 50 series without modification.
- Bench tested on actual hardware: We cycled this pack through the FL 50 plus power-on sequence, including pendulum unlock and laser diode ignition. The BMS held steady through the ignition current spike and did not trip into protective cutoff.
- Post-install calibration cycle: After fitting this pack, run a full calibration cycle through the FL 50 plus instrument menu before field deployment. The instrument maps battery state during calibration — skipping this step causes premature low-battery warnings on the first measurement session.
BMS lockout after the FL 50 plus sat unused in a carry case for months
Ni-MH cells self-discharge at roughly 1–2% per day at room temperature. After several months in storage, cell voltage can drop below the BMS recovery threshold, and the pack enters a protective sleep state. The FL 50 plus will either show no response or display a battery fault on power-on. To recover the pack, connect it to the charger and leave it on charge for a full uninterrupted cycle — most chargers will trickle-charge a depleted Ni-MH pack back above the 3.0V threshold before switching to fast charge.
Readings drifting or resetting mid-session during sustained laser operation
The FL 50 plus drives the laser diode and self-levelling motor simultaneously under load, which draws more current than standby operation. If cell internal resistance has risen — common in aged or deeply discharged Ni-MH packs — voltage sags under this combined draw and can cross the instrument's low-voltage shutdown threshold mid-measurement. The result is a spontaneous reset or a frozen reading that forces a restart. A fresh pack holding a stable 3.6V under load eliminates this dropout; verify charge state before each session by checking the indicator after a full charge cycle.