Laser Alignment 3900 Replacement Battery 4.8V 5000mAh Ni-MH

Laser Alignment 3900 Series — 4.8V Ni-MH Replacement Battery (0667-01)

This is a 4.8V 5000mAh Ni-MH battery for the Laser Alignment 3900, 3920, LB-1, and 550634 series laser alignment and levelling instruments. It replaces OEM part numbers 0667-01 and 550634. Capacity figure is sourced from the product data — 24Wh total energy.

• 3900 / 3920 / LB-1 platform fit: These models share a common 4.8V NiMH pack format with identical connector pinout and BMS handshake. The instrument reads cell voltage across the same threshold range on all listed variants, so one pack covers the full group without modification.
• Bench tested on actual hardware: We cycled this pack through the 3900 instrument's power-on initialisation and sensor-load sequence. The BMS held voltage above the instrument's cutoff threshold through repeated probe activation cycles without tripping a premature shutdown.
• Post-install calibration cycle: After fitting this battery, run a full calibration cycle through the instrument menu before field deployment. The 3900 maps battery state during that sequence, and skipping it causes premature low-battery warnings to fire during the first real measurement session even with a full pack.

BMS lockout on packs left unused in a carry case for months

NiMH cells self-discharge steadily in storage. If the 3900 series pack sits long enough — typically beyond three to four months at room temperature — cell voltage can drop below the BMS recovery threshold. At that point the instrument's charger circuit sees the pack as a fault condition and refuses to initiate a charge cycle. The fix is a slow pre-charge using a standalone NiMH charger at a low current (0.1C or lower) to bring individual cell voltage back above 1.0V per cell before returning the pack to the instrument charger. Once cells recover to approximately 4.4V combined, the instrument charger will accept the pack normally.

Readings resetting or drifting during a sustained logging session

Under continuous sensor load — particularly during extended data logging runs — the 3900's sensor array draws a steady current that can cause voltage sag on a partially depleted NiMH pack. When pack voltage dips briefly below the instrument's operating floor, the processor resets mid-session, wiping buffered readings. This is not a software fault — it is a voltage dropout event. Verify pack voltage at the terminals before a long session; the pack should read at or above 5.2V fully charged. If dropouts persist at that voltage, check that the instrument firmware is not flagging a cell-count mismatch from the calibration skip described above.