About this replacement
Theradome LH40 / LH80 Series — 3.7V Li-ion Replacement Battery (INR18650-1S1P)
This is a 3.7V, 3400mAh Li-ion cell replacement for the Theradome LH40, LH80, and LH80 Pro laser hair growth helmets. The INR18650-1S1P format is the factory cell used in the helmet's internal battery pack. Swap it in when the original no longer holds a full charge or the helmet cuts out mid-session.
- LH40, LH80, and LH80 Pro compatibility: All three helmets run the same single-cell 3.7V architecture and share the INR18650-1S1P cell format. The BMS handshake, connector layout, and voltage rail are identical across the range.
- Bench tested on actual hardware: We cycled this cell through the LH40 charge circuit and confirmed the BMS accepted the cell without a protection-fault lockout. Charge termination and discharge cutoff both triggered at the expected thresholds.
- Port care after scalp-contact sessions: The LH40 sits against the scalp, and moisture from sweat or topical treatments can wick into the charging port. Before reconnecting the charger, check the port is fully dry — even a thin film of moisture can trigger a BMS protection fault that mimics a dead battery.
Why the LH40 cuts out before the battery indicator hits low
The LH40 runs 678 laser diodes simultaneously. That combined draw creates a brief voltage sag at the cell level each time the helmet cycles its diode array. An aged cell with elevated internal resistance sags past the BMS undervoltage threshold before the fuel gauge registers low, so the helmet shuts off with charge still showing. A fresh cell with lower internal resistance handles the same load without triggering that cutoff.
Helmet shows charging but capacity never recovers past 50%
Daily top-off charging without full discharge cycles causes gauge drift in the BMS — the reported state-of-charge drifts away from the real cell capacity. The helmet charges to what the BMS thinks is 100%, but actual usable capacity has shrunk. To recalibrate, run the helmet until it shuts off on undervoltage protection, then charge uninterrupted to full from a cold start. If capacity still reads low after two full cycles, the cell itself has degraded and replacement is the correct step.