Petzl E36NAO Replacement Battery 3.7V 2600mAh Li-ion

Petzl Nao / Nao+ / E36NAO — 3.7V Li-ion Replacement Battery (ICR18650-35)

This is a 3.7V, 2600mAh (9.62Wh) Li-ion cell that replaces the ICR18650-35 in the Petzl E36NAO, Nao, and Nao+ headlamps. These headlamps are used in mining, caving, and technical outdoor work where consistent light output matters. A degraded original cell causes the headlamp to step down output early — swapping this cell restores full driver performance.

• Nao series compatibility: The E36NAO, Nao, and Nao+ all run the same 18650 cell format with the same voltage rail and connector. Petzl's reactive lighting driver in the Nao+ communicates charge state over a single-cell BMS handshake — this cell meets that spec without modification.
• Bench tested on actual hardware: We cycled this cell through the Nao+'s reactive lighting driver at full output. The BMS held voltage above the driver's 3.0V cutoff through a full discharge curve with no premature stepdown.
• Single-cell replacement note: The Nao series uses one 18650 cell, so there's no multi-cell balancing concern here. What matters is not mixing this new cell with a partially discharged old one if you have a spare pack — always run this cell as the only power source from a full charge to avoid confusing the headlamp's charge indicator.

Petzl Nao+ reactive driver stepping down output before the battery indicator warns you

The Nao+ uses a reactive lighting driver that continuously adjusts brightness based on ambient conditions. When the cell voltage sags under high-draw conditions — typically below 3.4V — the driver steps output down before the low-battery LED triggers. This isn't a fault; it's brownout protection keeping the driver stable. A worn cell with high internal resistance sags faster, so you see the stepdown much earlier in the charge cycle. A fresh cell at 2600mAh holds voltage above 3.4V for significantly more of the discharge curve, keeping the driver in its upper output range longer.

Headlamp flickering or cycling through modes at end of charge

If the Nao or Nao+ starts flickering or rapidly cycling modes near the end of a charge, the cell voltage is dropping below the driver's minimum threshold and recovering slightly — causing the driver to restart repeatedly. This happens with aged cells whose voltage bounces around 3.0–3.1V under load. It's not a driver fault and the headlamp isn't broken. Switch to the lowest output mode immediately to stabilise the driver, then recharge — if it happens at the start of what should be a full charge, the cell needs replacement.