Fenix Spare batteries

Overview

Fenix Spare Batteries are protected rechargeable lithium-ion cells supplied in two distinct sizes to suit different Fenix lighting products: the 21700-format ARB-L21-5000 and the 18650-format ARB-L18-3400.

Both are 3.6V Li-ion cells, but they are not interchangeable - the light in question must physically and electrically support the specific cell format. The ARB-L21-5000 is the larger of the two, measuring 21.5 x 76mm and weighing 73.5g, while the ARB-L18-3400 measures 18.6 x 69mm and weighs 50g.

Each battery includes a built-in protection circuit to guard against short circuits, overcharging, over-discharging and overheating, along with pressure-relief vents for an added layer of safety. Neither the standard ARB-L21-5000 V2.0 nor the standard ARB-L18-3400 has a built-in USB-C charging port, so a separate charger is required unless the battery is charged inside a compatible light.

Perfect For

These spare batteries are ideal for anglers who rely on a Fenix torch or headlamp during long night-fishing sessions and want a backup cell ready to swap in when the original runs low.

The higher-capacity ARB-L21-5000 (5000mAh) suits Fenix lights built for 21700 cells, such as the PD36R V2.0 and C7 V2.0, and is the battery supplied with the HM70R headlamp. The ARB-L18-3400 (3400mAh) is the correct spare for 18650-powered lights, including the HM65R-T V2.0. Always match the spare battery to your exact light's specified battery size rather than choosing whichever cell has the higher mAh rating.

Technical Insight

The protection circuitry built into both cells is what allows them to be used safely in high-output lighting - it monitors for short circuits, overcharging and over-discharging, while the pressure-relief vents provide a further safeguard against internal faults. Both batteries are rated for 500 charging cycles.

The ARB-L21-5000 supports a higher maximum output current of 7.5A and a maximum charge current of 5A, reflecting its role in more powerful modern torches, headlamps and lanterns. The ARB-L18-3400 has a lower 2A maximum charge current, in line with its smaller 18650 format. Both can be charged using a recommended 1A current, either via a compatible external charger or inside a suitable Fenix light.