Fenix ARB-LP21 Power Extender: Extend Flashlight Runtime

By Fenix Flashlights | World Leader in Fenix® Sales | Fenix Lighting USA | Published: 2026-10-17

Category: How-to Guides

Learn how to set up the Fenix ARB-LP21 Power Extender to extend your Fenix HM75R headlamp runtime and charge other devices via USB-C.

Quick answer

The Fenix ARB-LP21 Power Extender is a 5000mAh 21700 battery pack designed specifically for the Fenix HM75R Headlamp. To extend runtime, charge the ARB-LP21 fully via USB-C, connect it to the HM75R using the headlamp's power input, and secure it with the integrated belt clip. It also works as a standalone power bank for other devices.

What the ARB-LP21 Power Extender Does

The ARB-LP21 is not a universal battery—it is engineered for the Fenix HM75R Headlamp. It houses a 5000mAh 21700 battery and connects to the headlamp to dramatically extend runtime. Beyond headlamp use, it functions as a standalone charger for other devices via its USB-C port, so you can top up a phone or other small electronics in the field. Its IP68 waterproof rating and secure belt clip make it suitable for all-weather use.

Step-by-Step Setup and Usage

Follow these steps to get the most from your ARB-LP21. Each step includes a common mistake to avoid.

Steps to Extend Runtime with the ARB-LP21

Charge the ARB-LP21 fully: Connect the ARB-LP21 to a USB-C power source using a compatible cable. The integrated 5000mAh battery will charge; a full charge takes several hours depending on the power source. Common mistake: using a low-power USB port that slows charging. Use a 5V/2A or higher adapter for faster charging.
Connect the ARB-LP21 to the HM75R Headlamp: Plug the ARB-LP21 into the HM75R's power input port. The headlamp will draw power from the extender, extending runtime. Checkpoint: ensure the connector is fully seated. A loose connection can cause intermittent power.
Secure the ARB-LP21 with the belt clip: Clip the ARB-LP21 to your belt, backpack strap, or pocket using the integrated belt clip. This keeps the cable from snagging and reduces strain on the connector. Common mistake: letting the extender dangle by the cable, which can damage the port over time.
Use the ARB-LP21 as a power bank for other devices: Disconnect from the headlamp and connect your phone or other device to the ARB-LP21's USB-C port using a compatible cable. It will charge your device. Note: the ARB-LP21 is optimized for the HM75R; charging other devices may be slower than a dedicated power bank.
Monitor runtime and recharge as needed: The ARB-LP21's 5000mAh capacity roughly doubles the HM75R's runtime, but actual results depend on brightness level and conditions. Recharge the ARB-LP21 when depleted. Checkpoint: if runtime seems short, verify the ARB-LP21 is fully charged and the headlamp is not in a high-drain mode.

Product facts

Fenix ARB-LP21 Power Extender: Engineered specifically for the Fenix HM75R Headlamp, this power bank dramatically extends runtime with its integrated 5000mAh 21700 battery. It provides a dedicated, high-capacity power source for the HM75R, ensuring longer runtimes without needing to swap headlamp batteries.
Fenix ARB-LP21 Power Extender: Beyond powering your headlamp, it functions as a standalone charger for your other devices via its USB-C port. You can use it to charge a phone or other small electronics, making it a versatile accessory for camping or emergencies.
Fenix ARB-LP21 Power Extender: Designed for durability and convenience, it features a secure belt clip and a robust IP68 waterproof rating for reliable, all-weather performance. The belt clip keeps the extender accessible and secure, while IP68 rating means it can withstand rain, splashes, or accidental immersion.