Fenix ARB-L21-5000U battery on white background
Fenix ARB-L21-5000U battery being recharged via built-in usb-C charging
Fenix ARB-L21-5000U battery transparent image showing multiple layers of protection
Fenix ARB-L21-5000U battery pressure relief vents
Fenix ARB-L21-5000U battery specifications chart
Fenix ARB-L21-5000U battery on white background from the side
Fenix ARB-L21-5000U battery on white background showing charging port

Fenix ARB-L21-5000U Rechargeable 21700 Battery

Regular price$26.95
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The Fenix ARB‑L21‑5000U is a high-capacity 21700 rechargeable Li‑ion battery delivering 5,000 mAh of reliable power for high-drain flashlights and gear. Equipped with a built-in USB‑C charging port, you can recharge it quickly with common USB‑C cables—no separate charger required. Ideal for extended runtime and convenience.

Safety is front and center: it features comprehensive protection circuitry (overcharge, over-discharge, short-circuit, and overheat protection), as well as pressure relief vents for safe energy release. The button-top design ensures compatibility with a wide range of devices. Rated for 500 charge cycles, it maintains dependable performance over time.

Key Specifications:

  • Capacity: 5,000 mAh, Nominal Voltage: 3.6 V
  • Charge Rate: USB‑C port, up to 5 A max (standard 1–2 A recommended)
  • Output: Up to 8 A discharge
  • Safety Features: Multi-layer protection circuitry, pressure relief vents
  • Longevity: Up to 500 full charge cycles
  • Design: Button‑top format, 73 mm long, ~21.6 mm diameter, ~72 g
  • Warranty: 12‑month manufacturer coverage

A robust and convenient power solution—perfect for tactical flashlights, headlamps, outdoor gear, and emergency devices.

Shipping Note: Lithium ion batteries are considered "dangerous goods" per US transportation regulations and can often only be transported via ground shipment. Orders containing loose batteries will typically take much longer than predicted to be delivered. Delivery times of 14+ days are not uncommon.  

  • 5000mAh Capacity
  • USB Type-C Charging
  • 500 Charging Cycles
  • Protection Circuits in Anode
  • Overcharge Protection
  • Over-discharge protection
  • Short Circuit Protection
  • Overheat protection
  • 3 sets of overheat protection
  • Pressure relief valve
  • Microporous PTC Cathodal Block
  • 3.6V USB Charging - USB-A to USB-C Cable
  • Single Port Charger
  • 1 Amp/2 Amp at 10 Watts Max
  • Maximum Output Current 8A
  • Weight: 2.56 oz / 72.5g
  • 2.99" Height
  • 0.85" Diameter
  • **USB Cable NOT Included**
  • One Fenix ARB-L21-5000U USB-C Rechargeable 21700 battery

Overall rating: 4.805936 / 5 from 438 reviews.

AI Generated Review Summary

The Fenix ARB-L21-5000U Rechargeable 21700 Battery offers high-capacity power with a 5,000 mAh capacity, USB-C charging, and comprehensive safety features. Customers praise its exceptional performance, long life, and quick charging. The product is well-regarded for its quality and value for money.

Summary topics

  • Battery Performance: 2%
  • Battery Quality: 8%
  • Battery Life: 3%
  • Charging Speed: 12%
  • Battery Power: 4%

Review topics: [].

Review highlights

  • "Excellent quality, customer service, and shipping."Raymond P.
  • "As a nightwatch I must say, Fenix makes a great high luminance light with awesome rechargeable batteries."Denis L.
  • "Built-in charger and charge port is genious"Marcela L.

Reviews

"Exactly what i thought i was getting. Took awhile to get but I was in no rush. Wanted OEM batteries to keep charged and on back up. Modern day flashlights are insanely powerful in small packages but at the highest setting they dont last long. Get get everything so its nice having back up batteries and these will do. USB charging with all the protection included. Don’t want to use cheap batteries in nice flashlights. Thats not nice to them , give your lights the fuel to run at 100%."

Jason (5/5)

I’m unclear about what I’m

"I’m unclear about what I’m about to say, but it’s something that confuses me. The battery is 5000 mah, however when I charge it with a smart charger and it only charges up to ~4000 mah when it indicates “ FULL” I’d lov to hear an explanation of why that is."

Gary S. (3/5)

Not what they use to be

"I’ve bought at least ten different head lamps and twice as many batteries. They claim 500 charge cycles. The last ten batteries I’ve bought arrived completely drained. They don’t hold a charge. Customer service told me I wasn’t using the correct usb port. Lithium batteries should be stored at 60 percent capacity not dead. Very disappointed"

Billy m. (1/5)

Dandy

"Much needed extra battery."

Stephen A. (5/5)

Don't buy!!!

"I was given one as a gift and the battery went bad in 2 months"

Mike (1/5)

Spare Lithium battery for longer outings

"Haven’t used the battery yet due to winter but expect the additional charge available to be invaluable for longer bike packing adventures."

Steven R. (5/5)

The best battery,. I've used

"The best battery,. I've used this battery on going 4-yrars now and am very happy. . . Dale"

Dale R. (5/5)

Best one yet👍🏼

"Awesome battery, long lasting. I like it because it has its own charging port. I only buy my batteries from fenix and all my lights. The qualities of this batteries are second to none. They perform very well."

Jonathan C. (5/5)

Battery

"Doesn’t work with head lamp"

Wyatt N. (1/5)

Fenix has great products at

"Fenix has great products at great prices."

Todd W. (5/5)

Q&A