The FR6 is a 1.5 volt AA lithium cell using lithium iron disulfide chemistry, the same 14.5 by 50.5 mm size as a standard AA. Energizer sells it as the L91 (Ultimate Lithium AA). It is a drop-in replacement for an alkaline AA that lasts far longer in high-drain and cold-weather use.
FR6, side view, drawn to scale: 14.5 mm across, 50.5 mm tall.
Voltage
1.5 V
Size (D×H)
14.5 × 50.5 mm
Chemistry
Lithium iron disulfide (Li-FeS2)
Capacity
~3000 mAh
Same battery, other names
Every code below is the exact same cell as FR6. Any of them will fit and work identically.
FR03 / L92 (AAA lithium): The AAA version of the same 1.5 V lithium chemistry; smaller case, lower capacity, same advantages.
Buying and replacement notes
The FR6 (L91) fits anywhere an AA alkaline (LR6) goes and starts at the same 1.5 V nominal, so it is a straight swap. Its advantages show up under load and in the cold: it holds voltage where alkaline sags, weighs about a third less (roughly 15 grams versus 23), runs from about -40 to 60 C, and keeps most of its charge for around 20 years on the shelf. That makes it the standard choice for trail cameras, outdoor locks and anything left unused for months.
One caution: a fresh lithium AA reads closer to 1.8 V off-load, higher than an alkaline. Almost all AA devices tolerate this, but a few voltage-sensitive electronics run warm or misbehave on lithium, so check the manual for gear that specifically calls for alkaline. Lithium AA cells are primary (single-use) and must not be recharged.
Commonly used in
digital camerastrail cameraswireless door locksoutdoor sensorshigh-drain flashlightsGPS units
⚠️ Keep away from children. Swallowed button batteries, especially 20 mm lithium coins, cause severe injury within hours. If a child swallows one, call the 24/7 National Battery Ingestion Hotline (US) at 800-498-8666 or poison control at 800-222-1222 immediately. What every parent should know.
Frequently asked questions
Is an FR6 the same as an AA battery?
Yes, in size and voltage. The FR6 is a 1.5 V AA cell at 14.5 by 50.5 mm, sold as the L91 or Energizer Ultimate Lithium AA. It drops into any AA slot in place of an alkaline LR6. The difference is chemistry: lithium iron disulfide instead of alkaline, which lasts longer under high drain and in the cold.
Can I use lithium AA batteries in any device?
Almost always. Lithium AA cells match the AA size and 1.5 V nominal, so they work in nearly everything that takes an AA. The one exception is a small number of voltage-sensitive devices that specify alkaline, because a fresh lithium AA reads about 1.8 V off-load. Check the manual if a device warns against lithium.
How long do lithium AA batteries last?
Much longer than alkaline in demanding use. In a digital or trail camera an FR6 can outlast an alkaline AA several times over, and it keeps roughly 90 percent of its charge after years in storage thanks to a shelf life near 20 years. In low-drain devices like clocks the gap is smaller and alkaline is cheaper.
Are lithium AA batteries rechargeable?
No. The FR6 (L91) is a primary, single-use cell and must never be recharged; trying to charge it is unsafe. For rechargeable AA power you want NiMH cells, which are a different 1.2 V chemistry. Lithium AA is the choice when you want maximum runtime and shelf life from a disposable cell.