AA, 14500 and 18500 cells look alike but differ in size, voltage and chemistry. Learn how solar lights work and why 3.2V LiFePO4 18500 cells power so many of them.

Walk the battery aisle and AA, 14500 and 18500 cells can look almost the same. They aren't interchangeable, though. Size, voltage and chemistry decide what a cell can power, how long it lasts and whether it's safe in your device. This guide explains the differences and why so many solar lights now use rechargeable 18500 cells.

Shop: IFR18500 Lithium Iron Phosphate 3.2V Rechargeable Battery (8 Pack)

AA vs. 14500 vs. 18500: the quick comparison

Cell	Size (approx.)	Typical chemistry	Nominal voltage	Typical capacity
AA (alkaline)	14.5 × 50.5 mm	Alkaline, single use	1.5V	~2,000–3,000mAh at light loads
AA (NiMH)	14.5 × 50.5 mm	Nickel-metal hydride, rechargeable	1.2V	~2,000–2,800mAh
14500	14 × 50 mm (AA size)	Li-ion (3.7V) or LiFePO4 (3.2V)	3.2–3.7V	~400–1,000mAh
18500	18 × 50 mm	Li-ion (3.6–3.7V) or LiFePO4 (3.2V)	3.2–3.7V	~1,000–2,040mAh

The names are just dimensions. "18500" means 18mm wide and 50.0mm long, and "14500" means 14mm by 50mm, the same size as an AA. That matching size is the trap. A 14500 fits in an AA slot but delivers 2.5 to 3 times the voltage, which can burn out a device built for 1.2–1.5V. Only use 14500 cells in devices that say they support them.

Why an 18500 instead of AA cells?

An 18500 is 18mm wide instead of 14.5mm, so it holds far more material than an AA-size cell. With lithium chemistry it also stores much more energy per cell, since one 3.2V LiFePO4 cell does the job of two or three NiMH AAs in series. Fewer cells means fewer contacts to corrode outdoors, a simpler circuit and a smaller battery compartment.

Shop: Panasonic NCR18500A 2040mAh 3.8A 18500 Battery

How a solar light works

1. Daytime charging: the small photovoltaic panel on top turns sunlight into current, and the controller sends it into the battery.
2. Dusk detection: when panel voltage drops (or a light sensor sees darkness), the controller stops charging and switches the LED on.
3. Night run: the battery drives the LED until morning or until it reaches its low-voltage cutoff.
4. Repeat: at sunrise the panel starts charging again. That's one full charge cycle every day, around 365 cycles a year.

That daily cycling is why the battery matters so much. A solar light battery charges and drains every single day, often in summer heat or winter cold.

Why solar lights use 18500 LiFePO4 (3.2V) cells

• Voltage matches the LED. A white LED needs roughly 3V to light. One 3.2V LiFePO4 cell can drive it directly. A single 1.2V NiMH AA needs a boost circuit to step the voltage up, and that circuit wastes energy.
• Cycle life. LiFePO4 (lithium iron phosphate) is known for long cycle life. Quality cells commonly handle well over a thousand cycles, which suits a battery that cycles every night.
• Heat tolerance and safety. LiFePO4 is among the most thermally stable lithium chemistries, a real advantage in a sealed housing sitting in direct sun.
• Flat discharge. LiFePO4 holds a steady voltage for most of its discharge, so the light stays evenly bright instead of slowly dimming.
• More runtime in the same footprint. The wider 18500 body fits more capacity into a short compartment.

Replacing solar light batteries: match the chemistry

Before buying replacements, check the label on the old cell or inside the compartment:

• Says IFR, LiFePO4 or 3.2V: replace with a 3.2V LiFePO4 cell of the same size, like our IFR18500 3.2V 8-pack or, for AA-size lights, our IFR14500 3.2V solar light batteries.
• Says 3.7V or Li-ion: use a 3.7V Li-ion cell of the same size, such as our 18500 3.7V 1400mAh flat top or the higher-capacity Panasonic NCR18500A.
• Says 1.2V, NiMH or AA: stick with NiMH AAs such as our EBL 2800mAh rechargeable AA 8-pack. Don't swap in a 14500, since the higher voltage can damage the light's controller.

Never mix chemistries, old and new cells, or different capacities in the same device.

Shop: IFR 14500 3.2v Battery (8 pack) Rechargeable - Solar Lights and More

Safety and certifications

Lithium cells should come with documentation: UN38.3 transport testing, IEC 62133 safety testing for portable cells, and a Safety Data Sheet (SDS/MSDS). You'll find the test documents for the cells we stock in our 18650 Data Center. For general handling, see our battery safety guidelines.

Shop solar and small-format cells

• IFR18500 LiFePO4 3.2V rechargeable battery (8 pack)
• IFR14500 3.2V LiFePO4 battery, rechargeable, for solar lights
• Panasonic NCR18500A
• 18500 Li-ion 3.7V 1400mAh flat top
• EBL rechargeable AA 2800mAh (8 pack)

Buying for a solar lighting product line? Request a bulk quote.

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