18650 Battery vs AA Battery
Both 18650 and AA batteries are cylindrical batteries that differ in size and that come in various types of battery chemistries, thus differing in the battery voltage, capacity, and other charging/discharging characteristics.
In order to simplify finding the batteries for their devices, some manufacturers ship their 18650 devices (mostly flashlights) with an AA battery holder that allows AA batteries to be used in the 18650 battery compartments. But, is this actually a good idea?
Published: December 13, 2021.
18650 and AA Batteries Features and Specifications
18650 batteries are rechargeable lithium-ion batteries featuring physical dimensions of 18.6 x 65.2 mm (~0.732 x 2.567 inches), a nominal voltage between 3.2 and 3.7 volts, and a nominal capacity between 1800 and 5000 mAh.
Since lithium-ion batteries are relatively sensitive to charging and discharging conditions, some 18650 batteries feature a small Battery Management System (BMS) that protects the battery from unwanted events - such 18650 batteries are called "protected 18650" batteries.
Also, 18650 batteries differ in the form of the positive terminal - there are "flat top" batteries and "button-top" batteries.
Which type suits your needs, You have to find it by reading the manual of the device you have.
Since lithium chemistries don't suffer from memory effect, 18650 batteries may be recharged as soon as one can put them in the charger, assuming that they are not too warm or too cold.
AA batteries feature physical dimensions of 14.5 x 50.5 mm (~0.571 x 1.988 inches) and are available in numerous chemistries as shown in the following comparison chart:
|Chemistry||Common Name||Rechargeable||Typical Capacity (mAh)||Voltage (V)
|Zinc Carbon||R6, 15D||No||600 - 1600||1.5|
|Alkaline||LR6, 15A||No (Mostly No)||1800 - 2700||1.5|
|Li-FeS2||FR6, 15LF||No||2700 - 3300||1.5 (1.8 max)|
|Li-ion||14500||Yes||600 - 2000+||3.6 - 3.7|
|NiCd||KR6, 1.2K2||Yes||600 - 1000||1.2|
|NiMH||HR6, 1.2H2||Yes||700 - 2800||1.2|
|NiOOH||-||No||2200 - 2700||1.5 (1.7 max)|
|NiZn||ZR6||Yes||1500 - 1800||1.6 - 1.65|
"Standard" AA battery is a non-rechargeable alkaline AA battery featuring a nominal voltage of 1.5V and a typical capacity of 1800-2700 mAh.
Note: one of the oldest AA battery chemistries is Zinc-Carbon chemistry with a nominal voltage of 1.5V and a typical capacity of 600-1600 mAh. But, since they feature a relatively high self-discharge rate and are prone to leakages, they are mostly replaced by alkaline AA batteries, although they are still available on the market.
Because of differences in size and chemistry, 18650 and AA batteries are able to store different amounts of energy:
- high-drain 18650 battery, 3.7V 2000 mAh: 7.4 Wh,
- high-drain 18650 battery, 3.7V 4000 mAh: 14.8 Wh,
- alkaline AA battery, 1.5V, 1200 mAh: 1.8 Wh,
- lithium-ion AA (14500) battery, 3.7V, 1500 mAh: 5.55 Wh,
- NiOOH (Nickel Oxy-Hydroxide) AA battery, 1.5V 2500 mAh: 3.75 Wh,
- NiMH (Nickel Metal-Hydride) AA battery, 1.2V 2000 mAh: 2.4 Wh, etc.
As one can see, the main difference between 18650 and AA batteries is the difference between the amounts of stored energy, with some models of 18650 batteries being able to store 14.8 (or even more) Wh of energy, while the best AA batteries are unable to store more than 5-6 Wh of energy.
So, if You are going to replace the single 18650 battery with the battery adapter holding one AA battery, be aware that the output power and the amount of energy are going to be decreased.
Personally, it may be great to have an option to choose between the batteries, but the 18650 battery simply stores much more energy than the AA battery and should be a preferred choice for many devices and gadgets.