Battery FAQs
by TigerhawkT3

1. Cell? Battery?
2. What do protected/unprotected mean in regards to Lithium-Ion?
3. What are the different types of Lithium-Ion?
4. What's up with these five-digit batteries?
5. Why can't I use alkalines in more lights?

1.
Q: Cell? Battery?
A: A cell is a single "piece" of power. For the alkaline chemistry, for example, a single cell has a voltage of 1.5V. For NiMH, it's 1.2V, and so on. Cells can be combined in series or parallel to form a "battery" of cells.

2.
Q: What do protected/unprotected mean in regards to Lithium-Ion?
A: As mentioned in the "what is double-tapping" question, a "protected" Li-Ion has a small electronic circuit integrated into the cell packaging. It protects against common dangers, such as overcharge, overdischarge, short-circuit (overcurrent), and temperature. These cells are safer to use, both individually and in batteries. They are less likely to ignite and cause personal or property damage, a phenomenon known as "venting with flame." Unprotected cells do not have this protection circuit, so they can have more capacity and current capability than protected cells. Some cells, such as 10440 (AAA), are too small to accomodate a protection circuit and are only available in unprotected form. Users must decide whether they prefer to stick with protected cells or accept the bigger responsibility and danger associated with unprotected cells.

3.
Q: What are the different types of Lithium-Ion?
A: Lithium-Ion is a rechargeable family of cell chemistries. Ordinary Li-Ion types are referred to simply as Li-Ion, RCR, or by their metric size designations (see the question "what's up with these five-digit batteries?"). When they have just been charged, they have an open-circuit voltage of about 4.2V (make sure your charger terminates at 4.20V at the most). They have a nominal voltage of about 3.7V, after a bit of use and under a moderate load. You should recharge them once they reach about 3.4V-3.5V. To make Li-Ion a more suitable replacement for primary (nonrechargeable) CR123As, with their 3V nominal voltage, some manufacturers add some sort of circuit or dropping resistor into their 16340-sized cells, giving them a lower voltage intended to mimic that of primaries. These have reduced capacity compared to primaries or even ordinary 3.7V Li-Ion, but they serve their purpose. A relatively new type of Li-Ion is LiFePO4, or Lithium Iron Phosphate. With a resting voltage of about 3.2V-3.3V, it has lower voltage (and slightly reduced capacities) as compared to the classic Li-Ion discussed above, can handle significantly more current, and is safer. For example, CR123A-sized LiFePO4 cells can handle about 5A, whereas the same current would require an 18650-sized Li-Ion cell or greater. LiFePO4 is available in 16340 (CR123A), 14500 (AA), and 18650 sizes.

4.
Q: What's up with these five-digit batteries?
A: Cells of various types can be referred to by their standardized size codes. The first two digits are the cell's diameter in mm, the second two digits are the cell's length in mm, and the "0" usually found at the end indicates a cylindrical cell. An ordinary AA cell, for example, is 14mm in diameter, 50mm long, and cylindrical, so it would be a 14500. Other common sizes are 10440 (AAA), 26500 (C), 16340 (CR123A), 17500, 18500, 14670, 17670, 18650, and more.

5.
Q: Why can't I use alkalines in more lights?
A: Alkalines can't deliver current nearly as well as other chemistries, such as NiCad, NiMH, lithium, or Li-Ion. A AA cell, for example, can't really do better than 4-500mA. Any more, and it will only provide a tiny fraction of its advertised capacity. The other chemistries mentioned above can provide high current quite reliably throughout their advertised capacity.