Li-Ion Protected Cells

'Protected' cells have a small protection circuit built into them to prevent them from being over charged, over heated, shorted, or over discharged. Because protected cells are about 3 mm longer than unprotected cells, they do not always fit lights and chargers, so it's important to determine if they are going to work with your equipment in advance.

Fenix ARB-L2S 18650 3400 mAh batteries $21.95

Panasonic NCR18650B Protected Batteries: $16.95

These are the Ferrari of cells, about the best money can buy. They have the highest mAh capacity currently available. The protected version weighs 48g, has circuit protection and is too tall (69.4mm) to run in the power packs, but fits the Spark headlamps and chargers.

Spark Protected Batteries

Spark Li-Ion rechargeable Battery 18650 3100mAh $14.95. This battery is manufacturered from a Japanese Panasonic cell, which along with Sanyo and GP, are considered to be the top 18650 batteries available.
- Capacity: 3100mAh 
- Nominal voltage: 3.60V 
- Charge voltage: 4.20V
- Charge current: 1650mA
- Charge time (Std.): 3 hours
- Discharge cut-off voltage: 2.50V
- Discharge current: 620mA
- Discharge cut off current: 30mA
- PCB Protection trip curent: Above 10A 

Spark Li-Ion rechargeable Battery 18650 2600mAh: $12.95. These are made from Japanese Sanyo cells.

- Capacity: 2600mAh
- Nominal voltage: 3.7V 
- Charging voltage protection: 4.2V 
- Protect circuit: 5A 
- Discharge cut-off voltage: 3.0V
Testing of this battery shows it has very good discharge capacity of up to 2.2 Ah while maintain a minimum voltage of 3.6v at a 2a current draw, making it suitable for the SX5 headlamp.

From independant test site: "This battery uses aSanyo cell, this is a good quality cell and as can be see on the curves the two cells does track very nicely. There will be no problem with series applications of this cell. The high voltage can be an advantage in smaller (i.e. 3-4 watt) lights where the battery replaces two CR123 cells. The PCB current limit is placed about 30% above the cell specification, this is a good value. All in all a very good battery. 

14500 Protected Cells

Nitecore 14500: $12.95

These are very good quality cells with built in low voltage cut-off so they cannot be over discharged. Due to the protection circuit, they will not fit into the AA Battery packs for the Wouxun radios.