Modern high-power flashlights use lithium-ion batteries.

Well, battery isn’t the right word.  The correct term is cell.  A battery is made up of several cells.  An example is the battery for a cordless power tool.  If you open it up, you’ll find several cells inside making up the battery.

Most flashlights use a single cell for power. 

If you’ve ever looked at lithium-ion cells you’ve probably been baffled by  the numbers that they’re listed  with.  18650, 14500,18350, 21700 etc.

It’s easy.  The numbers are the basic dimensions of the cell in millimeters.  The first two numbers are the diameter, the last three numbers are the  length to one decimal  place, for some reason.

18650 therefore means that the cell is 18mm in diameter and 65mm long.

You’ll also sometimes see three letters  in front of the number.  Usually, ICR or INR.  I stands for lithium ion. The middle letter is the  cathode material. C being cobalt, N being  nickel, M being manganese and  V being vanadium.  And the R tells you that it’s a cylindrical cell.

Different size cells also have different amounts of  energy they store.  It’s measured in milliamp hours (mAh).  To define that, if you have a electric circuit that uses 1 milliamp, a 1mAh cell  will take   1  hour to drain. 

Lets compare two fantastic cells (these are my recommended  cells in these sizes by the way).  The 18650 Samsung 30Q and the 18350 Keeppower UH1835P. 

The Keeppower holds  1200mAh while the  Samsung at the  same diameter and less  than twice  as long holds 3000mAh, nearly  three times  as much.   So capacity is linked more to the construction of the cell rather  than the size.  Bigger cells hold more energy but it's not necessarily proportionate to the dimensions

We’ll continue talking about batteries next week.  Stay tuned…

Got anything you want to know about flashlights?  Let me know in the comments and I'll add it to my How To Speak Flashlight list.