Please clarify 18650 battery rating

[gcbryan]

18650 batteries have a 4.2V nominal rating. Yet I hear that under load it's more like 3.7V or some lower number.

Other batteries such as CR123 are 3V and that's the number you use to figure runtime. With 18650 you don't use 4.2V ...why is that the case only with 18650 batteries. I understand that you could get 4.2V right off the charger so you need to take that into account when designing a driver or when picking at led to use but for runtime what is the number you use for this battery...4.2V or 3.7V or what?

Also, while I'm asking questions... when a battery pack is listed as being wired as 5 in series and 2 parallel is this 7 batteries or 10?

 

[amigafan2003]

Also, while I'm asking questions... when a battery pack is listed as being wired as 5 in series and 2 parallel is this 7 batteries or 10?

5s2p would be 10 cells.

I'll let someone else answer your other question over 18650 cell voltage.

 

[VidPro]

to understand the loaded voltages of 123s
use the many wonderfull discharge graphs.
http://www.candlepowerforums.com/vb/showthread.php?t=195712

and for 18650s
http://www.candlepowerforums.com/vb/showthread.php?t=117117

then do your own graph :-) where you assemble your project on "the bench" and see what you get.

5S 2P is

|--{===]{===]{===]{===]{===]--| <-- 5 cells in series
|--{===]{===]{===]{===]{===]--|
^ by 2 sets of them in parellel.

10, good thing i didnt have to do math, pictures :-)

 

[hoongern]

18650 batteries have a 4.2V nominal rating. Yet I hear that under load it's more like 3.7V or some lower number.

Other batteries such as CR123 are 3V and that's the number you use to figure runtime. With 18650 you don't use 4.2V ...why is that the case only with 18650 batteries. I understand that you could get 4.2V right off the charger so you need to take that into account when designing a driver or when picking at led to use but for runtime what is the number you use for this battery...4.2V or 3.7V or what?

Also, while I'm asking questions... when a battery pack is listed as being wired as 5 in series and 2 parallel is this 7 batteries or 10?

All cells drop in voltage when in use. The higher the load you put on them, the more they drop.

Primary cells (CR123, Alkalines) tend to be specified at their resting voltage (when not under a load) - 3V and 1.5V respectively. However, under load, they will drop. A CR123 under some load would maybe only be 2.3V, and the alkaline might be 1V.

Rechargeable cells (Ni-mh, Li-Ion) tend to be specified at their average voltage throughout a discharge. However, their resting voltage, off the charger, is higher.

There is a lot more technicality involved, but that's a very simple summary.

As already mentioned by VidPro, looking at voltage graphs will let you know a lot.

However, I think it's also useful to look at say these graphs of CR123 tests, where you'll see that they aren't actually 3V when in use. Much lower, depending on how much load you put on them.

(If you are interested, here are some graphs of Alkalines and here for Ni-Mh)

 

[jasonck08]

As stated above, the more current you pull from a Li-on cell (or probably any battery) the more the voltage will drop.

For example, during a discharge at 1A for a 18650, the average voltage might be around 3.6v throughout the discharge curve. At 5A discharge the average voltage might be 3.2 or 3.3v.

 

[mdocod]

It has been in my experience that primary cells tend to be label rated closer to the open circuit voltage of the cell. While secondary cells tend to be rated closer to their average operating voltage under expected loads.

I'm not sure why this is the case, but I have found this to be true in more cases than not.

calculating approximated run-time requires a lot of information. The behavior of flashlight itself plays a large role. Study the discharge charts of cells done by others and work from there.

You'll see that most CR123 tests are in the 2.2V-2.8V operating range depending on load.