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CR123A primary cells, when BRAND new, fresh out of the box, measure about 3.2V per cell. However. When you put a load on them, that voltage does not stay 3.2. This is the case with ALL batteries. They have an open circuit voltage, and they have a working voltage. Depending on how much you demand from the cell (in amps) will determine how much variation there is between open circuit voltage and working voltage...
Please Refer to Silverfoxes charts of CR123A discharge characteristics here http://www.candlepowerforums.com/vb/...ad.php?t=67078
You will see that at 1 Amp, most cells almost immediately sag to 2.5V.
Now, most standard 9V lamps are from 1.1-1.3 Amp.
The HO-9 is about 1.5Amp. At 1.5Amp we can expect the average CR123A cell to sag to about 2.4V within a few seconds of being fresh. 3X2.4V=7.2V
So.. when designing high power tactical lamps, they take this into consideration.. They need the lamp to be able to survive that initial high-voltage spike of the first few seconds, but they try to make the bulb in such a way that it will run strong and bright far below 9V, because in reality, the bulb will rarely if ever actually get 9V after the voltage sag of the batteries under a load.
Take a look under "lamp specifications" at this page:
http://www.lumensfactory.com/specifications
you'll see they have designed the target voltage for 9V lamp to be 7.6V. smart eh?
Also
take note of the EO-9, which is a 7.2V lamp... this is because it draws 2.0amps and sags the voltage of the cells down even further. they took this into consideration when designing this lamp, by doing so, it is brighter and whiter off of the cells it was intended to be used with.
Now... Here's the deal withe Li-Ion.
Yes they do only come off the charger at 4.2V (8.4V total for a pair in series). But as you can see above, the fresh voltage of a cell says very little about the way it works under a load. GENERALLY speaking,
Lithium Ion cells suffer from less voltage sag than Lithium Primaries. As a result of this, the ideal voltage range for running a 9V tactical lamp is reached without a problem. Now.... Ordinarily speaking. The ideal setup is to use a 3 cell light like the C3 or 9P or something. This way you can use larger cells. For example. A pair of 17500 size cells are a direct replacement for a set of 3 CR123s. The larger higher capacity cells will also perform as well or better than the original configuration of 3 CR123s..
When you step down a "notch" in size and try to run powerful 9V lamp assemblies off of small RCR123 cells, depending on the cell, you do loose a little bit of performance and a lot of runtime trying to get so much out of so little... but if compact size and bright are important, then we make these sacrifices. As far as real world use goes, you wouldn't be able to tell much different between a HO-9 on a pair of RCR123s to a HO-9 on 3 CR123s... in fact, it's hard to distinguish the difference between LFs HO-9 and EO-9. The human eye just isn't a very good light meter.
On a pair of BIG Li-Ion cells like 17670s, the HO-9 runs at about 1.6Amp. On the small RCR123s, it runs closer to 1.5Amp.... So a bigger set of cells will run a little brighter, but you can't see much difference. For long runtime, many people are using "4 cell" sized bodies (length of 4 CR123A cells) that are bored to accept 18mm diameter cells... a pair of 18650s gives about 2200mAh capacity instead of the ~550mAh you get from a pair of RCR123s, So they can drive even more powerful lamps, or run the regular ones much longer. But at the sacrifice of space... [WOLF-EYES and PILA brand lights come from the factory with 18mm diameter cells in mind, no boring neccessary, if you are new to the world of Li-Ion, you would do yourself a great favor to start with one of these brands, being able to support the extra mm diameter cell, adds about 30-50% capacity and current capability to your configuration, you can also piece together a Surfire Style Light using many of the available Surefire heads, bezels, tailcaps, and lamps, in conjunction with Leef or FM bodies sold at Lighthound. The result is Surefire Like quality with support for 18mm cells. this is generally the most expensive, but also the most rewarding, and adaptable option available.]
It's important to note that rechargeable cells are often rated differently than primary cells. Specifically, most primary cells are rated at their "fresh" open-circuit voltage output, while most rechargeable cells are rated at their "nominal" or average output into a load. So a
CR123 primary is rated 3.0V, but in reality, under a load, it is usually around 2.25V-2.75V depending on how demanding the load. A Li-Ion cell is rated 3.7V, but in reality, it actually HOLDS 3.7V or higher into most loads. [food for thought time]:The same holds true in comparing Alkaline primary cells to rechargeable NIMH cells. In reality, even thought the NIMH is only rated 1.2V, it will often hold a higher voltage through most of a discharge than it's "1.5V" Alkaline cousin.