Why use 2x123 batteries like 17670/68's when 2 123's will do?

What is the advantage (if any) of using 1 larger battery?

The one larger battery is a Li-ion rechargeable battery. The 18650 does give some lights longer runtimes than 2 cr123s. Some of us get the larger types cheap or free since they are used in laptops and other electronics. In the long run these batteries are cheaper to use for most users.

Many led lights can use the 17670, while only a few can use the slightly wider 18650.

Sverre

Ahh, now I see why people mention boring thier lights out to fit the 18650's, but don't you loose the high voltage advantage of 2 123's? 3.7v x 2.... what voltage are those larger batteries?

Is there a compairson chart somewhere??

Those 17xx0's and 18xx0's have the same voltage as RCR123s but they have much higher capacity. In the case of 18650s, it's close to 3 times as much, thus lasting almost 3 times longer.

These cells are normally used as subsitutes in LED flashlights. In a LED flashlight, there's circuitry to tune the supplied voltage to the forward voltage of the LED used. Hence, the "voltage loss" as seen by the LED itself isn't as much as you think. The decrease in brightness, if any, as a result isn't that noticeable given our logarithmic eyesight, but the gain in run-time is substantial.

Like Ciam said, but in slightly different words...

If a light will run on both 2x123 and 1 single 3.7v li-ion rechargeable, then, the light has some type of regulation. You don't really lose the "high voltage advantage", which I'm guessing you mean the 6 volts of the 2 primaries versus the 3.7 volts of the rechargeable, because the regulating circuit will, for the most part, keep the voltage at a constant level. In the case of Lux V ("5w") lights, even 6 volts isn't really enough to light the LED--the circuit will boost the 6 volts of the primaries or the 3.7 volts of the rechargeable to the needed voltage and keep the current constant.

Another thing that comes into play is the internal resistance of these two different types of batteries. The li-ion batteries have less internal resistance and can supply the needed current more effectively. That's why you sometimes see people running their 3x123 incandescent lights off of two rechargebles without loss of performance. It's because the 9 volts of the 3 primaries actually sags under the high current draw of the bulb (the voltage drops when trying to meet the demand for a current that is beyond it's ability to effectively deliver) making it more like the 8.4 volts of the two rechargeables (the 3.7v batteries charge, fully, to 4.2v). So, the two rechargeables, with less internal resistance, will be able to deliver the voltage and current more effectively, which makes up for their difference in voltage.

I've got a 3X123 light that actually runs BETTER on two rechageables! It will keep the brightness closer to maximum for a longer period of time than will the primaries.

Hope this helps.

Considering all of the recent reports of fire and explosion when using multiple 123s in series, using one cell is probably safer if you intend to pull a lot of amps, even when taking into account that amperage will necessarily double when voltage is halved.

I realize that many li-ion tool and notebook battery packs use unprotected cells in series with an external protection circuit, but then you don't normally carry those around in your pocket just inches from your 'nads

The voltage of the 18650's and 17670's being lower isn't a problem in lights meant to use them. They often use lower voltage bulbs that use more current, that will make up for it. LED lights will usually use a circuit to either lower or increase the voltage as needed for a certain application. In many of those cases the voltage of the 18650 being lower isn't an issue.

Have a look here to see why a single 18650 can be better than a pair of CR123a's or RCR123's. https://www.candlepowerforums.com/threads/134254 and here https://www.candlepowerforums.com/threads/128188