cr123 getting hot

[Lolaralph]

I'm powering a P7 mag mod with a shiningbeam 2.8-6v driver with two primary 123's. The two cells get pretty warm in use, is this normal or am I drawing too much from the cells and risking something bad
Any opinions appreciated

 

[VegasF6]

Maybe.
Is it this driver?
http://www.shiningbeam.com/servlet/the-132/3-dsh-Mode-Regulated-Circuit-Board/Detail

So, on high that driver should pull 2.8 amps from your batteries. I don't know what you are using, hopefully they are quality primary cells, not made in china. Let's take for instance a set of Duracell CR123A. Here is the data sheet: http://lib.store.yahoo.net/lib/theshorelinemarket/DURACELL-CR123A-ULTRA-DATASHEET.pdf
They are rated as 1550 mAh but, as you can see from the continous discharge graph, that is at 1000mA draw. Now, look at this graph so kindly provided by SilverFox. It is located a bit more than halfway down the page. You can see at 2.5A he is only getting ~22 minutes of runtime, and less than 1Ah.

If you only look at the label you see 1550 mAH and you figure OK, I am pulling less than 3 amps, ergo, less than a 2C load. 2C is fine, though it is getting borderline. But, at 22 minutes we see in reality this is a 3C load. And that is at 2.5A but your draw is actually a bit higher.

The most obvious giveaway is that you say they are getting hot. Lithium cells shouldn't heat up during normal usage, so even if you were within safe boundries I would still be concerned. Perhaps though the led heating the flashlight body could be heating the batteries, but, I doubt it.

In addition, the resting voltage of 2 primary CR123s is probably closer to 6.6V, a bit high for that driver. But, at high current I am sure the voltage sags more than enough to not be concerned about it.

If you are going to use a 7135 based linear regulator (that is what you have) then your best battery solution is a single cell, 18650 or larger. You won't get a ton of runtime before falling out of regulation, but it is safe.

 

[Lolaralph]

Thanks VegasF6, I really appreciate the input, I had been running the light on an 18650 but stuck the two primarys in just to compare output, it sure seemed alot brighter on the primarys and doing a tailcap draw check with my DMM they showed 2.4 amps draw where the 18650 was only showing around 2.0-2.2 amps. Anyone have any suggestions as to how to safely get 6.0 volts to the driver?

 

[Lighthouse one]

There is a reason why high power lights often use 2 18650 batteries. You could try an IMR 18650. They have less resistance, and can draw 10 amps, but they only have 60% of the capacity that a regular 18650 has. I really wouldn't keep using the 2 123 batteries.

 

[Chodes]

The driver will not draw 2.8A from your batteries.

If it is delivering a true 2.8A , that is to the LED at a lower voltage than your 2x 123 cells.

They are probably delivering about 5.8v to the driver, which is not 100% efficient , which is then providing about 3.7v or so to the LED (whatever volts is required to draw 2.8A)

That 2.1v or so headroom means (assuming vf of LED is 3.7v) means you have about 1.6 times the required volts. If the driver was 100% efficient ,
divide 2.8A by 1.6 and that's about 1.75A from the cells.
Taking driver inefficiency into account , say 80% , it will be more like 2 - 2.1A.

2-2.1A at start up, increasing as cells deplete is a high load for primary 123s. Surefire M6 draws about 2.5A from it's 123 primaries and that is considered about the limit.

 

[VegasF6]

Chodes, it is a linear driver, not switch mode.

 

[Lolaralph]

Thanks to everyone who has responded, all the excellent info is greatly appreciated, and will be put to good use. This modding is sure a continuous learning experience.

 

[Chodes]

Chodes, it is a linear driver, not switch mode.

Oh , OK. The cells will be getting hot from the load and the diver will be getting pretty hot too..

Using a single 18650 would be a much better choice - it will handle the load and the driver will run cooler.