Battery Level and Laser Performance

[Arc]

I would like to know if the following is true. I have a laser using an LM317 based driver. The laser has multiple diodes and driver for each diode. Total current draw is 2A and when the batteries are fully charged they give 8.4V (2 Li-ions in series). Now I have read in a couple of different places that the optimal voltage to feed into this type of driver is 7.2V.
My question is will fully charged batteries at 8.4V give more laser performance as compared to when their charge level drops to 7.2V or will the output be the same. I can't seem to tell any difference but I don't have an LPM so I can't be sure.

Cheers

 

[Cyparagon]

The lm317 is no-doubt configured for constant-current. This means that performance will not change at all until the input voltage is outside a certain range that depends on the load voltage and the heat sinking on the IC. Generally speaking, the lm317 will provide proper regulation provided the input voltage is at least 3.5V above the output voltage. If these are red or IR diodes you're talking about, the lm317 will provide good regulation until the lithium cells are drained. If it is another color, lm317 was a bad driver choice and will not work reliably.

tl;dr: no change for red or IR.

 

[Arc]

Thanks very much for your reply. What about the voltage under load. For example when the battery voltage is 7.4V without load, then under load the voltage is only 5.9V. Will this then be reducing the performance of the laser diodes? Note that I do believe the minimum voltage for this driver is 6V but not entirely sure if this is 6V under load or no load.

And when you say that performance will not change until a threshold is reached, will this be a sudden noticeable change like going from bright to dim all of a sudden or will performance just gradually fall away as voltage keeps dropping?

 

[Cyparagon]

Current will fall gradually when the voltage reduces beyond that point, because the IC falls out of current regulation.

If your voltage sags that much under load, one or both of your cells are defective.

 

[Arc]

They are both very new and high quality Panasonic cells that I purchased in store from Battery World. How much should the voltage sag under 2 amps load?
Note that I have never had any issues with these cells, they have always worked perfectly, have consistently great battery life, they discharge at the exact same rate every time and they always charge up fine and consistently in terms of charging time. Also I've never discharged them below 3V or charged them past 4.2V and always use a Nitecore Intellicharger i4.
I also have many cheap cells from ebay and they all sag a bit more under load compared to the Panasonics. More notable though, the Panasonic cells last twice as long as the cheap ones.

Based on the above do you still believe they are defective? And is there anything else I can check to see if they are healthy or not. Is this correct for the internal resistance?
Ri=(VNL-VL)/IL=(7.4V-5.9V)/2Amps=0.75 Ohms

And if so is this a healthy value?

 

[Cyparagon]

How (where, exactly) are you measuring the voltage under load?

Ri=(VNL-VL)/IL=(7.4V-5.9V)/2Amps=0.75 Ohms

That is correct, assuming you've measured everything properly. I would expect <150mΩ per cell.

 

[Arc]

I'm measuring the voltage directly from the batteries.

So if it is correct then 750mΩ is 375mΩ per cell right? Then would this mean that these cells are unhealthy since its more than double that of the 150mΩ you mentioned?

 

[Cyparagon]

And you're getting the 5.9V reading when the cells are under load? Can I see a picture of the device, please?

 

[Arc]

Yeah sure here is what it looks like and you can also see the voltage readings. The volt meter panel is connected directly to the batteries via a momentary switch.

 

[LarryDFW]

Those aren't Panasonic cells in your photo.

Panasonic cells should NOT be dropping that much under the 2Amp load you described .

LarryDFW

 

[Cyparagon]

They're probably cheap chinese cells. They're good enough for lower power things, but you either need a larger (18650) cell, or a higher quality cell. Google "IMR 16340" if you don't want to use the larger cells. Staying away from brands with FIRE in the name will increase you chances of getting a decent cell.

 

[Arc]

They actually didn't look like this when I purchased them. They were black and had a protection circuit. I had to remove the protection circuit to make them fit into this device as well as my laser pointer which is in a CreeC6 host. And to remove the protection circuit I had to remove the outer black shrink wrap leaving the grey wrap. They looked very legit when I bought them inside the original packaging. Also Battery World is a very reputable vendor here in Adelaide. I haven't been able to find any information suggesting that they sell cheap chinese cells disguised as quality ones. I'd be very surprised if they actually sold me fake cells for top dollar.
Do you think that in removing the protection I damaged the cells?? I did it very carefully. They have always worked very well for me giving literally double the battery life compared to various fire brand cells that I have. They also seem to provide plenty of power. Also I know that I get very close to 1500s of "on time" from a full charge of these so called panasonic cells as measured from direct testing. This equates to (1500s/3600s/h)x2000mA=833mah. Now they are rated for 850mah. This is very close to the rating and thus a sign of legitimacy. These tangsfire 18350's I have for example are rated for 1500mah and only give around 750 seconds of use in this device or 416mah. At a lower draw I'm sure they'd come closer to their rated capacity though.

Also since it's dropping to 5.79V under load which is below the 6V minimum for lm317, will this be affecting laser performance? I actually notice a loss of performance (just by eye) when the cells are down to about 7V no load which would be about 5.5V under load given the sag. At this point I consider the cells to be drained hence I get 833mah going from 8.4V down to 7V. Apart from this sag I have been very satisfied with these cells and have always considered them to be worth the extra 300% premium I payed compared to my fire brands.

Please help me figure this one out somebody.

 

[dubious speculation]

Obtaining quality brand name cells from a reputable seller is a good starting point. I have not heard of
Battery World before but I do know that the ebay seller supersports600 supplies genuine AW cells.

AW 18350 IMR Li-Mn 3.7v. They are quite expensive compared to their retail value, but at least it's from a local Australian seller.

 

[Arc]

Yeah I actually happen to own a pair of AW IMR Li-Mn cells. I bought them from the seller seller"vapsol-australia" with 99.4% positive feedback.
I tested them in my laser that draws 2 amps and the voltage sag under load was 1.2V rather than 1.5V for the "panasonic". Slightly better but not so great is it? These must be fake cells too then huh?
Do you think if I buy a pair of AW IMR from supersports600 then I will get cells that don't sag so much? How much should they sag under 2A load? About half a volt?

 

[Cyparagon]

1.2V each? I'm starting to think you're measuring the voltage wrong again. Is it at the battery terminals, or through some wiring? What exactly are you using to connect the two cells in series?

 

[Arc]

I'm using steel to make the connection that puts the cells in series, all connections are very solid. The voltage is measured from the cells as in the same positive and negative terminals that the drivers connect to for power. Yeah it is measured through wiring. In the 2nd photo I uploaded you can see the volt meter on the battery lid and the wiring that connects it down through to the teminals of the cells.
It's not 1.2V each, I meant total for the 2 cells in series. I checked again and it's going from 8.11V no load, to 6.98V under load, so only 1.13V actually.
How does this sound now for the quality of the cells? Also the panasonic were sagging 1.5V total for the pair.