Voltage sag new VTC5A

[dl12345]

This is my first single battery DNA75 mod - my other mods are DNA200s (Boxer 133 and Releaux).

With a fully charged brand new VTC5A I'm noticing substantial voltage sag. At 10A current, the voltage is dropping to 3.4V (see photo). On Mooch's battery test of the VTC5A, this only happened when discharging at 40A.

Is what I'm seeing "normal" or indicative of a faulty (or possibly fake) battery, or am I just misunderstanding these graphs?

[retird]

Do you get a similar sag with the other batteries you use in this device?

[Spector NS5 RD]

where did you buy the cells from? here's a few screenshots of one of my VTC5A's. one at 75w and the other at 48w, like yours. both with a .33 ohm load. what kind of sag do you see at 75w? what brand mod is this? 
 
 

[dl12345]

This is a VT75 nano. Yes, I do notice a similar sag when using a Samsung 25R - pack voltage drops to about 3.3v - 34v on a 10A load using a 0.5ohm coil. It happens on the initial draw during the preheat phase (65W preheat, 9 hard). Of course on my other mods the sag is much less noticeable since they're using 2 or 3 cells. Just not sure if this is normal for a DNA75 or points to a battery issue.

[Spector NS5 RD]

TBH, if you bought the cell from a reputable seller, the sag is consistent with my VTC5A cells. the DNA 75 is a power hungry board, as are most other single 18650 75w+ mods. you will see more sag due the fact you have only one cell to feed the DNA 75 board, where as 2-3 cells would spread the load across them. i'd say you're good to go. 

just a side note, you can check to see what your "cell soft cutoff" is set to. on my 18650 DNA 75 mods, i lower the value to 2.65v - 2.7v. what that does is slightly pushes back the "weak battery" bar flashing too early and eeks out a little extra cell life.

[VapingBad]

You are not drawing 10 A, the board efficiency is 85% so 48 W at the 510 requires 56.5 W from the battery (48/0.85) and at 3.4 V that would be 16.6 A (56.5/3.4).  That drop does seem very excessive and getting it with other batteries I would suspect contact resistance (dirt or weak spring) or resistance in the electrical path between the battery and board like cold solder joint or if it has a common ground loose board mount screws etc.

[dl12345]

ChunkyButt200 said:

TBH, if you bought the cell from a reputable seller, the sag is consistent with my VTC5A cells. the DNA 75 is a power hungry board, as are most other single 18650 75w+ mods. you will see more sag due the fact you have only one cell to feed the DNA 75 board, where as 2-3 cells would spread the load across them. i'd say you're good to go. 

just a side note, you can check to see what your "cell soft cutoff" is set to. on my 18650 DNA 75 mods, i lower the value to 2.65v - 2.7v. what that does is slightly pushes back the "weak battery" bar flashing too early and eeks out a little extra cell life.

Thanks, it's really helpful to know that the sag is consistent with your cells. You also confirmed what I surmised: that is to say that the sag is so noticeable because it's a single cell device. My soft cell cutoff is currently set to 3v, so I'll lower it as you suggest. 

[dl12345]

VapingBad said:

You are not drawing 10 A, the board efficiency is 85% so 48 W at the 510 requires 56.5 W from the battery (48/0.85) and at 3.4 V that would be 16.6 A (56.5/3.4).  That drop does seem very excessive and getting it with other batteries I would suspect contact resistance (dirt or weak spring) or resistance in the electrical path between the battery and board like cold solder joint or if it has a common ground loose board mount screws etc.

It actually did have loose board mount screws which I had previously tightened up, so these readings are with the tightened screws. Perhaps I should clean the board contacts. I've already cleaned the threads which came with quite a lot of manufacturing residue out of the box.

This is a Hcigar VT75 nano, so the ground connections from the 510 plate and the negative battery contact are via a thread (instead of a soldered connection), which to be honest I don't really like - I'm considering soldering the board ground directly to the 510 base plate but to do that I'd need to drill a hole in the plastic mount that sits just below the 510 plate. The engineering in this mod is ill thought out.

[VapingBad]

If that drop is at 75 W preheat the battery would be at 26 A @ 3.4 V  75/0.85 = 88.23 W, I = P/V = 88.23/3.4 = 25.95 A

[dl12345]

VapingBad said:

If that drop is at 75 W preheat the battery would be at 26 A @ 3.4 V  75/0.85 = 88.23 W, I = P/V = 88.23/3.4 = 25.95 A

62W preheat (sorry, I said 65W earlier but I just checked and it's set to 62W because this should give a theoretical max load of around 20A....)

However according the escribe screenshot I posted the current draw shows just over 10A at the 3.4v level, right? Hence my concern....

[VapingBad]

dl12345 said:

62W preheat (sorry, I said 65W earlier but I just checked and it's set to 62W because this should give a theoretical max load of around 20A....)

However according the escribe screenshot I posted the current draw shows just over 10A at the 3.4v level, right? Hence my concern....

No that is the current at the 510, at the battery you need 62/0.85 = 73 W and when the battery is at 3.4 V the board would be drawing 21.5 A.  Theoretical max load if your low cut off is 2.75 V for 62 W the current requirement would be 26.5 A, for 75 W this would be 32.01 A.  As the battery voltage drops more current is required to provide the power.   Hope that helps.

[dl12345]

Got it, yes that helps a lot. So with a 62W preheat for 1s I'm operating the battery close to it's maximum rated continuous current and slightly over when hitting the soft cell cutoff.

Looking at the numbers it would probably be a good idea to decrease the preheat slightly to around 58W so that a voltage close to the soft cell cut-off of 2.75v doesn't cause the battery to exceed the maximum rated 25A continuous current discharge during the preheat period.

Thanks for the advice, much appreciated.