Weak battery warning with new Molicel P42A

[dl12345]

Every so often, randomly it seems, I'm getting a weak battery warning with my Dovpo twin 21700 mod using two brand new Molicel P42As. Not sure what's causing it - the problem happens even at well near to 100% battery.

My preheat is set to 200w (dual coil SS304 claptons @ 0.26v) punch 11 and then normal power is 120w

My battery setup is below

Anyone have as to what in my setup might be causing this? I doubt I have dud batteries as I bought them from the official UK distributor

[retird]

You are using just 2 batteries thus the max output is reduced from what a 3 or 4 cell device will do...  this datasheet may help...

https://downloads.evolvapor.com/dna250color.pdf

 

[dl12345]

2 hours ago, retird said:

You are using just 2 batteries thus the max output is reduced from what a 3 or 4 cell device will do...  this datasheet may help...

https://downloads.evolvapor.com/dna250color.pdf

 

Thanks, I'm coming from a 3-cell device so I was aware that power should be reduced, although according to the spec sheet you so kindly linked, it should be capable of 200w on a 0.26 ohm coil if I'm reading it correctly (the chart on page 6)?

Unless I should be referring to the Nickel chart instead of the Kanthal chart, in which case it looks like it maxes out at around 140w?

That said, I'd previously reduced the pre-heat to 150w and I still get the same symptom...I'll try decreasing pre-heat below 140w and see if it fixes things....failing that I'll need to make a coil with a lower resistance than 0.26 ohms it seems.

[retird]

Connected to Device Monitor in EScribe you can see how the device is managing watts/temp/battery/battery sag and etc. while firing 

You are not using Kanthal from what you posted which is stainless steel.... wondering if it is just battery sag while firing??

 

[dwcraig1]

On the DNA250C with 2 cells I don't know the maximum voltage output but I'm pretty sure that isn't much more than a DNA250 running on 2 cells which is 6.2 volts.

[dl12345]

40 minutes ago, retird said:

Connected to Device Monitor in EScribe you can see how the device is managing watts/temp/battery/battery sag and etc. while firing 

You are not using Kanthal from what you posted which is stainless steel.... wondering if it is just battery sag while firing??

 

Max voltage draw and battery sag values highlighted. Does this shed any light?

 

[dwcraig1]

It looks to me that you are hitting the set temperature and the board is throttling down the power.

Your lowest voltage drop on the pair of cells appears to be 7.08 volts, half being 3.54 volts, well above the 2.75 volt cutoff.

Could you post the entire screen with the boxes checked like this snippet?

[dl12345]

I'll need some time to try and replicate the problem. What I posted was a normal puff, where the weak battery message did not appear. It seems quite random. so I'll probably need to try for a while before I can record the problem - I'll do as you suggest and check a bunch more boxes

[dwcraig1]

Your image show 6.54 volts output, this will be dependent on the cell's charge state. But lets just assume that is the boards maximum voltage output. Then figure in your .26 ohm SS coil. As the coil heats up the resistance will increase a bit, let's guess to 0.29 ohms.

 https://www.the12volt.com/ohm/ohms-law-calculators.asp

Keep in mind that as the charge on the cells go down the voltage output will never be higher than the sum of the voltage drop.

Example: cell voltage drops to 2.9 volts per cell under load. Max voltage output will not exceed 5.8 volts. If your settings call for more than that the error may show at end of puff.

[dl12345]

21 hours ago, dwcraig1 said:

Your image show 6.54 volts output, this will be dependent on the cell's charge state. But lets just assume that is the boards maximum voltage output. Then figure in your .26 ohm SS coil. As the coil heats up the resistance will increase a bit, let's guess to 0.29 ohms.

 https://www.the12volt.com/ohm/ohms-law-calculators.asp

Keep in mind that as the charge on the cells go down the voltage output will never be higher than the sum of the voltage drop.

Example: cell voltage drops to 2.9 volts per cell under load. Max voltage output will not exceed 5.8 volts. If your settings call for more than that the error may show at end of puff.

The error always shows at the beginning of the puff, never the end. 

However, using that calculator and solving for V

And keeping my fully charged mod connected to Escribe the whole day, so I have something like 160k data points, I can see that maximum voltage sag takes the battery pack down to 7.4v at the point of maximum sag if I use a preheat of 200w.

So it's not hard to imagine that with a less than full battery, even 80%+, that it could sag below 7.3v and trigger the weak battery message during the preheat phase.

This would be consistent with the symptoms I'm experiencing (mostly happens at the beginning of a puff when I have not taken one for a while, so the coil has cooled down).

So what I really should be doing here is keeping the preheat power no higher than probably 140w..I'll need to experiment and look to see what the battery pack sags to when it's reaching 30% charge or lower to determine what would be the maximum setting for preheat.

Thanks for the help @dwcraig1 and @retird

 

Edited July 14, 2021 by dl12345

[dl12345]

A couple of more calculations solving for multiple variables seems to show that I can't really rise above 164w on a 2 cell capable of 30A continuous output either while remaining above battery cutoff voltage, unless I'm mistaken?

The Molicell P42A can do brief spurts up to 40A according to Mooch's test, but for continuous output he rates it at 30A.

So it looks like I could lower the coil resistance to 0.184 and stay within battery cutoff voltage and rated amps at power up to 164w

Am I getting this right - I'm trying to solve here for the situation when the battery pack is reaching empty...

Edited July 14, 2021 by dl12345