Parallel Mech Mod - Watts Law

[iz_thewiz149]

Hi all,

I’m thinking about purchasing a parallel mech mod, most likely the Dovpo Clutch X18. Now I’ve recently been using a single 21700 mech mod for a few weeks which has been fantastic, but the only disadvantage is that it’s a single battery mech and I’m usually getting half a day out of one battery (Samsung 30T), which is not necessarily a bad thing and this was expected. I have a married pair of these batteries and they’re constantly been discharged down to around 3.5V before switching out and charging externally.

I have two sets of Molicel P26A which are both married and are currently used exclusively for a dual battery regulated mod. I’m planning on using the Molicels for the parallel mech mod, but I’d like to check my arithmetic when it comes to calculating watts law on this device.

Molicel P26A - 4.2V / 25A CDR
Coil resistance - 0.14 ohms
Ohms law - 4.2/0.14 = 30

Now, where I’m confused here is firstly the the combined current of the two Li-Ion batteries in parallel. If the combined amount of the two Molicels in parallel is 50A, then is the ohms law calculation of 30 actually safe when in parallel? I have 20 amps of CDR headroom theoretically. Or, because the cell load is now split in parallel, and assuming it’s a 50% split, is the real figure more like 12.5A per cell? Because if it is in fact 12.5A then I’ve exceeded the CDR of both cells.

Secondly, watts law.

What is the correct equation for determining watts law in parallel?

1. 4.2V X 50A (combined current)
   
   
2. 4.2V X 25A (single cell current)
   
   
3. 4.2V X 12.5A (theoretical split current)

I have been reading and viewing Mooch's resources, which is invaluable especially for mech users. He makes the point that with parallel, when the cells are split the actual split percentage is theoretical, meaning that factors such as the state of the mech & battery contacts, or atomiser threads and pin will affect the split load per cell. Meaning it may not necessarily be an even 50% split load. Which I think makes parallel somewhat confusing over a series mech. And without a reference figure for watts law, I can't get an accurate calculation.

Thanks for the read. Hope to get some clarification on this.

 

[mimöschen]

With a 0.14ohm coil you're drawing 15amps at 4.2V from each cell in a parallel mod which equals
15A x 2 x 4.2V = 126W.

But these are just theroretical values, because under load the fully charged P26A's voltage will immediately sag to about 3.9V.
So your equation is more like:
3.9V / 0.14ohms = 27.9A
That's 13.93A per cell.
13.93A x 2 x 3.9V = 108W

As always voltage and ampdraw will decrease further with each consecutive puff you take.

 

[iz_thewiz149]

Thanks for your reply.

If I understand correctly, you used 15A as a reference for watts law by halving the ohms law calculation from 0.14 ohms:
4.2 / 0.14 = 30
30 / 2 = 15

This makes sense if that’s the true parallel current. The Molicel P26A CDR is rated at 25A (estimated by Mooch), but since the intent is to run these in parallel, am I still referencing the 15A current per cell to confirm that I won’t be exceeding the CDR?

Thanks again.

 

[Z-Lee]

Left side of the chart - you're going to be drawing about 15 theoretical amps per battery.

 

[UncLeJunkLe]

These videos may be of some help to you

 

[iz_thewiz149]

View attachment 959419

Left side of the chart - you're going to be drawing about 15 theoretical amps per battery.

Thanks for your reply.

Can I ask how you came to the 15A figure?

 

[iz_thewiz149]

With a 0.14ohm coil you're drawing 15amps at 4.2V from each cell in a parallel mod which equals
15A x 2 x 4.2V = 126W.

But these are just theroretical values, because under load the fully charged P26A's voltage will immediately sag to about 3.9V.
So your equation is more like:
3.9V / 0.14ohms = 27.9A
That's 13.93A per cell.
13.93A x 2 x 3.9V = 108W

As always voltage and ampdraw will decrease further with each consecutive puff you take.

Thanks for your reply.

If I understand correctly, you used 15A as a reference for watts law by halving the ohms law calculation from 0.14 ohms:
4.2 / 0.14 = 30
30 / 2 = 15

This makes sense if that’s the true parallel current. The Molicel P26A CDR is rated at 25A (estimated by Mooch), but since the intent is to run these in parallel, am I still referencing the 15A current per cell to confirm that I won’t be exceeding the CDR?

Thanks again.

 

[iz_thewiz149]

These videos may be of some help to you

Thanks for your reply.

Mooch's resources have been invaluable to me, and they definitely assisted me before purchasing my first mech. Although he makes a point that with parallel systems there are theoretical values, which I think is what makes determining a reference figure difficult. Single cell mechs in contrast are much easier in determining a single reference.

 

[Ed Brown]

@iz_thewiz149, I didn't know that VxA = Watts is known as Watts Law.

Ya learn something every day!