A Question for Any Electrical Engineers (or experts in electronics)

[BigEgo]

(mods I put this in "General" to reach a wider audience. If you feel it needs to be moved, that's fine.).

I have often wondered something about batteries. I can do the math, but since I am not a trained EE who understands these circuits at a more fundamental level, I feel I am missing something. My question involves series vs. parallel batteries.

You often hear people say things like "parallel gives more battery life and series gives more power." However, the math does not support this claim on regulated mods (which use boost circuits). The equation for finding amp draw on a regulated mod is simple since we have control over the wattage:

I = P/V  (Amps = Power / Volts)

Remember on a regulated mod, there is an input and an output circuit. We cannot confuse the two. As far as battery life/safety is concerned, we only care about the input voltage/amperage. What goes on at the output circuit (at the coil) is not indicative of what's coming out of the batteries (except for watts. My understanding is that it's "watts in and watts out.")

Here's a "thought experiment." Let's say we have 2 mods (with the same board) except one is wired for series batteries and the other for parallel batteries. Let's say we plan to use Samsung 25R's in both. These batteries are 2500mAh, 3.7v nominal with a max continuous discharge of 20A. Let's say we want to determine which mod has the longest battery life. We run them both at 100 watts for our test. Therefore, let's do the math:

Parallel:  100w / 3.7v = 27A

Series:  100w / 7.4v = 13.5A

(Let's assume 100% efficiency for the sake of simplicity).

So, the series circuit draws half the current that the parallel circuit does. This makes sense because a series circuit doubles the voltage which also lowers the current needed. (Remember that battery life is determined on how many amps is being drained from the battery).

A better way to illustrate this is to use watt/hours instead of mAh to determine battery life. That equation is:

mAh * voltage/1000

So, let's do the math:

Parallel:  5000mAh * 3.7v/1000 = 18.5 watt/hr.

Series:  2500mAh * 7.4v/1000 = 18.5 watt/hr.

The math tells us that the battery life will be exactly the same between series and parallel at a given power level. Remember that two 2500mAh in parallel gives us 5000mAh and two 2500mAh batteries in series still gives us 2500mAh (but with doubled voltage).

So my questions are:

1) If you were designing a dual battery regulated mod, what is the advantage of series over parallel (if any) or vice versa?

2) Is series "safer" for higher power levels? I am assuming that parallel batteries doubles the amperage output (20A max to 40A max), so it should be able to handle just as high of power as a series box, no?

I just feel there is something I am missing here. Why do most regulated mod manufacturers choose series over parallel? The math tells us they will perform exactly the same. I feel there is something deeper in the engineering of the circuits that makes one better than the other but I don't know what that is.

Is it out of fear that if one battery in parallel fails that the other battery will be overstressed? That's all I can come up with.

 

[bussdriver]

I believe it has to do with cost and simplicity of circuit design. In a parallel mod, there is 3.7 volts available. No problem as long as the needed voltage to the atomizer is lower than this voltage. A simple "buck converter" is used to give the output needed.

The problem arises when more than the battery voltage is needed. If 6 volts were needed for the coil to achieve the desired output, then the 3.7 volts is not sufficient. In this case, a "boost converter" is used. The 3.7 volts is boosted upwards to achieve the necessary voltage.

Bucking the excess voltage is rather simple and the circuitry needed can be small. Boosting the voltage requires a more complex circuit, and the components are much larger. When high wattage outputs are desired much more space is needed for the boosting circuitry.

Enter the series battery connection. With 2 batteries, 7.4 volts is available. With 3 it rises to 11.1 volts. With the higher voltage from the batteries, a simple buck converter can handle the job; less heat, less space, and better efficiency which will lead to better battery life.
At the higher current (power) outputs some vapers now seek, the cost and size of the mod is greatly reduced.

 

[BigEgo]

Makes sense. I had read before that bucking was more efficient than boosting, so that would explain it.