Single cell vs Dual Cell Li-ion (3.7v vs 7.4v)

Hi
Need some advise.
I am making a portable device using Li-Ion batteries and battery life is my #1 priority.
My load on the battery is a MCU which is continously running and have a constant load.
In parallel to the battery I have a audio amplifier which can operate at 7.4v (louder) or 3.7v (softer)

Assuming I need a high performing audio (loud), it seems to be better to have my battery cells connected in series to provide 7.4v and add ~$1 for the additional buck regulator.

Did I get my calculations right?
I am assuming buck regulator efficiency of 90%.
Available cells are 50mAh. Parallel will give me 3.7v-100mA or series 7.4v-50mA.

Comments?
I am wondering based on the maths, why cellphones never implemented 7.4v series cells instead to have good audio and battery life? Assuming buck regulator costs are manageble?

Using series batteries results in higher costs and lower energy density. You don't really need high voltage to get more output power, you can just reduce your transducer impedance as well.

mtwieg said:

Using series batteries results in higher costs and lower energy density. You don't really need high voltage to get more output power, you can just reduce your transducer impedance as well.

Click to expand...

Hi, yes there are cost associated with series batteries. However from the calculations, it has the same energy density.

Two 50mAH 3.7v in parallel = 100mAh - .37Wh
Two 50mAh 3.7v in series = 50mAh - .37Wh

Right if you compare two cells in series vs two cells in parallel the effective energy density is the same (assuming the conversion efficiency of whatever DC-DC stage you use is the same). But you would never actually use two cells in parallel, but rather choose a single larger cell. And that will have somewhat higher energy density than two smaller cells, simply due to manufacturing constraints.

Thanks for the reply.
If my device (MCU/CODEC/etc) all operates at 2.8v, is it still worth it to use a buck regulator at 2.8v driven from Li-Cell operating from 4.2v to 3v?
With a constant load of ~100mA, with a buck regulator at 2.8v with 90% efficiency, I would be saving ~15mA at the battery at 3.7v, more at the higher voltages, less at the lower.
I had always thought that buck regulators would be most beneficial when input-output voltage differences are large, I will be quite surprised if 90% efficiency can be achieved with less then .3-.5v between input and output.
Comments?

use a synchronous buck, that will make it efficient.
Also you could do a sync boost if your output is above or same as input volt

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