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I'm working on prototyping a power bank, to be used for charging mobile devices, smart watches, ect.. It needs to be 10'000mAH.

My initial thought, since this is just a prototype and doesn't have to be in a final manufacturable state, was to use a single lithium polymer pack, 10'000mAH and at 3.7v. I would then use a boost regulator, to boost it to 5v, and use the 5v to charge the devices.

This approach is the simplest approach, and should work in theory.

I started looking at existing models, and >90% of power banks use cylindrical lithium ion cells, used in parallel and in series. The <10% that do use lipo's use them in series, at 7.4v or even higher. I assume they are doing this, because it is more efficient to reduce the voltage, than it is to increase it.

My thought with multiple cells, is that it will make it difficult for me, not controlling my battery sources, getting cells which match enough to use together.

Is my initial approach that wrong of an idea, and if so, what are your suggestions to use multiple cells (lipo or lithium ion)?

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I understand that you're asking about how to make your pack's capacity useful. One cannot say that your initial approach is wrong unless a requirement is specified. No, it's not necessarily wrong, it depends what you want to achieve.

Let's say you want to charge a phone with a battery which has a capacity of 2000 mAh and for charging it asks for 5V and 1A. If you connect your initial setup to this device, then your pack's drain current will be 5 W/3.7 V=1.35 A. You can divide the bank's capacity with your phone battery's capacity only if your battery charges to 3.7 V, because what you supply to your phone is not necessarily what reaches the battery, it first passes through the charge management circuit. If the phone's battery charges at 4.2 V, for example, then it means that the load battery will have a current of 5 W/4.2 V=1.19 A flowing through it. Your bank would be able to charge the cellphone fully less than 4 times and a half as opposed to 5 times that you would obtain if you divided the pack's capacity with the load battery's capacity.

If I assume that your pack is made of 5 3.7 V Lithium-Ion batteries connected in parallel, each having a capacity of 2000 mAh, I can state that if you connected them all in series, you would be able to charge your phone fully 7 times+ using a buck-boost converter, so you can keep charging even after the pack's output voltage is <=5 V. Note that I didn't include losses in my calculation. I believe that's what happens to:

The <10% that do use lipo's use them in series, at 7.4v or even higher

Lithium-Polimer is more dangerous than Lithium-Ion, because it can catch fire or explode more easily if it is not handled properly, but its charging process is less difficult in comparison to the other type. I think the choice should be based on your experience.

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