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I am creating a power bank with 3000mAh 18650 cells in a 3S5P configuration. This will give me 189Wh, 12.6v, and 15A. Assuming an iPhone 8 has a battery of 1821mAh or 6.96Wh, how many times it can be charged?

I found that it is 15A/1.821A or 8.2 times if there is no loss

But why can't it be 189Wh/6.96Wh or 27.1 times?

Awais Saeed
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    The latter is correct assuming no losses. – winny Feb 02 '23 at 11:48
  • @Awais Saeed. Your title doesn't match your actual question. Please change it to match your actual question. – Davide Andrea Feb 02 '23 at 15:11
  • Where do you get the current figures from ? – Rohit Gupta Feb 03 '23 at 04:29
  • @RohitGupta, I was believing that the correct answer would be `189Wh/6.96Wh`. But, people on Quora were suggesting the other way around and that didn't click me. The current of any phone can be found online and current of cells from the datasheet. – Awais Saeed Feb 03 '23 at 04:42

2 Answers2

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I am creating a power bank with 3000mAh 18650 cells in a 3S5P configuration. This will give me 189Wh, 12.6v, and 15A.

Three series cells implies 4.2 volts per cell (12.6 volts ÷ 3) and, that isn't sustainable. It will rapidly reduce towards maybe 3.75 volts per cell then remain somewhat stable for a decent period: -

enter image description here

Image from Li-ion Voltage Analysis.

So, more realistically you should be using 3.75 volts to calculate power. Then, you should consider that anything close to removing most of the available charge is probably going to impact the lifetime of your power bank.

Depending on how you want to work this, you might only want to remove 60% of the energy available. You should definitely read the data sheets for the cells you are using and, decide what you can realistically achieve vs cell-longevity.

Your effective useful power output might only be 15 amps x 3 x 3.75 volts x 60% for instance. Probably something around 100 Wh but, like I said it depends on the cell and other information in its data sheet. You also need to factor in the buck conversion efficiency (if one is required).

Maybe read what The Battery University has to say about discharge rates and available charge for Lithium Ion power cells.

I found that it is (15A/1.821A) if there is no loss

You are using 15 amps load current and 1821 mAh inappropriately. The first is figure is current and the second is current x time --> it's a meaningless figure of "something" per hour.

Andy aka
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  • The SOC plot was very interesting because it was new to me. As for the last part of your explanation, I have some confusion. If a cell is of 3000mAh and 5 of them in parallel will give me a total of 15000mAh, then why 15000mAh/1821mAh is inappropriate? Both seem to be current times Time. – Awais Saeed Feb 03 '23 at 04:31
  • It's your abuse of units that's the problem. When you said 15 A you meant 15 Ah etc.. Engineering is about attention to detail. – Andy aka Feb 03 '23 at 09:29
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But, why can't it be 189Wh/6.96Wh?

Who says it can't?

That is correct: 189 Wh/6.96 Wh = 27 times.

Given the inefficiencies in various DC-DC converters, it will be less than 27 times, but still in that ball park.

Davide Andrea
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