How big mAH laptop powerbank should I buy if I need to charge my laptop twice?

[shivajikobardan]

I'm wondering how much big of a powerbank should I buy if I want to charge my laptop twice to 100%?

My laptop:

Dell inspiron 15 5567

Please help me do the math.

I'm from Nepal and these are the powerbanks available in Nepal. (No good quality products available as purchasing capacity is low).

https://www.daraz.com.np/catalog/?q=laptop+powerbank&_keyori=ss&from=input&spm=a2a0e.11779170.search.go.287d2d2beprFOy

[tom66]

Your laptop battery is 42Wh, going by Google search.

Powerbanks are almost universally specified with Ah at 3.7V, so equivalently your laptop is the same as a 11350mAh @ 3.7V powerbank.  To charge it twice, you'd need at least 22700mAh, plus around 10% to account for the loss during the charging process.  So around 25000mAh is required.

Your laptop also needs to support USB-C charging.

The other thing to check is whether the capacity is genuine - a lot of powerbanks like about the capacity.  And there are a lot of fakes around.  Are you able to return it if it is lying about capacity?  The going rate for Li-Ion cell is at least $150/kWh, the 25Ah pack will be 92Wh, so it is very unlikely a genuine powerbank will cost less than $20 USD equivalent.

[SiliconWizard]

The other thing to check is whether the capacity is genuine - a lot of powerbanks like about the capacity.  And there are a lot of fakes around.  Are you able to return it if it is lying about capacity?  The going rate for Li-Ion cell is at least $150/kWh, the 25Ah pack will be 92Wh, so it is very unlikely a genuine powerbank will cost less than $20 USD equivalent.

Yep. Many of the "cheap" (read: under $100) power banks rated above 15Ah are scams. There are numerous tests around showing that a typical sub-$100 battery rated 30Ah is actually just barely over 10Ah.
Physics can't lie. 100Wh of Li-ion batteries will take significant weight and volume. Just have a look at the typical energy density for Li-ion and do the maths.

$20? Almost anything sold under $100 for > 20Ah is likely to be a lie in practice.

[tom66]

Yep. Many of the "cheap" (read: under $100) power banks rated above 15Ah are scams. There are numerous tests around showing that a typical sub-$100 battery rated 30Ah is actually just barely over 10Ah.
Physics can't lie. 100Wh of Li-ion batteries will take significant weight and volume. Just have a look at the typical energy density for Li-ion and do the maths.

$20? Almost anything sold under $100 for > 20Ah is likely to be a lie in practice.

Yes, a fair point: cell price is one thing, but the integration is costly too.  I found that a search for trustworthy 20Ah or 30Ah powerbanks returns a price around 2-3x what I quoted, which allows for the casing, power board, cell integration, retail costs and manufacturer profit. 

[shapirus]

plus around 10% to account for the loss during the charging process.

I'd say at least double the 10%, because the dc/dc conversion, most likely, happens in both the powerbank and the laptop.

Then there are losses in the wires and in the cells.

So I'd probably account for about 30% total losses.