Will this be safe to test? Homemade Laptop Power bank

[robbesegers]

Hi everyone, i'm new to this forum. (English is not my native language)
I'm planning on building something that can power my Macbook Pro Retina from a normal mobile portable power bank.
I know it's not ideaal because of the small capacity of a powerbank (+-20000mAh) but I don't need it to power my device for days.
When i'm at school, i'm using my macbook constantly (Graphic Design), and in most of the buildings we don't have wall sockets.
I know that laptop power banks exist, but they are really expensive.

I was thinking to buy these things:

- LM2577 power converter module like this: https://www.amazon.com/gp/product/B008HMETBE?psc=1&redirect=true&ref_=oh_aui_search_detailpage

- Macbook pro repair cord cable: https://www.amazon.com/gp/product/B00XTTUFXG?psc=1&redirect=true&ref_=oh_aui_search_detailpage

- A male to female DC adapter ( Just to make it easier to connect wires from the power converter to the repair cord cable )

- A simple usb cable


The next step is cutting open the usb cable, and connecting the positive and negative wire to the Power Converter's input.
After that i'll connect the repair cord to the output, with the dc adapter between them.

Will this be safe to use with my macbook?

 

[Alec_t]

Welcome to EP!

Will this be safe to use with my macbook?

What is the peak current demand of the Macbook?
What input voltage does the Macbook require?
What is the voltage output of your powerbank?
Will your Macbook recognise a non-Mac power supply?

 

[robbesegers]

Welcome to EP!

What is the peak current demand of the Macbook?
What input voltage does the Macbook require?
What is the voltage output of your powerbank?
Will your Macbook recognise a non-Mac power supply?

The voltage output is my powerbank is 5V (2.4A)

My macbook's specifications are 4386mA and 12236mV ( it uses the 85W macbook charger) and it will recognise non-mac power supply.

I don't know the peak current demand..

 

[Gryd3]

The voltage output is my powerbank is 5V (2.4A)

My macbook's specifications are 4386mA and 12236mV ( it uses the 85W macbook charger) and it will recognise non-mac power supply.

I don't know the peak current demand..

You have a problem already most likely...
If you are unfamiliar with the power calculation it is volts*amps.

The powerbank is 5V @ 2.4A which is 12Watts
The macbook is ~12V @ 4.3A which is 51Watts!

The powerbank is underpowered even with a converter.... you will need something beefier.
If you are lucky... the macbook won't actually *draw* 4.3Amps which means you can get away with a less capable power bank... but if the macbook momentarily draws too much current it could shut-down or damage the powerbank.

Please keep in mind that the 20,000mAh power bank may not be rated for what you expect either...
20,000mAh @ 5V (hopefully) is 100Wh ... and if the world was a perfect place, that would be enough capacity to run your laptop for 2 hours... but there are losses when you convert voltages/amps and more losses when you drive them too hard.
If the power bank manufacturer is being funny with numbers... the 20,000mAh may be for the 3.7V cell inside the bank. If this is the case you only have 74Wh to work with which is an hour and a half... but remember, these are *perfect* results assuming the macbook is pulling 50W all the time.
These numbers will change greatly in the real-world...

(ie. bank on actually getting about 80% of the capacity, and find a way to measure the average draw of your laptop if you want to know how long it can last)

 

[Alec_t]

So, the Mac needs >4.3A. The DC converter in the link is rated for 2.5A output only .
The converter would have to boost 5V to 20V (the charger output voltage), so even if it were 100% efficient it would draw at least 4.3A x 20V/5V = 17.2A. That seems beyond the capability of your powerbank.

 

[hevans1944]

Best to carry a looong extension cord with your MacBook Pro Retina and power it from a wall outlet. That's what my wife does with her MacBook Pro Retina when we travel. At home, the wall outlets are located conveniently close to the MacBook power supply. I suppose transferring to a school more attuned to the 21st Century, with plenty of wall outlets for everyone is not an option.