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What is the right way to connect the cells of this battery pack, and tips on making a power bank? Answered

Greeting makers,

I have a few questions about connecting Battery cells, and making a DIY Power Bank.
And I'd like to hear your opinion on it.

I have 12x Ni-Mh 1.2v 3000mAh AA betteries laying around. They are brand new and I thought of using them to make a Power Bank.

Since most phones use 5v for charging I wanted to connect 4 cells in series giving me 5v and 3 of these packs in parallel for an end result of a single 5v 9000mAh Battery Pack.

But when I started to connect them I realized that there are 2 ways it can be done, and I am not sure which is the right one.
Is there any difference at all? 

I will add a picture below, as it says more than thousand words.

Furthermore I would like to ask for your opinion on how to proceed making a functional power bank. Just wiring the cells straight to an Usb port doesn't seem like a smart idea. 

What would I need to make sure it is safe for the devices it charges?
And how could I make the Power Bank rechargeable via Usb?

Thanks in advance.



Best Answer 3 years ago

1. You alsways select your battery voltage higher than what you actually need - if it is for this purpose.
2. Charging from a USB port is not recommended as legally it can only supply a max of 500mA.
A USB3 port can handle up to 2A but still not recommended.

You are best of to combine the batteries to a higher voltage that would also suit a commercial charger, like the ones for RC models.
7.2 or 9.6V are pretty much standard.
Since your batteries are already over the 2A region I don't see a real need for a parallel configuration.
To always get 5V you need a dedicated voltage regulator.
And if you need it for the phone or tablet then also make sure to supply the correct voltages on the data lines of the USB port - check Wikipedia for the USB charging standard for detailed info.


Answer 3 years ago

Thank you very much for the reply.
So (6x2) 7.2v 6000mAh, with a 5v voltage regulator would be a better approach? The parallel configuration is to extend the capacity.
A phone battery is usually around 3000mAh so without a parallel config it would be barely possible to charge it up one time, and that looking from an ideal perspective.


Answer 3 years ago

Sure you can double the packs to get more AH in parallel but keep in mind that this is not an ideal case.
Each cell also discharges (even when there is no load).
Take the battery packs from old cordless drills as a bad example.
They did the same in the early days to get the capacity higher, the result was that most battery packs failed within 6 months of use.
Each parallel pack of batteries can and will have a different internal resistance and self discharge - same for the individual batteries in this pack.
Similar problem when cahring such packs.
Since all have different values you can and will get a pack that won't be fully charged - in return it discharges itself and the other packs faster and over time it all just fails to provide the juice you need.
If it would be that simple the RC guys would not constantly scream for higher capacity batteries ;)


Answer 3 years ago

Alright, thank you very much for all the info. It is very appreciated.
I will make sure to use those cells in some different way then. :)


Answer 3 years ago

And the question about connecting them still stays. Which of the two ways would be more stable?