I'm absolute beginner in electrical engineering and I need help in building a powerbank using three different batteries with different characteristics.

I have a sudden passion to implement this project because it's very interesting to me and I want to create my own powerbank and don't buy a new one branded by Xiaomi etc.

What I have now is 3 batteries:

  1. Biggest one (black color) it is a used battery. I came out of a Asus Nexus 7 2012 and its capacity decreased by big amount of cycles of charging/discharging. It has strange black black yellow white red red wire and I don't know exactly where is + and - and I have no tool with help which I can detect it. How can I determine its conbections? Its capacity is 4325mAh, 3.7V and 16Wh (as it described on photo below.)
  2. Middle size one (silver color.) It is a new battery (about 5 charges/discharges) and it was used in a case-powerbank for my iPhone (it accidentally crashed and I've extracted the battery.) Its capacity is 5000mAh, 3.7V and unknown Wh (I'll update my question once I get it.)
  3. Small size one (silver color.) It is a new battery (about 10 charges/discharges) and it was used in powerbank for my iPhone (it accidentally crashed and I have extracted the battery.) Its capacity is 2020mAh, 3.7V and 7.68Wh or 7.88Wh

I want to unite them all sequentially so all capacities will be added (4325+5000+2020.)

Only one my requirement is I need 1 usb port for this powerbank and it should be 1 ampere (non high voltage) because it's completely enough for my iPhone 6s.

I'm going to print a case on 3d printer from plastic.

Is it possible?

Can you give advice me, please, what I have to buy and what I need to learn and may be some blueprint and some voltage balance board(correct me if I'm wrong) and some other boards and where I can buy them (quality board, please.)

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closed as too broad by Leon Heller, winny, Dmitry Grigoryev, JRE, DoxyLover May 15 at 7:30

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    \$\begingroup\$ Series connection is not possible of mismatched battery capacity and parallel must all be the same voltage chemistry, \$\endgroup\$ – Sunnyskyguy EE75 May 14 at 10:40
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    \$\begingroup\$ Besides being overly broad, questions about which products to buy (and where) are off topic. \$\endgroup\$ – JRE May 14 at 11:20

It is possible, but not advisable.

First a little background information:

  1. Putting batteries in series adds the voltages together. This does not add the capacities together.
  2. You put batteries in parallel to add the capacity. This applies to current as well as capacity.
  3. You normally put batteries of the same size together to make a larger one.

Lithium batteries are not something you want to play with. If you mistreat them, they can (and will) catch fire. And, it is almost a given that you will make a mistake in what sounds like your first project.

It is possible, but you will have to do a lot of things that will make it simpler (and cheaper) to buy a proper power bank.

  • You will need a separate charge controller for each of your batteries.
  • You will need a management circuit for each battery. This monitors the discharge state and disconnects it from the load when it is too low. It also has to protect against charging a low capacity battery from a higher capacity battery.
  • You will also need to control the discharge rate of each battery separately. Each battery type has a maximum discharge rate (maximum current it should be allowed to deliver.) Three batteries, three discharge rates.

For each of those components, you will have to provide proper settings matching your three batteries.

That's a lot of information to collect, and for at least one battery you don't even have the basics, let alone the specifics.

If you ever get all that together, you need a boost converter to convert the voltage from the (nominal) 3.7V to the 5V normally needed as the input to your cellphone or other device.

All of that can be dealt with, but it is a lot of stuff to deal with.
I'm a hobbyist who has built a lot of stuff, and I wouldn't do it - too dangerous for too little gain.

The engineers here wouldn't do it, either. So much to do to make work and make it safe that it isn't worth the bother. They'd specify new, identical cells and use battery controllers appropriate to just that one type.

Long story short:

Don't do this. It isn't as easy as it looks, and you could burn your house down.


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