# Help designing a circuit for DIY powerbank

Discussion in 'The Projects Forum' started by Kaur Palang, Apr 12, 2016.

1. ### Kaur Palang Thread Starter New Member

Apr 12, 2016
3
0
Hello
I am looking into making myself a powerbank based on 18650 cells I'm planning to salvage from old laptops. But I want it to be kind of a group bank meaning it would be able to charge 10 devices simultaneously. During the research I did yesterday, I figured out some things. Note that these are my understandings, if there are inaccuracies, please correct me. Also I am a complete newbie to electronics so please be patient

Now, things I've concluded:
1) I need 10 amps to provide the 10 ports with it's maximum power output of 0.9 amps because amperage gets divided between outputs while voltage remains the same.
2) I need to run the cells in series/parallel to achieve 7.4 volts (to be reduced to 5) and high mAh capacity.
These conclusions are based on http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

I would like to implement a RGB LED array to indicate current storage of a so-called section (marked with gray on image). So when one section is full, it's LED would be green and eventually change to red as the cells empty (or section dies). I found some things called rheostats, but I don't understand how they work alone and in combination with LEDs.

I'd also like the powerbank to have passtrough charging capability.
I've left the scheme incomplete since I do not know how to show some things. (USB connections (series/parallel), rheostats and LEDs, passtrough charging.

Any help is greatly appreciated!
TheCreepySheep

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
Welcome to AAC.

1. To draw 10A you need cells that can handle 6A. Paralleling them can give 12A. 2A extra so the batteries will last longer. There are 8000mAH cells now.

2. Rheostat cannot solve the problem

3. You should use new cells. Salvaged cells will create problems when you parallel them. To parallel, cells must be identical in every way.

4. To make a RGB indicator you would need to power the indicator separately. If the cell dies so will the controller if not powered separately.

5. A uC will make the cell management task easier and simpler. Like to indicate cell charge and disconnect is the cell voltage reaches lower limit.

There a lot of issues but with your limited input this is all I can say

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
Oh yes, To charge you need a proper li-on charger that can do CC and CV charging. Taking into account the capacity of cells you chose

4. ### Kaur Palang Thread Starter New Member

Apr 12, 2016
3
0
Thank you for your quick reply, I get most of your suggestions, but I do have some questions.

1. How can I determine how many amps a cell can handle? And will a 10 Ah cell with 4.2V be a problem?
4. Now that I think about it, having a blank LED is a neat way of distinguishing a dead cell from an empty one. Assuming I see the low charge
5. What does "uC" stand for?

TheCreepySheep

5. ### Kaur Palang Thread Starter New Member

Apr 12, 2016
3
0
I found some corrections myself.
Firstly, I do not need 5 V, I think I can very well charge a phone with 3.7 volts.
I discovered that if I look for batteries from vaping pages, the discharge rate is displayed as opposed to Amazon and eBay

6. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
Latest phones will not charge if below 5V

7. ### Dr.killjoy Well-Known Member

Apr 28, 2013
1,210
158
Right now I am working on my own design for the same project .. Most battery pack use a number of 18650 cells in a parallel configuration which is the not greatest setup but it's cheap and easy .. The cells are charged at 4.2v @1amp hence why these units take so long to charge when the standard charge rate is 4.2v @1amp per single cell .. Also the cells will need a protection circuit to make sure not to damage the cells by over discharging .. The cells will only output 4.2v max which is not enough for the cell phone to charge properly and so you will a boost converter to change 4.2v to 5v .. Don't forget the resistor network for the data pins in order to get a higher charge rate .. For me I was sticking with the Samsung 1.5amp resistor network vs Apple 2.1amps setup..

Here is a great video to help you understand a little more

I hope this help out..

Last edited: Apr 13, 2016