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Building an xxl Powerbank with fast charging and solar option
#1
Hi,
 
I’m planning to build an xxxl power bank to charge some mobile devices. Once or twice a year we are going on a camping trip with around 6 people. I am not sure if is use the right board category.
 
Currently, I am harvesting some 18650 cells from notebook batteries which I got from my office.
I think I build a 4s 20p battery pack.
 
It would be nice if I could charge the power bank very fast: sometimes we have access to a public power socket on the camping ground, so I would like to get as much power into it as possible in a short time. So I could use a 20V 8.5A notebook changer for example.
 
I also want to add an option to change the powerbank with my existing solar panel (https://de.aliexpress.com/item/Portable-...61069.html)
 
I think I should use this BMS: https://de.aliexpress.com/item/4-S-40A-L...937191.htm
 
Then I have to set the voltage to the 16.8V with this module: https://de.aliexpress.com/item/1PCS-Electric-Unit-High-quality-C-D-C-CC-CV-Buck-Converter-Step-down-Power-Module/32806874450.html
Or can I use this with that fancy display:  https://www.banggood.com/de/MPT-7210A-Al...56572.html


Greetings from Germany
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#2
schmo, what you have in mind is more or less matching to your needs.
Thge last one is not a fancy display, but a charge controller which is probably for Lead acid batteries (Bleiakkumulatoren). The Limits for the Voltage ranges between Lead 12 cells/24 Volts and LiIion 7s do not really good match. If you take an MPPT then look for one for LiIon.
The panel is not simply usable, because its Voltage is too low for a 7s system. 18 Volt panels are intended for an MPPT to 12 V batteries.
so far my 2 (Euro)cts.
Also from germany....
1 kWp in Test
4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
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#3
Cells that can discharge at high amps can also charge faster with less heat than low-amp cells. I would suggest coordless tool batteries, as opposed to laptop cells.
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#4
When working with salvage cells, you don’t really have the cell to cell consistency to safely throw high currents at it.  You might be dealing with high resistance cells which should be treated with lower drain currents.

That said, with a 20 cell parallel count, you could probably throw 20 amps at it as its still under 1a charge rates per cell.  Probably within an acceptable ranges.  Up to you to research and determine what you have.

Generally speaking, higher your charge and discharge rates are, the more important internal resistance and cell matching becomes.  This is why balance RC chargers are pretty much required in the RC hobby space.  Their C rates require it from a safety and battery management standpoint.

Edit:  I noticed you mentioned a 20v charger… you would most certainly over-volt and over-charge your 4s pack using something like that. You are better off using a inexpensive hobby charger to manage such a thing pack, and use the 20v as a supply for the charger.
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#5
Could he still use the laptop charger with a buck converter to reduce the voltage to a 16.8v output or 16v to keep the batteries under 4.2v and check the cells voltage when close to 4.0v ?

I am new to this as well so it would be nice to have a charging system that will cutoff before 4.2V and not be a rc charger. I like the thought of using old power supplies but i know you loose the balance factor without a rc balance charger or buying bms for all the 4s packs i will make down the road.

Wondering for a similar project with cells i have left over that are under 2AH.
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#6
(06-28-2019, 07:32 PM)JBonesaw Wrote: Could he still use the laptop charger with a buck converter to reduce the voltage to a 16.8v output or 16v to keep the batteries under 4.2v and check the cells voltage when close to 4.0v ?

Yes, you could. The problem is you are limited on how many amps the buck converter can put out. The power brick might be rated at 10A, but the buck might only be able to do 4A. There are some bucks that can do higher, but they are getting higher priced. So, there's a balancing act, I suppose.

You could also parallel bucks to increase output. But you would probably want to use diodes on the bucks to keep them from powering each other on their outputs. And then measure voltage after the diodes as there will be voltage drop
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#7
I use an IBM laptop power brick, to power a cheap Tenergy hobby charger. That would be an ideal solution IMO.
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#8
I have a pc power supply for at home to run 4 lito kala 500 for 18650 cell testing, but I would like something portable for when i take my ebike around.
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