Big battery bank project. Circuit related questions?

Hello everyone!

I have a project in my mind and I would love some guidance and some ideas if possible.
I already have a lot of 18650 batteries in an “acceptable” state (1800-2100mAh each) and I am planning to make a large battery bank, for use in camping mostly (smartphone charging, led bulbs, electrical air pumps 12v, etc)

I did read a lot of articles in here about battery banks, but i didn't find something as complex as my idea.
I have already made a 12v battery pack with 12x18650, 3 rows in parallel, of 4 in series in each row.
My goal was to operate a 12v air pump for inflatable mattresses. It worked like a charm! (Also created with instructions from here! Thanks for that!)

But since my “DIY disease” does not leave me in piece… I want to build a large enough pack that can support the following:
1) 12v car plug
2) 5v/1A + 5v/2.1A USB plugs
3) charge circuit for the battery pack, so I will no longer have to remove the batteries to charge them separately
4) switches in each plug
5) led voltmeter also with switch
?) at some point I would like to add a circuit to charge it with a solar charger or a mini wind turbine that i will create at some point with my 3D printer, but for now let stay with the basic stuff!

For the case i will use a modified screw organizer case, with those classic 18650 black holders from ebay. (photo below)
The case will fit almost 8 to 10 holders (4x18650), so we are talking for 32 to 40 batteries...

My main concern is the circuit design.
I have done a few simple things in the past, but this is a bit more complicated and that is why I want your guidance/ideas.
How to safely connect all those things together or maybe suggest a different plan/design, cable thickness, etc...

Links gathered so far:
I am thinking about 4S in a row. Is that too much? Or should i consider 3S?
3S Li-ion Charger PCB with BMS

For charger... something like this maybe?
12.6V 5A Charger

optional... maybe i could fit sothing like that, for laptop charging?
150W DC-DC Boost Converter

for 5v 1/2.1Amp charger i m looking at this.
5A Charger Step down Module

for usb/12v plugs/voltmeter i am considering this option.

Thanks in advance for your time!

Picture of Big battery bank project. Circuit related questions?
Downunder35m7 months ago

Don't want to spoil your work but I would advise against such big projects.
LiPo's still love to cause troubles in the form of failing or going up in smoke.
To charge them in a big pack you need multiple safety features along the way, including temp control for the cells, internal short circuit protection and so on.
Hard enough with selected and identical batteries, next to impossible with batteries in various stages of their life cycle.
For anything 12V I really prefer to use deep cycle batteries.
Cost a few bucks more than your normal car battery but they last and offer the best discharge voltage to power ratio if you ask me.

the_phantom_boy (author)  Downunder35m7 months ago

The pack should be portable, and with bike/car or even UPS batteries this is not possible. Also i have charged/discharged all of those batteries a lot of times just to make sure nothing is wrong with them and every single of them respond great in high current charge. No heating, no nothing. (the ones that did heat up even a bit, are of the table).

And of course i will be monitoring them very often to check for issues.

Disregarding your DIY desires and being practical you can buy a 4 cell lipo battery intended for radio control use (hobby king). AND you can buy a safe charger for it.

You really need to be able to sense each cell and make sure they are not over charging so charging in a block like a car battery isn't an option that you should be doing.

Most Rc battery chargers have an option to power from a 12 volt source so you can run from a car battery or a 12 volt solar cell array - assuming it can supply the current.

Skill builds up over time and the best skills are built on a firm foundation ie theory. You can fined lots of web sites that will take you through Lipo charging principles.

Small projects build skill knowledge and confidence. Then embark on larger more ambitious projects.

the_phantom_boy (author)  rickharris7 months ago

I appreciate the warnings and i do understand the risks involved!

I already have an IMAX B6 charger, which i use to check the batteries, and will continue to use to charge that "device" until i am confident at certain level that i can plug it in a bigger power supply to charge it.

But, I will use those batteries since i already have those batteries!
I am not interested to give an extra amount of ~100usd to buy new ones!
If this is the case... i can buy a new powerbank that already give what i want!

But that is not the point of this project!

I did read a lot about charging those things and i understand that those good quality 18650 cells that i have (LG, Sony, etc) are much better than those flat lipos, since those batteries are more prone to failure.

So... about the circuit, either i use 18650, or other type of batteries... can you give me a few basic info?