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12V and 5v(USB) battery pack build. help.

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Pexy13:
Greetings!
My brother asked me if I could build him a battery pack with 18650 cells that he got from work.
He wants a 12V output and a 5V usb output. There are 8 cells in total.
I was thinking of doing something in these lines:
1) Connect 2 batteries in parallel and then 4 pairs in series.
2) Get a 4S BMS for charging.
-something like this: https://www.ebay.com/itm/3-4S-BMS-PCB-40A-Li-ion-Lithium-Battery-Charging-Protector-Board-18650-Module/253624191159?hash=item3b0d2e08b7%3Am%3AmfmyTfdOFEIrRDk_IhsnsxQ&frcectupt=true&fbclid=IwAR09bEndrc7S-h7QbTzAPwEeS2zlA6ZarHvIYWIaxgtcFvGB5SzJxzp1l4c
or this: https://www.ebay.com/itm/3S-4S-5S-3-7V-Polymer-Lithium-Battery-PCB-BMS-Protection-Board-Balance-Charging/332718178699?hash=item4d778c818b:m:mU0FwbeSu6Qs1LoApTvfXxA&frcectupt=true
3) Use a 12V and a 5V regulators on the output of the BMS to have a regulated 12V and 5V USB outputs.
- something like this for the 5V:
https://www.ebay.com/itm/USB-Charger-12V-24V-8-35V-to-5V-8A-DC-DC-Buck-Step-Down-Power-Supply-Converter/202518994580?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D58654%26meid%3Dee3b6b3e02cf40b59563f36bbdaa12cc%26pid%3D100005%26rk%3D5%26rkt%3D6%26mehot%3Dag%26sd%3D381374423467%26itm%3D202518994580&_trksid=p2047675.c100005.m1851
- something like one of these for 12V:
https://www.ebay.com/itm/XL4016-DC-CC-8A-200-280W-Step-Down-Buck-Converter-7V-40V-To-1-2-35V-Power-Module/192337508944?hash=item2cc8357a50:m:mKFxXwzaQeLUQA8Maz0fJpQ&frcectupt=true

Am I going in the right direction and would something like this work?
Do you have any recomendations. What would you change and why?
All suggestions are welcome.

Thanks,
Sven

ogden:
None of eBay links work for me. You shall note that BMS boards are not chargers, they are *protection* boards. You need current-limited charger anyway in addition to BMS, better it is voltage-limited as well because most BMS will trip too late - when battery is long overcharged. You don't want to overcharge because it reduces battery lifetime.

Pexy13:
I fixed the ebay links.
He will use a bench power supply to charge them  anyway but thanks for the info.

ogden:

--- Quote from: Pexy13 on June 13, 2019, 02:47:15 pm ---He will use a bench power supply to charge them  anyway but thanks for the info.

--- End quote ---

Well... I would not do it. You don't want to leave *any* chances for Li-Ion charging to go wrong - because it may end with fire. If current capability of that supply exceeds max charging current of cells - you badly want to introduce switching charge regulator with adjustable both voltage and current. Those 4s "balanced" BMS looks good - in case they work as promised and tresholds are within specs. 5V USB board looks very good, just make sure to use screw terminals. BTW if you are getting used 18650 cells - make sure to measure their capacity using smart charger and balance your battery bank well.

Pexy13:
So you think that I should also add a charging regulator? Im not completely sure what to look for so if you would send me a link to the regulator I would appreciate it.
I have a battery charger I used for my RC cars and stuff so i can check the capacity whit it to balance the pack.

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