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first battery pack - hard to get started...
Hey everyone!

i want to make my first battery pack just for the fun of it and gain some experience.

First charing: 
lets say i built my battery with a bms.
How do i charge it? do i need a special charge controller for this or can i just connect + and - to my lab power supply, set it to 4.2V and 2A and let it run till the bms cuts of when the battery is full?

lets say i have a 6s BMS.
do i need to add a 6s battery or is it ok to add a 4s battery?
dont know if there is a conflict about the voltages !?

Which bms is this?:
I opened a e-bike battery and tested the cells.
i also collected the bms but now i cant remember which specs the battery had..
can you tell me some specs of the bms?
i know the battery was li-ion, 36V and had 9000mAh - can you tell which P and S this was?

[Image: i4VtQT8.jpg]

BMS balance:
I was looking so some cheap china bms.. but all the cheap ones are balancing with 50mA...
is it only balancing at 50mA or can i also draw only 50mA out of it?
Is this ok for a first test-pack or are 50mA useless?
Any recommendations for good and cheap china bms?

I know a lot of questions but i would really appreciate it if some can bring light into the darkness Smile
I think nobody is answering, because the questions are too broad and unspecific. Next time post what you're building, how many cells, voltage, etc etc...

Yes, you can charge a lithium cell / battery with a lab power supply. It's generally advisable to use the cells at a lower setting than specified in the spec sheet, particularly when using older cells. I'd use no more than (4.15v * CellsInSeries) and (0.5A * CellsInParallel).

Do 6S BMSs support 4S usage: it'll depend on the BMS, but most likely no.

It's not advisable to reuse an "unknown" BMS outside of its original usage. You have no idea what it does / requires.

Cheap China BMS: The really cheap ones are only good for toys and flashlights. If you're building a larger pack (ebike, powerwall, etc), get one with programmable parameters like this:

Is 50mA balancing enough? If you have brand new quality cells, you won't need virtually any balancing. If you have shitty cells you will need more balancing. If you have reasonably good cells tested for self discharge, then 50mA is enough to keep a 100p cells pack in balance (at least for me).
The previously mentioned programmable BMS makes all the difference here. Most cheap ones start balancing when cells start going over 4.2V... ie plays catch-up when it's basically too late. The programmable ones can be set to start balancing at much lower voltages, thus have much more time to get the cells balanced.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
Agree with @ajw22 that your question does not have enough detail. So guessing a bit - assuming your dealing with a small battery 50-100cells kind of thing, I started by using my Accucell 6 hobby charger. It can be used with or without balancing - but mastering its features, and using balance etc is as good a starting place as any to understand what is needed.
Many of my packs, big and small, dont have a BMS.  Especially some of my packs I made when I started because I have RC / hobby chargers which can handle the balancing for me.

Instead I focused more on the pack construction and the application.  Because of my applications and how I use my packs (portable power), I still deal with charging & balancing external to the battery pack using my balance charger.

That said, a well made pack, with good cells that are somewhat matched, I dont really have any significant balance issues to even worry about. I do agree with the above post about mastering its features and just REALLY understanding lithium batteries puts you well ahead of most people.  These days I focus more on monitoring than BMS and balancing.

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