How do you correctly charge a lithium battery pack through a bms?

How do you correctly charge a lithium battery pack through a bms?
Posted by Ehaze on 10 Feb, 2022 08:19
Being new to EE and battery building, I was watching a video where guy tested some of the common cheap BMSs from amazon. In the video he said the common wall power supply cant properly charge a lithium battery pack .
So I am wondering what charger or system does one get?I have seen many videos on ppl building packs , but they never test them to show the long term charge and discharge rates .
I thought I would just hop on amazon or check out a wal-mart and look for a wall charger , but now I have more questions .
Would be nice to come across a smart BMS for the smaller packs .( i have seen a few diy BMSs but they look way to demanding to build from small parts )

#1 Reply
Posted by Siwastaja on 10 Feb, 2022 09:21
Who knows?

BMS can mean anything. Look at the specs to see what it actually contains.

"Common cheap BMS from amazon" likely is comparable to having nothing at all. At some point in its lifetime, the main power switch MOSFETs blow in short circuit, after which it becomes just a piece of wire.

Charging requires active control of current, and a product called "li-ion charger" does that. A CC-CV lab power supply can also do that, if configured right. A common wall-wart won't do it.

BMS rarely includes the charger. Amazon BMS definitely won't. So what they do have is just an emergency security layer in case of haywire charger. Which, on Amazon BMS's is very well known to just blow making a short circuit, not protecting against anything.

But again, this all depends on what the BMS is, is this single cell or multi-cell pack, how the BMS and charger communicate, and so on. This should all be in the product specifications and documentation, but decent BMS and charger systems of course cost orders of magnitude more than random AliZonBay boards.

#2 Reply
Posted by Ehaze on 12 Feb, 2022 07:44
The video was just using one of the small bms with 2 cells, I dont personally have any BMSs from amazon I just have ones i scavenged, but now I dont know what kind of charger to look for. I tend to scavenge old power tools so I guess i can star there ?

When I think of BMSs I think of something that allows for wall charging a battery pack, but lets say something small like 2 to 12 cells , 1 to 3s parallel and or series .

#3 Reply
Posted by Siwastaja on 12 Feb, 2022 07:50
BMS usually means a system which measures cell voltages and pack current, and either contains, or controls an external, disconnection switch. Additionally, BMS may control charger and loads in a more soft way than using the disconnection switch, leaving the switch for emergency use only (secondary layer of protection).

Very rarely is an actual charger contained under the term "BMS".

But BMS can mean anything, and this is due to culture of emphasizing importance of BMS without understanding anything at all. This leads to cargo cult engineering, "have BMS" tick in a box design.

Charger is basically a CC-CV power supply, with some minor extras like CV timeout, begin-of-charge voltage qualification, temperature limits.

#4 Reply
Posted by BeBuLamar on 12 Feb, 2022 18:05
I am sorry for my ignorance but what is BMS?

#5 Reply
Posted by Siwastaja on 12 Feb, 2022 19:57
Battery management system. It manages a battery. Like a manager of a pop star, it can actually mean anything.

#6 Reply
Posted by Ehaze on 14 Feb, 2022 07:48
As a beginner it is so confusing, there never seems to be a clear answer. One video will show you how to build a BMS, and another will say these are not BMSs and wont charge the cells,but wont go into details on how to charge the cells.

These BMSs are in most battery powered devices. There all kinds of mass produced battery powered devices that use cheap BMS we use every day without thinking about it

Is there a slap on and go product out there for smaller packs?

#7 Reply
Posted by Siwastaja on 14 Feb, 2022 08:01
Designing those is just non-trivial. Even professionals often get them wrong. I did some work analyzing failed BMS designs.

If you are a beginner, you should not rely on any tutorial, at all. Instead you should be building solid understanding on how li-ion cells actually work, what are the actual use cases, and then start gaining understanding how to manage them. This way, you get all the answers you need, and then you'll see most of the internet "knowledge" is pure BS, or total cargo cult engineering.

If you just want to use a li-ion cell in your product, your best bet is to buy a "protected cell", an existing charger module, and hope they are properly engineered. I'm thinking about some Adafruit-like stuff. I have no idea if Adafruit products are any good, but even Digikey sells them so I kind of assume they have weeded out totally unsafe designs. Hopefully. At least the risk should be smaller than with some random Aliexpress/Amazon/Ebay crap.

#8 Reply
Posted by Ehaze on 14 Feb, 2022 12:48
I am slowly cutting through the BS , thats why I just ask more specific questions.

I actually just bought some protected cells from jag35 but retail they are kind of expensive, it would be cheaper just to buy the actual boards.
The main issue Im having trouble with is the market side.There doesn't seem to be any consistent product to buy from the USA that isnt a 3 month wait.
You would think they would make more small plug and play device that can balance charge up battery packs and have a easy to read display. I have seen a few products but nothing stands out so far!

So from what i gathered is it might be best for smaller packs to buy a hobby charger to balance charge, use the cheap amazon bms for protection.
A BMS should allow for more mobility and use through standard outlets. Like if Im out on a trip and i need to charge my pack from a wall at a library or rest stop , I dont wan to have to carry a big Charger with me .

#9 Reply
Posted by Siwastaja on 14 Feb, 2022 16:17
One option is to completely skip that balancing stuff. It's totally optional. If any design relies on it for "safety", then it's not safe anyway. Balancing is only for maximizing long-term capacity (because the fullest cell defines high-voltage cutoff, while the emptiest defines low-voltage cutoff), but if the cells are good stuff and not some random rejects or fakes, chances are high the pack never loses significant capacity due to imbalance.

So while balancing (done right) makes sense, you can ease your task by forgetting about it, first.

All imbalanced packs I have opened up have been in systems with balancing BMS (which, obviously, has then failed to do its job). The few without any BMS at all, for example by Robert Bosch - perfect balance even at end-of-life.

#10 Reply
Posted by Ehaze on 22 Feb, 2022 14:18
I guess when PPL refer to BMS they think of the on board kinds , but a BMS is just a way to manage your battery , there are a few ways to get the same result .
What confuses me the most is the actual balance charging feature, so I thought the b1 b2 b3 ect where the balance leads but that just puts them series??
I guess every boards is a bit different. I think I am confusing balance charging with just regular charging.
I thought if you put 3 cells in series and put a charge on the end +and - it wouldn't charge the pack correctly?
I guess if I had to discharge through B+ and B- it wouldn't get the PCB protection, I had the idea it would not be be safe to charge that way?

SO in the attachment can I just charge on B+/- ? I thought that would not work?
I have a Tenergy Smart charger set to 16v that says it will cut off when charged, now that you mentioned it I guess this BMS doesnt have balance charging anyway. It has thermal sensors and B1B2B3.

I was thinking about bypassing the BMS for charging and using an RC 16v 1-4s charger, It can monitor and show each packs status. I would just use the BMS chip for low voltage protection.
BUT then I could just use a low voltage protection device ( there are a few options ) I am not sure which one I would choose and I wish there where more pre-made plug and play options that allowed for easy set up and monitoring ( there are some diy ones that are bit too complicated and rewire sourcing tiny parts )

So maybe you would have some thoughts on moving the charging from any board based BMS/PCb ( which PPl seem to not like anyway ) to a small RC style balance charger. I have one by Iflight its cheap and easy to use for up to 16v .

I also had an idea to make a modular battery pack from 4bay holders all connected by xt30s that allow it to be broken apart and connected to a power source individually and independently.
1 group of the holder are also protected by a chip so they have over and under discharge protection. I am wondering if I could use any 5v source for those ones , they will allow me to safely run a battery pack without any bms/pcb board .( IN THEORY)
Any thoughts are appreciated !

#11 Reply
Posted by jerryk on 23 Feb, 2022 05:45
Maybe this will answer a couple of your questions.
https://youtu.be/wIbHLacozFo

Jerry

#12 Reply
Posted by Ehaze on 28 Feb, 2022 19:44
ok this is basically what I have been doing.
I just pop the cell groups on a 4 bay, and then match then up as close as possible manually. Or i can charge the battery holder by connecting it to my 4bay with alligator clips and it charges like its 1 3.7v cell even though its 4 in parallel !

I started with drones so charging those battery packs are as simple as plugging in the plugs. Now I am trying to build a battery pack, I was assuming I couldn't just charge the series of packs.
I made a 12v 3s4p, I assumed I needed to balance charge them, BUT now my main question would be if it is OK to Charge the pack by the final positive and negative leads out? I can simply order a 12v smart charger that will shut off once it reaches max, I have a 16v one but I think it would be bad to use it for 12v as its only set to one voltage use .