Electronics > Projects, Designs, and Technical Stuff
Balance charging lithium cells
paulca:
Generally, as a rule of thumb, do not mix different capacity cells in series... or parallel for that matter.
If you watch through some of this guys videos he provides links to software designed for what you are doing. Basically you capacity test all your batteries, enter them into the software and it gives you the best way to lay them out in parallel and series. He also shows various ways he has used to balance them. His latest videos show an elaborate system where each parallel cell pack has a BMS itself and these chain together over I2C (or similar) to communicate with monitoring software.
paulca:
Also, just a polite reminder that Lithium cells are genuinely dangerous. Unprotected cells can get pretty upset if overcharged or shorted. The metal 18650s can still swell, rupture and spray flames. Just be careful and don't charge them unsupervised unless you are totally sure of your setup.
OM222O:
the protection is already taken care of. I'm just not sure about balancing them or not and if I decide to go down that route I want to minimize losses
Siwastaja:
--- Quote from: OM222O on April 30, 2019, 11:14:14 am ---there seems to be different schools of thought here like always :-DD
--- End quote ---
This is the classical difference between hobbyists consuming "for hobbyists" products (RC toys), and li-ion battery analysis, pack construction and management professionals, such as myself.
The mantra of "absolutely needing balancing" is ubiquitous on forums, yet hilarious if you have ever looked at real-world products. Apparently the world's biggest and most well known companies have no idea what they are doing. HobbyKing School of Engineering knows better!
Of course, if your target is just a hobby, then it might be wise to listen to what other hobbyists say, and use the designated hobby products; they have a wide audience testing the products.
If you are actually designing a product, go with the real-world information from the industry. It's a long road anyway. Completely ignore any RC hobbyist information, Battery University, and similar sources. Peer reviewed scientific studies are better, although slow and inefficient way to learn. If someone is talking about "li-ion and LiPo", it's a good telltale sign already they have no idea what they are talking about.
Even better, do teardowns of commercial and industrial li-ion packs. You'll see many different approaches!
As said earlier, non-balanced, even non-monitored packs are absolutely ubiquitous in consumer and industrial products; especially in 2s and 3s, but fairly commonplace up to 6s. I have measured a few BOSCH power tool packs that, after End-Of-Life of years of hard use and highly increased DCR, are still in balance within 10mV at top, using Samsung SDI cells. They do not include any electrical connections to the series taps. It's highly possible some units are worse, and do imbalance, but it seems they still don't go up in flames. OTOH, we have seen quite a few incidents of flames and early destruction of cells in laptop packs which do include balancing.
In general, if you don't know what you are doing and need to design a safe system, I recommend you do neither kind of a system. Keep learning, and in the meantime, use something off-the-shelf that everybody uses already; it's a good way to hobby around, but having used a battery analysis toy from HobbyKing to measure a Chinese toy battery from the same place doesn't magically make you an expert on battery tech.
paulca:
--- Quote from: Siwastaja on April 30, 2019, 01:47:29 pm ---If you don't know what you are doing, I recommend you do neither kind of a system. Use something off-the-shelf that everybody uses already; it's a good way to hobby around, but having used a battery analysis toy from HobbyKing to measure a Chinese toy battery from the same place doesn't magically make you an expert on battery tech.
--- End quote ---
So if I show you batteries out of balance I would clearly be making that information up and we are both hallucinating as clearly they don't go out of balance in your world. Laptop packs don't fail either because of cell imbalance.
Also, "Hobby" batteries are most often used in different ways, surprisingly far more intensely. You show me a power drill that will pull 100A, 200A peak and drain it's battery pack in under 7 minutes. Then show me the person who expects that pack, cooled, charged and flying in the air gain in under 30 minutes.
I'm sure that if you buy expensive matched, low resistance, batch matched cells for £4 each and you charge them at low rates and discharge them at low rates and seal them in a can preventing anyone accessing them, they MIGHT remain in balance for years. However not all of us live in the world where we want to spend £65 on a new battery pack.
On your advice I will now use "CHARGE" mode at 2C rate and not "BALANCE CHARGE" mode when I charge my LiPos. As you state I am completely safe doing this. So I will. I assume it's okay to do this while my daughter sleeps upstairs and leave them unattended, yes?
Also, we had the conversation before on LiIon versis LiPo, we agreed I think that while it has nothing to do with the chemistry, that marketing and branded has polluted that completely (LiPo just means pouch cell these days) and there is little to now way to tell, asides a datasheet what the actual chemistry is in both cases. In consumer grade batteries, typically LiPo = High current, fast chargeable pouch cell. LiIon is generally a low current, hard cased, slow charge battery. Neither really are those chemistries/technologies anymore, but they definitely NOT the same type of cell either.
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