Electronics > Projects, Designs, and Technical Stuff
Balance charging lithium cells
KL27x:
--- Quote ---So, lithium ion cells tend to be almost good enough to require no in-system balancing whatsoever. This creates the awkward "it depends" situation, necessitating real system level understanding.
In reality, sometimes balancing is needed, sometimes not.
--- End quote ---
In my opinion, the balancing is rarely necessary for increasing cell life, regarding longevity or number of cycles. But it can very slightly increase the cell capacity right after a full charge. When I say slightly, I mean very slightly, and this applies mostly if the battery is used immediately after charging it. So balancing would be important for something like an EV, where you have massive weight/space in the batteries. Or a cell phone, which it going to be on 24/7 and sucking battery right after you unplug it. (Not that a single cell battery ever needs to be balanced, of course).
But in many other uses, it is not going to do a whole lot of improving. Say for something you recharge then put away. And it may be more effective just to buy batteries with a larger capacity to start with and/or adding more and/or larger batteries.
This is because my experience is that the bad/weak cells start to self-discharge faster, but it doesn't appear to be constant/linear. The bad cells seems to quickly SD from 4.20 down to say 4.00-4.08. So even if you go through the hoops to get all cells in a series to 4.20, the bad cells lose that last bit of full charge relatively fast, anyway. So in balancing them, you are not necessarily fixing something that is constantly drifting farther out of whack until it is causing some major problem. Those individual cell voltage discrepancies will reappear on the very next charge. You will have to re-balance them almost just as much on every subsequent charge cycle until you replace the weak links, and you are doing that only to reclaim a relatively small increase in the total charge.
This is somewhat limited in sample size. I'm probably over-extrapolating and oversimplifying and jumping to some conclusions. People are still designing products that occasionally catch on fire. But I wonder how much of that is simply due to manufacturing faults in the cells, themselves.
edit: Hmm, I guess the way to describe it is that assuming there IS a creep over time issue that is actually corrected by the occasional cell balancing, this issue might be riding on top of a larger discrepancy that is not getting "fixed" in the same way.
OM222O:
if what you are saying is correct, then one should be much more concerned with the health of their pack, rather than it's balance. I.e: measuring individual cell voltages and comparing how much they discharge and warn the user if one or a few of them are degrading much faster than the others ??? this will prevent some major failure and the batteries are cheap enough to replace anyways!
KL27x:
I'd put it that
IF one is more concerned about pack life than capacity per charge, it MIGHT be in your best interest to only balance the cells every X charge cycles rather than on every single charge cycle. But I don't know for sure if that is the case, even if this is correct under at least some specific set of circumstances. :)
This also assumes to some extent that the battery might be maintained by cell replacement rather than fully replaced. Like if your parameters say the device must run for X time for the battery to still be considered good, the one that is balanced every time might meet that for a higher number of cycles but in that time suffer slightly more decay/wear on the good cells... but no one cares, because at that point the entire battery will be replaced, anyway. The one that is not resistor-bled-overcycled as often might drop below that X limit sooner due to that weak cell not being corrected on every cycle.... but it might be in better shape after some maintenance including replacing individual cells.
jbb:
It may not happen very often, but if you get a small self discharge imbalance in a series pack then you can lose huge amounts of capacity. I’ve had this happen to a couple of infrequently used packs; you take a few hundred mAh out and one cell cuts off with undervoltage. Then you change it and another cell cuts off with overvoltage.
For your own use, you could probably ignore it and see what happens.
For mass production, you’d run the risk of customer dissatisfaction and needing to replace packs.
In any case, LiIon and LiPo packs should probably include under and over voltage protection for every series cell. Because fires are bad.
Siwastaja:
--- Quote from: jbb on April 29, 2019, 04:07:05 am ---In any case, LiIon and LiPo packs should probably include under and over voltage protection for every series cell. Because fires are bad.
--- End quote ---
On the Internet forums, everybody "knows" this is always required, otherwise a fire ensues.
In reality, BOSCH products do not go up in flames, despite not having any series cell monitoring whatsoever.
Most fire incidents are due to cell manufacturing defects, typically in single-cell applications.
But, your distinction between monitoring and balancing is a correct and an important one. Cell voltage monitoring may be important for both safety and functionality. Balancing is a secondary improvement which may or may not be used with cell voltage monitoring, to prevent pack capacity reduction caused by imbalance (where OVP and UVP events are caused by different cells). Balancing rarely "protects" the pack.
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