Electronics > Projects, Designs, and Technical Stuff
Balance charging lithium cells
Siwastaja:
--- Quote from: OM222O on April 30, 2019, 11:14:14 am ---but I'm still not sure how a pack can remain balanced when cells have different capacities
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They will be balanced only at one point - typically at "top". Everywhere else, they'll be out of balance, until you are at top again - necessitating per-cell* cutoffs for charge (OVP) and discharge (UVP). But they do remain balanced (at that same point) because same current runs through every cell, and they all charge and discharge the same number of Coulombs. (Coulombic efficiency being around 99.99%, with little difference between the cells.)
So when we talk about "balance", what we really mean is balanced at one point.
*Cell as in series element. Multiple parallel cells count as a single cell
Now, to came back to your question about redistributive balancing; if you really have a pack where capacity differences are large, you would (theoretically) want "dynamic balancing" - balancing current which is high enough to make large balance differences during discharge cycle. I.e.:
Cell 1: 100Ah
Cell 2: 90Ah
Cell 3: 110Ah
"Normal balancing": all are at 4.20V at the same time. Pack capacity is 90Ah, when Cell2 hits 2.80V (or whatever you use).
"Dynamic redistributive balancing": All are at 4.20V at the same time. While discharging, charge is moved from Cell3 to Cell2. Pack capacity is 100Ah. All cells hit 2.80V at the same time.
Now, unless you discharges are super slow, you'll quickly see you need enormous balancing power to achieve this. Making the cost higher (easily at least around $5-10/cell, even if optimized for cost in masses), also weight (transformer magnetics). So, such systems are mostly of academic interest, they almost never make economic sense; even in second-life applications.
The largest completely repurposed pack I have built was a 85V, 250Ah (around 21kWh) pack for an EV, some cells were damaged by overdischarge (by a redistributive, expensive, broken-by-design BMS, by the way!), meaning I had bigger-than-normal differences on leakages. Still, 20mA balancing current (resistive) was enough to keep the system in balance. In any case, I carefully tried to match the capacities, just by measuring all the cell capacities beforehand and working up combinations where the capacities match as closely as possible. This is the preferred way instead of dynamic redistributive balancing: you'll have much lower-cost system, with less extra weight and complexity. So, instead of 90, 100, 110Ah, you would have 99, 100 and 101Ah, resulting in a 99Ah pack.
If you are building a large second-life pack out of small cells, such matching is fairly easy to do. Remember that if you parallel different cell brands/types/conditions, you risk problems with current sharing, so may need to derate both charge and discharge current.
Siwastaja:
--- Quote from: paulca on April 30, 2019, 02:08:31 pm ---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.
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That's not at all what I wrote; a complete strawman. I have developed several li-ion balancing BMS systems; including the redistributive ones asked by the OP, because, yes, balancing is often needed. Please reread carefully; if you are unable to understand, it will be your issue, not mine. Thank you.
paulca:
--- Quote from: Siwastaja on April 30, 2019, 02:16:42 pm ---
--- Quote from: paulca on April 30, 2019, 02:08:31 pm ---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.
--- End quote ---
That's not at all what I wrote; a complete strawman. I have developed several li-ion balancing BMS systems; including the redistributive ones asked by the OP, because, yes, balancing is often needed. Please reread carefully; if you are unable to understand, it will be your issue, not mine. Thank you.
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It just seemed from how brutually you attacked "hobbiests" on their understanding of lithium batteries that they don't go out of balance. You seemed to suggest that professionally developed products don't even consider balancing as there is no need to.
I frequently (after a sunny week) run a laptop from RC LiPos charged off a solar panel to use up excess solar power from a battery only system. They are always balance charged, but as they are unprotected high current packs (I don't know if you can protect packs capable of 100A constant). I run a LiPo alarm on the pack which cycles through cell voltages so I can literally watch the pack unbalance. Within about 5 minutes of running the laptop they will go out of balance by a few tens of mV. By the time the pack is flat the total output voltage is greater than 3.0V per cell, but one cell will set off the alarm as it's below 3.0V per cell.
In fairness the lipo alarm (a voltage sensor, LED display and smoke alarm sounder) is running off cell 1. But without that they still go out of balance and cells 2 and 3 are not equal either. Maybe this could come down to cheap hobby lipos. However I have seen similar tests with expensive LiPo packs from top name brands. Maybe if you approach LG or Samsung and make a commerical order for exactly matched and batched cells you might get better, I don't know.
Once thing to consider and you might experience this in an EV, when tightly packed into a pack the middle cells are exposed to higher heat and this, I believe changes their internal resistance which under high load will cause imbalance to build up pretty quickly. Although, if you are dealing with 18650s this is probably less of a concern as their total pack density is poor compared to tightly packed LiPo flat "slab" cells shrink wrapped into packs.
Siwastaja:
--- Quote from: paulca on April 30, 2019, 02:35:02 pm ---brutually you attacked
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Sorry 'bout that, I can see my poor choice of words...
--- Quote ---You seemed to suggest that professionally developed products don't even consider balancing as there is no need to.
--- End quote ---
... very hard to see how you would make this interpretation, though. Your claim, instead, was absolutely clear: balancing is absolutely necessary, always. This is untrue.
--- Quote ---I frequently (after a sunny week) run a laptop from RC LiPos charged off a solar panel to use up excess solar power from a battery only system. They are always balance charged, but as they are unprotected high current packs (I don't know if you can protect packs capable of 100A constant). I run a LiPo alarm on the pack which cycles through cell voltages so I can literally watch the pack unbalance. Within about 5 minutes of running the laptop they will go out of balance by a few tens of mV.
--- End quote ---
I see. You are mixing up two things here (see my previous reply). Having different cell voltages outside the "balance point" (typically top), doesn't mean the battery should be called "unbalanced". Try charging it up without balance charger, and you'll see the voltages match again! It was (top) balanced all the time! Now, do it 1000 times (and/or wait for 5 years) and you may be developing an actual imbalance; for this, balancing is needed. It would never happen during one cycle, unless the battery is dying in your hands. Now, if you started to balance during this discharge, you would be out-of balance from the top, and would need to balance again in opposite direction during charge! This would be a massive waste.
To avoid confusion, it's better to talk about capacity mismatched pack, and not use the term "balance" to describe what you are seeing.
--- Quote ---By the time the pack is flat the total output voltage is greater than 3.0V per cell, but one cell will set off the alarm as it's below 3.0V per cell.
--- End quote ---
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Yes - necessitating either cell-level monitoring (Low-Voltage Cutoff), or safety margin (i.e., use 3.10V instead of manufacturer-specified 2.50V or whatever). The smaller the number of cells in series, the more a single cell contributes to the total voltage; hence, 2s untapped packs are very common, and over 6s, all packs tend to use cell-level monitoring (highly recommended unless you have a very good reason not to). Neither case still requires balancing.
--- Quote ---Once thing to consider and you might experience this in an EV, when tightly packed into a pack the middle cells are exposed to higher heat and this
--- End quote ---
This is true. Higher temperature increases leakage (self-discharge) current, causing slow imbalancing. Large systems are difficult to thermally couple, again necessitating balancing. Luckily, large packs are expensive anyway, and tend to have some complexity in any case, so adding a simple cell-level management system with simple resistive balancing tends to become a no-brainer.
Note that still this is a phenomenon that happens slowly (during years) if not handled. Don't mix it up with varying voltages during a cycle; it will be back "in balance" again, even without balancing. In fact, trying to balance "mid-cycle" causes imbalance; hence balancers typically work with full battery, during the end of charge cycle, or, if they are smart enough (such as one of my BMS design was), they sample the balance at the end of charge, then can do the actual dissipation work at any state. This was my trick of reducing the balancing current from the 500 mA of the competitors down to 40mA, yet still do the same work (or even more), making thermal design and safety easier to achieve. Yes, I saw a report of stuck-on 500mA balancer resistor burning a car. I was actually able to fuse the cell connections with 100mA fuses.
paulca:
I my case my LiPos are all high current discharge packs with no cell protection. I'm not sure you can get cell protection for a 5Ah 30C cell capable of 150A, I'm not sure how you would switch that kind of current without heavy bulky mosfets. I have seen people take the LiPo alarms I have and disconnect the sounder and connect a relay, but that's also bulky and heavy.
In their real application the brushless motor speed controller connects to the balance lead and watches cell voltages. When one falls under 3.30V (normally, but configurable) it starts a "soft shutdown" in that it limits your power progressively. This usually gives you 30 seconds to a minute to make a controlled landing. Hard cut off happens at a lower voltage and forces you to do a dead stick landing which can be messy with a plane and very difficult and usually fatal with a heli.
If I was to try and test "top balance" I don't think my charger would permit it. In "CHARGE" mode with no balancing it will charge until a single cell his 4.20V and then end the charge, regardless of the other cell voltages. I could I suppose try this and if it is top balanced, the others will be with in 10mV or so. Obviously not on the pack running the cell monitor from cell 1.
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