My opinion of balancing LFP batteries is based on my personal experience with LFP house banks that I built using native large prismatics with the cells staying within balance for years, I am not a big fan of driving an LFP battery into the charging hockey stick region any more than it truly requires. That said, and putting that technical opinion aside, I do adhere (grudgingly) to the LFP battery manufacturer’s technical guidance.
I have been doing more digging on balancing and received a graduate level lesson from a colleague who worked for a major self-contained battery manufacturer and is still loosely associated with their technical side.
So with that background, I posted a question on a marine forum aimed at a fellow professional with a deep understanding of LFP self-contained batteries and asked him why we as an industry feel we have to balance LFP batteries?”
His response was a master class in LFP cell balancing:
Passive balancing.
Cell balance turns on at I believe 3.4 with a Delta greater than 0.020.
After absorption is complete and cells fall to resting voltage or float of 13.5 then there is no cell balancing.
Should you get a new battery that has been sitting or was not well balanced at assy (it happens) then it can now be “cooked” into balance by holding it at some elevated voltage until it is balanced. For instance I just had a customer that threw his bank of 4 Epoch V2 Elite 24 volt batteries in and on his first charge he got a cell overvolt when under BMS control.
Since the absorption for the 24 volt batteries are now 28 volts. To avoid cycling I had him turn on “limit max charge voltage” in DVCC and set that voltage .2 volts under absorption, so 27.8. When you do that the battery will stay in absorption because it never reaches the 28.0 volts needed to cycle to the next phase and lower CVL to 27. I had him leave it overnight and this morning all 4 batteries had a Delta of about .020mv @ 27.8 v. Then turn the voltage back so they cycle normally and on subsequent cycles they will balance to 0.20mv @ 28.0.
If you wanted to refine the balance more you can repeat the same thing at a higher voltage and get the split to 0.20 @ 28.4 etc.
And yes once they are balanced like that they should stay aligned relative to whatever anomalies are between each cell. And that could be a long time. It’s really a question of how well matched and batched the cell are as well as just the overall quality of the cell as to how long they may go and not need balancing. And of course, dont forget you cant check for good balancing unless the voltage is pushed into the upper knee levels. The higher they are pushed the more the mismatch is uncovered and exposed and the more you can act on that disparity. Balancing at 13.5 wont get you anything. Even 13.6 may barely begin to show disparities so that balancer may act on something. Thats why the .020 delta is added as a requirement.
If cells aren’t matched and batched right or one develops an issue it may fall outside the “authority” capability of the balancer to keep pace. I had a Vatrer battery I tested recently, and it seemed to have a small cell issue. The balancer was somewhat keeping up. But I bet that particular battery wont make 10 years. probably not 5. Once a cell falls outside the capability of the balancer it wont take long for the battery to become unusable.
Active balancers always give me pause…lol. Yes they have more authority and balance quickly. But a cheap active balancer can also be used to disguise crappy cells..lol. Sometimes I wonder if bad cells are disguised by powerful active balancers, which are cheap. Obviously if given the choice of top tier, well matched cells or mediocre/poor cells and a powerful active balancer to reign them in…ill take the good cells and passive balancer. That’s the recipe for a long lasting battery.
Yes, Charlie I do believe we may be balancing our packs more often than may be needed. Thats one of the reasons we lowered the absorption to 14.0 volts (3.5vpc). To lower the overall time spent at any higher voltage if it’s not required to maintain balancing. There certainly is no reason to use anything higher for gained capacity and it will also not effectively reduce charge time to full capacity.
The problem is we really don’t know the condition of our cells when the balancer is keeping pace. It may be right on the cusp of losing control. Or it could be that on subsequent absorption cycles the cells are so well matched that the balancer doesn’t kick in at all or when it does it may be for seconds. Other lesser packs may ride the balancer the entire time in absorption and just squeeze out of absorption just in range. Or just out of range for that matter.
Of course the cell batching isnt just based on internal resistance and capacity. There are other factors such as over voltage potential. As another engineer puts is “This is overhead power necessary to drive kinetic processes in the battery”. And this “overvoltage potential” changes in the cell based on temp and age. So if one area of the battery is hotter than another it can also affect cell balancing over time, especially at higher C rates.
Ya know that Balmar SG200 gauge that had the gimmicky “State of Health” reading? The one that seemed to always say 100% State Of Health no matter what? It just dawned on me…someone could use a monitoring program to monitor the balance time and cell voltage in absorption, and you could probably project some State of Health reading based off that. Maybe dont call it State of Health. maybe call it “Cell mismatch Health”. If the balancer has to run on each subsequent absorption cycle full time and its barely maintaining the delta set in the algorithm IMO that battery is not likely to make it 10 years.
**Danger** cell mismatch approaching balancer capability limits!
:ROFLMAO: Of course I am kidding. No battery maker would add that since it would result in additional warranty claims prior to the prorated window..lol.
Well, Victron might. They are good like that.
Anyways, I think as long as you aren’t pushing the absorption voltage up too high or holding for too long there is little harm in balancing every cycle IMO. But for a quality cell pack it’s also likely it may not be required. I think Victron has a recommendation of 2 hours per month? Also I think as batteries age the more they will be leaning on the balancers to maintain a viable pack.
My recent Battle Born video is actually showing an example of that. The 4.5 year old BB 24 volt 50 AH batteries had cell issues that became more than the balancer could adjust for. The result was a quick death spiral one that state was reached. In case anyone wants to watch a bad, rambling video ill post it below.
So I really don’t have a super specific answer and I suppose it would be very dependent on what you have. As we all know the quality of these builds varies greatly.
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Since Victron guidance for the NG batteries is to balance for two hours every month is difficult to execute with a Multi or Quattro as the absorption time can only be adjusted by connecting a Mk3 USB and reprogramming the inv/chgr. To comply with this requirement would require reprogramming a WakeSpeed, Arco Zeus, or Balmar alternator regulator and the Multi or Quattro to adjust the absorption time to keep the system in the charging hockey stick area long enough to balance.
Question #1: Can control of absorption time and voltage for all Victron charging sources with DVCC engaged be made accessible via the Cerbo GX?
NOTE: There is a long standing maxim in ship maintenance; “If a maintenance activity is too difficult to accomplish, if it gets accomplished at all, it will not be properly performed.”
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As noted previously, I have been consulting on a boat with Battle Born batteries and have done a lot of research especially in light of the engineering/quality issues BB is currently experiencing. The technical documents for the BB 270Ahr battery states holding the battery at the absorption voltage for 60 to 90 minutes per 270Ahr battery string. Conversation with their technical support engineer recommends holding their batteries in the charging hockey stick region for 100 minutes/270Ahr battery. This particular bank has 4 parallel 270Ahr strings so to balance IAW the technical manual would require 240 minutes (4 hours) to 360 minutes (6 hours) or, per the tech support desk, 400 minutes (6.7 hours). This issue is further complicated because, even when a BB battery is equipped with the BB Intelligence feature, individual cell voltages cannot be observed.
Victron’s guidance for our NG batteries from the 24VDC/200Ahr NG Battery Tech Manual:
“Absorption time: 2 hours. We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such
as backup or UPS applications, and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the
balancer enough time to balance the cells properly.”
This is the same guidance for all NG batteries regardless of their OCV or C rating. It is not intuitive to me why there is no parameter such as X minutes/Ahr. It seems counterintuitive that it will take the same amount of time to adequately balance a single 12.8VDC/100Ahr NG battery as it takes to balance 8 x 25.6VDC/200Ahr NG batteries in a 1,600Ahr bank. But that’s what our guidance says.
These are pretty non-specific requirements which lead to a couple of more questions:
Question #2: How can we “tighten up” the requirement for how often an NG battery must be balanced and what is the parameter for balancing:
X minutes/100Ahr of capacity?
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A lot of issues regarding balancing of our NG batteries dissolves if we can actively observe individual cell voltages and take corrective action based on a parameter such as <30mVDC difference across the battery individual cells is acceptable. With this approach, the operator can periodically check the individual condition of the cells in the battery and take appropriate action using the results from Question #2 above. This would reduce the amount of time that a battery is in the charging hockey stick region which I think is important to maximize battery service life.
Question #3: Since firmware 3.67 provided the ability to observe the high and low cell in a battery, would it be possible to pick up all cell voltages on a per battery basis and display them on the Touch 50/70?
If this is possible, then the next step would be to add an algorithm that picks out those cells that are out of balance or getting close to out of balance and warn the operator.
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Looking forward to further discussion on the subject of properly and minimally balancing Victron NG batteries.