When does cell balancing happen?

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phildw56au

Member
Joined
Sep 10, 2021
Messages
11
Location
River Heads, Qld
Having owned a 2012 ZE0 24kWh for a couple of years now (30000km on the odo, 65% SOH), and having read too many forum posts that I hope will answer this question but don't, the thing to do is to just ask the question as clearly as I can ...

WHEN does cell balancing happen in the Leaf battery pack?

It seems like such a simple question, but I've never found a simple answer.

I understand the HOW of cell balancing in a Nissan Leaf battery pack - a switched resistive bypass path across the cell/s that are identified as being too highly charged compared with others in the pack.

I've looked at plenty of Leaf Spy Pro graphs, but I can only see that when the car is either charging or turned on. What I want to know is if cell balancing is happening ALL THE TIME, irrespective of what the car is doing.

There is so much mythology around the charging habits of a Nissan Leaf, particularly the one that says that every once in a while, you should charge it up to 100% (both by fast charging and/or by slow charging, depending on which myth is being shared at the time) because only then will cell balancing occur.

If my car is sitting parked on my driveway, doing nothing but sunbathing, isn't that an ideal time to be balancing cells, irrespective of the state of charge of the pack?

Thanks in advance of any and all contributions to my better understanding.
 
Balancing happens much of the time. I think that charging to 100% is different because the car is sitting still while charging, unlike while driving, and that this allows the BMS, which can only handle tiny currents for cell balancing, can do the most effective work because the cells aren't charging and discharging to move the car while it's trying to balance them.
As for sitting still while off, I may be wrong about this, but I think that the BMS is not enabled for balancing then.
 
phildw56au said:
I've looked at plenty of Leaf Spy Pro graphs, but I can only see that when the car is either charging or turned on. What I want to know is if cell balancing is happening ALL THE TIME, irrespective of what the car is doing.
The short answer is: NO.
Cell balancing only occurs when "plugged in"...and even then only when not charging (e.g. full/complete).
The bigger issue is: the amount of current supported in individual cell branches (resistors) is so small that it is difficult to balance widely divergent cells/modules. For example, I replaced a (weak) module in my pack several weeks ago, and I am still no where near "balanced" (but at least that module is "strong" and not "weak").
 
There is no benefit to charging to 100% for anything other than extra range. There are a lot of drawbacks to charging to 100% including advanced degradation. MANY studies show up to a 300% increase in the life of the pack when only shallow cycling is done. Obviously that doesn't work for a car since we don't have the infrastructure to stop and charge every 30 miles so the ultimate goal is living in the middle but that is no different than eating responsibly.

Dessert is NEVER responsible but we all do it and if in moderation, its fine. Your battery is no different. For the day to day use when your needs only require 20-30 of your capacity, charge accordingly. Fully charge for that day trip that will exceed or approach your car's full capacity.
 
DaveinOlyWA said:
There is no benefit to charging to 100% for anything other than extra range. There are a lot of drawbacks to charging to 100% including advanced degradation. MANY studies show up to a 300% increase in the life of the pack when only shallow cycling is done. Obviously that doesn't work for a car since we don't have the infrastructure to stop and charge every 30 miles so the ultimate goal is living in the middle but that is no different than eating responsibly.

Hi Dave ... thanks for your input.

If, as you say, there's no benefit to charging to 100% for anything other than extra range, are you suggesting that cell balancing occurs under other conditions, or that cell balancing isn't an issue, or is unnecessary?
 
LeftieBiker said:
When LeafSpy shows cells in red, as it so often does, it is my understanding that those cells are being balanced.

Hi LeftieBiker ... thanks for your input.

I can be driving down the street and I'll see both red and blue bars on my Leaf Spy Pro graph. This would imply that cell balancing is attempted at least whenever the car is on, even if being driven. Is this what you're suggesting?
 
Stanton said:
The short answer is: NO.
Cell balancing only occurs when "plugged in"...and even then only when not charging (e.g. full/complete).
The bigger issue is: the amount of current supported in individual cell branches (resistors) is so small that it is difficult to balance widely divergent cells/modules. For example, I replaced a (weak) module in my pack several weeks ago, and I am still no where near "balanced" (but at least that module is "strong" and not "weak").

Hi Stanton ... thanks for your input.

If I only ever charge to 80%, does that mean that my cells will never balance?

I strongly suspect that your assertion is correct ... but there are plenty of people out there who are convinced that never charging to 100%, and unplugging the charger as soon as the charging is complete, are examples of 'Best Practice" in maintaining battery health.

Dare I say it ... your assertion makes sense in terms of making Nissan Leaf charging a simple thing to do (as Nissan would have intended) ... whenever you get home, plug it in, charge it up and leave it plugged in until you use it again ... with my only exception being (perhaps) charging it up to 100% when you know you're not going to drive it for a while.
 
I've always thought that balancing happens 'all the time' but I guess it would mean that some sort of controller was active so that would imply that it would happen all the time when the car is powered on in some form.

The balancing controller just needs to know the voltages of all the cells and that information is obviously available. More generally, battery packs can be top balanced (when at ~100% SOC) or bottom balanced (when at low SOC levels).

I don't see any reason the cells couldn't be balanced at any voltage level although it might not make sense or even be possible to do any balancing when the car is under a heavy load or in heavy regen. That would depend on the circuitry but even under a heavy load, balancing could probably still occur as any balancing current drained from individual cells would just put a slightly higher load on the cells with a higher voltage, thus draining them just a bit faster than cells with a relatively lower voltage.
 
I am going to offer some contradictory comments:
1. I think cell balancing happens whenever the car is charging or in ready mode. It may even balance when the car is parked since the LBC (lithium battery controller) is inside the battery case and powered from the battery. Since we have no way to communicate with the LBC unless the car is on or charging, there is no way that I know of to confirm whether it is active when the car is off.
2. I almost always charge fully and deeply discharge before charging again. I don't think capacity loss of the batteries in all three of my LEAFs is (was) any worse than other LEAFs used in my climate. Therefore, I suggest you charge and drive the car however fits your use needs without worrying excessively.
3. Cell balance does contribute to maximum range on a charge because charging stops when the highest cell pair reaches the maximum allowable voltage and turtle (or shutdown) happens when the lowest cell pair reaches the minimum allowable voltage. This means that the battery will store a little more energy if the cells are well balanced.
4. One thing I strongly recommend is to have the battery between 40% and 80% charge when the car will be parked for extended time. This will help to minimize the time car sits at high state of charge or really low state of charge.
 
phildw56au said:
LeftieBiker said:
When LeafSpy shows cells in red, as it so often does, it is my understanding that those cells are being balanced.

Hi LeftieBiker ... thanks for your input.

I can be driving down the street and I'll see both red and blue bars on my Leaf Spy Pro graph. This would imply that cell balancing is attempted at least whenever the car is on, even if being driven. Is this what you're suggesting?

I'm saying that balancing occurs whenever the conditions allow for it - when no serious loads or charging currents are present to make it impossible for the charge controller in the LBC to balance the pack with the very modest shunting it has available. If the BMS only top balanced the pack, then it would need much more robust hardware, with much higher currents, to do it.
 
phildw56au said:
Stanton said:
The short answer is: NO.
Cell balancing only occurs when "plugged in"...and even then only when not charging (e.g. full/complete).
The bigger issue is: the amount of current supported in individual cell branches (resistors) is so small that it is difficult to balance widely divergent cells/modules. For example, I replaced a (weak) module in my pack several weeks ago, and I am still no where near "balanced" (but at least that module is "strong" and not "weak").

Hi Stanton ... thanks for your input.

If I only ever charge to 80%, does that mean that my cells will never balance?
If your "charge stop" (e.g. "full") is set @80%, then the pack should balance after that if you leave it plugged in (although it may take several hours).
And yes, it is not good practice to charge to 100% and let the car "sit"...but 100% every once in awhile (to recalibrate the BMS) isn't a bad idea.
 
Stanton said:
The short answer is: NO.
Cell balancing only occurs when "plugged in"...and even then only when not charging (e.g. full/complete).
The bigger issue is: the amount of current supported in individual cell branches (resistors) is so small that it is difficult to balance widely divergent cells/modules.
I thought it has to balance all the time when not charging or discharging (driving or even in accessories mode listening to the radio). For the simple reason that as you say the bypass current is SO small. A figure of around 10mA comes to mind, with the bypass resistors being 340 ohms. Even staying plugged in may be enough to swamp the bypassing current.

So in my view, it's pretty much the opposite to what you stated. To me, bypassing must be happening all the time, but will only be effective when idle. I'm open to ideas on why I might be wrong.

Edit: Although when idle but still plugged in, there will still be differences in total cell currents, which might be just as effective as when it is idle. When driving, currents are so much higher than bypass current (by about 4 orders of magnitude) that I think Leftie is right. The forces actively pushing the cells near their limits are probably inducing far more unbalance than the very weak balancing currents can overcome. So while driving, I'd say that balancing continues but is essentially ineffective. Though I suppose you could say that even then, balancing is happening.
 
Well, we can believe LEAF Spy or not but its quite obvious to anyone who has it that balancing happens all the time and this "gotta be plugged in" or this or that is all nonsense.

As far as charging; There is no getting around the fact that Lithium prefers life in the middle. Obviously this is not feasible in a car where convenience getting places is THE reason for the car in the first place but some benefit can be had with some low key SOC management. There is no reason to not charge to full if that is what it takes but the other side of the coin is "how difficult is it to manage SOC to fit your immediate need?"

In the 24 kwh LEAF of the OP, I have to question whether you have enough capacity to do much of anything but in my case; I have a plus, a 10.2 mile daily commute and see no reason why I can't keep it within the 40-70% SOC range.

Now, some could construe it as a hassle to manage the charge since Nissan still hasn't seen the benefit of custom charge settings which is beyond me but you have timers and a big battery which means my target is 50% but TBH, I don't care if its 45% or 65%. Its close enough. I am soon to be 3½ years in on the one I have now and have done 23 full charges and it so happens #24 will be tomorrow night so even "I" do it.

But yeah, its all about whether you want "complete" convenience or maximized longevity. Rest assured that a million people parading up comments about how they have done this and that and the car is fine will still not change the basic physical properties of Lithium cells. The only thing they can do is affirm that Nissan is improving but that is all.
 
DaveinOlyWA said:
In the 24 kwh LEAF of the OP, I have to question whether you have enough capacity to do much of anything but in my case; I have a plus, a 10.2 mile daily commute and see no reason why I can't keep it within the 40-70% SOC range.

Hi Dave ... thanks for your input.

I use my Leaf for commuting in and out of the town that I live close to ... a round trip of about 32 km / 20 miles ... and I expect that it'll be able to do that for me for many years to come. I also manage to do that with the car charged up to 80%, but I'm quite ok with charging up to 100% when the mood takes me ... sometimes, a bit of extra shopping is in order.

What I'm not into is worrying about the minutiae of charging my car. Charging to 80% is easy; charging to 100% is easy; I normally leave my car uncharged (that is, not plugged in to charge) until the evening before I intend to drive it. This to me seems a simple way to achieve a 'best practice' outcome.

I liked a lot of what of what you are sharing in your complete post, but on this one point, I hope that I answered your question.
 
Thanks for all of the responses that I've received to my question.

Simply the fact that there have been a variety of proposed 'answers' to my question tells me that it was worth asking.

I'm now tending to think that cell balancing happens whenever the car is 'on' ... whether driving or charging ... as suggested by simply looking at Leaf Spy Pro while driving or charging. In both situations, you can see both red and blue bars on the graph, and the color is indicative of whether the bypass circuitry is on or off at the time.

Having had a close look at the circuit diagrams of the BMS/LBC that were shared by turbo3 in another post on this forum (they took some finding ... link is below) and having some knowledge of electronics and physics, I see no reason why this isn't entirely feasible.

https://www.mynissanleaf.com/viewtopic.php?p=380079#p380079

If I use a torch with a number of cells in series, and then put an appropriate value resistor (say 100 ohm) across just one of the cells, a couple of things will happen:
1 - The torch will still work when I switch it on,
2 - The resistor will still drain charge from the bridged cell, whether the torch is switched on or off,
3 - The resistor will not in any way affect the function of the torch, but that cell will run flat first if the resistor is left connected.

In the Leaf battery, assuming that my analogy is correct, switching on the bypass circuitry on any particular cell will drain charge from the cell, irrespective of whatever else is happening to the battery / cell at the time. Even when charging, the most current that can ever flow through the bypass circuit is determined by its resistance (430 ohm) and the cell voltage (max 4.1V).

Happy to continue the discussion if there is more to be shared.
 
Part of the picture, I think, is "when is balancing most effective?"
Differences between cells are much more pronounced at both the top and bottom ends of SOC. Thus, top-balancing would be more effective, even if some form of balancing is done at lower SOCs. Bottom-balancing, I think, is a different procedure. I used this method on my electric-mower project and it involved draining cells down near to minimum voltage and then re-iteratively draining small amounts of power until a stable voltage settled right at that bottom mark, separately for every cell. This worked quite well for keeping a LiFePo pack well-balanced for several years without further adjustments or BMS. Part of this strategy also involved somewhat under-charging the pack. I have read that bottom-balancing is more difficult with other chemistries so I'd be surprised if bottom-balancing is employed for LEAF.
I learned with my 2012 that the occasional 100% charge (plus balancing time) was definitely important to keeping the battery-control module calibrated; otherwise SOC indication could drop like a rock when driving down to low battery ranges. When I got the 2015, the 80% charge setting was no longer a thing and I just settled on charging to 100% all the time. Yes, even letting it sit for days at a time. The pack is much more robust than the 2012 and life is too short to play nursemaid to a battery.
 
Nubo said:
Differences between cells are much more pronounced at both the top and bottom ends of SOC.
That's certainly true for LFP batteries. But does it make much difference for NMC and derivative chemistries, other than at the very bottom where you don't want to be? At least not regularly for balancing.

Charge-and-discharge-voltages-at-025-C-in-dependence-of-SOC-for-the-NMC1-battery.png
 
I do a "long balance" on a salvage battery pack before I put it into a vehicle. I have a cable that plugs into the multi-pin connector on the battery. All that is needed is 12V +ve into ign and stay-alive, and earth into about 4 wires. My cable also has OBD plug, so I can monitor with leafspy

The definitive answer to OP's question, is "whenever the leaf is powered up (which includes while being charged) the cells are balancing"
 
xa, what a great idea. I am finally replacing some modules in a 2013 pack, and have been pondering how to get all the old cells as close as possible without taking them all up to 4.10 volts. Your technique is perfect--I will take all the cells down to 3.70, then let it balance for a while. The new modules will have greater capacity, so I will need to raise the voltage on those after using your technique.

Also, completely agree, as long as the BMS is being powered by 12 volt, then the shunt resistors are running. I have the LS adapter cable as well for connecting to the battery (Thanks Dala!)

Edit 3/28/23: This detailed dissection of the BMS concludes that: "The most interesting thing is that the LEAF BMS balances all the time.
Once cells are out of range the balance shunts start tuning on, will probably post a video tonight." https://www.diyelectriccar.com/thre...g-the-nissan-leaf-bms-serial-protocol.169225/

Further edit 3/28/23: This reference indicates that the each 430 ohm resistor (one for each cell) are activated by dedicated mosfets which in turn are controlled by the single controller chip. The controller chip is powered from by 12 volt. https://www.electricvehiclewiki.com/BMS/#Contactor_module
 
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