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Cell Balancing- What's the benefit? Best Way Without Battery Stress?
- Thread starter dafish
- Start date
I guess you mix the terminology?
-calibrating the BMS with balancing the cells?
I think that post from wk057 pretty much tells it all.
First of all, balaning is purely an act of equaling the voltage between cells (pack of cells in parallel) by either charging low cells or, as Tesla do, burnibg of the high cells.
As different cells have slightly different capacity, a perfect balanced pack at 50% SOC will not be in perfect balance at low and high SOC. If having perfect balance at 50%, the smallest capacity cell will be at a lower voltage at low SOC than the cell with larger capacity, which increases the imbalance number.
It will also gain voltage faster at high SOC and will tend to reach full voltage before the larger cells do, increasing the imbalance by being high.
Normally, a small imbalance at 50-70% do not matter. If the imbalance is high at full SOC, the battery can not be charged to the real 100% capacity as it need to stop the charge when the highest voltage cell is full at 4.200V.
Top balancing is done to allow all cells to reach 4.200V, or at least close to this, giving us maximum capacity.
A top balanced pack will have more imbalance at 50%, than if it was balanced at 50%.
For me using 55% as the normal charge level but charging to 100% or close at least ince a month for driving to work or my mother in law, I have had 4mV at these periods up to one month at 55%.
The imbalance do not increase during this month. Its more or less always 4mV if being around 30-55%.
One of the most common comments of the low SOC strategy is “your pack will get unbalanced”, but that is not true and also there probably is the classic misunderstanding/confusion to mix balancing the battery with “calibrating the BMS”.
We can ”calibrate the BMS” (help the BMS to estimate the battery capacity by showing the capacity to it)
We do not ”balance the BMS”
Neither do we ”calibrate the battery.”
So no need to think about the balancing, just as wk057 says.
ATPMSD
Active Member
I agree with @dafish, this is really not needed. If you want something easy to do, charge the car to 100% every now and then, then drive it soon thereafter. For example, change the car at night then drive it to work the next morning. At a minimum I do this quarterly. I would avoid letting the car’s SOC get near 0%. The car does have a safety buffer but if you drive it until it stops you are asking for trouble. People have gotten away with this many times, but this is what will kill a Lithium battery.
Thank you Aakee, for taking the time to help us all figure these things out. A few follow-ups if you don't mind:
1) You mention: "....It will also gain voltage faster at high SOC and will tend to reach full voltage before the larger cells do, increasing the imbalance by being high."
Does this mean LION cells show voltage increase non-linearly to MAH capacity? EG: at mid-levels of SoC, while charging, the voltage change is relatively minor, while at upper SoC, and again while charging, voltage change is more rapid? (not all that important, just trying to understand how LIONS work.)
Next, I ABC charge to 53%, have never charged to 100%.
2) Am I to understand that should I want to ensure cells are balanced for max range I should charge to 100%?
3) Would you care to make a SWAG at what the range I might be trading off if I don't cell balance at 100%? (I'm not gonna care about 1% or 2%, but I don't 5% or more might start to make a difference on a long winter trip.
Thanks!
-d
1) You mention: "....It will also gain voltage faster at high SOC and will tend to reach full voltage before the larger cells do, increasing the imbalance by being high."
Does this mean LION cells show voltage increase non-linearly to MAH capacity? EG: at mid-levels of SoC, while charging, the voltage change is relatively minor, while at upper SoC, and again while charging, voltage change is more rapid? (not all that important, just trying to understand how LIONS work.)
Next, I ABC charge to 53%, have never charged to 100%.
2) Am I to understand that should I want to ensure cells are balanced for max range I should charge to 100%?
3) Would you care to make a SWAG at what the range I might be trading off if I don't cell balance at 100%? (I'm not gonna care about 1% or 2%, but I don't 5% or more might start to make a difference on a long winter trip.
Thanks!
-d
0% will not kill the lithium battery.
Lithium batteries are happy at low SOC.
Our cars have a 4.5% buffer below 0% displayed. Even the true 0% at the end of the buffer is safe for the lithium battery.
The BMS has a safety function that disconnects the HV-battery when it reaches 0% true, or maybe slightly above.
This let the lithium battery to stop discharging at a safe level. When this happens, the low voltage battery stops being charged. Some (older Teslas) have lead acid LV-batteries.
As lead acid batteries do not like being discharged they get damaged if they discharge to much.
But the HV-batt is sleeping sweet at safe levels when this happens.
So 0% displayed is not dangerous for the lithium battery.
The low SOC limit when driving/traveling can safely be based solely on range anxiety.
This will keep even the lead acid battery safe.
I had my M3P below 10% at least hundred times. I was down to 0-1% several times and also to -2% and -1.8% twice.
Apparently it was not that bad for my battery.
As for the science and research, the SOC does this to calendar aging:
This is panasonic NCA cycled down to true 0%, from 100%, 90% and 80%
We can see that cycled like this a tesla LR that drives 400 km from 100 until it stops can do
250.000 km from 100%
320.000 km from 90%
400.000 km from 80%
Each time driven way below 0% displayed until it stops.
Conclusion, 0% does not kill the battery.
BTW: 0% is not completely empty but the safe discharge level set by the battery manufacturer. In Panasonic NCA, this is 2.5V/cell.
Thank you. Question:
My "Tessie" app reports I have a 78.7 KW usable capacity. My analysis suggests that either my BMS calibration is off, or my BMS is basing it's estimates off of more like 74%. The latter is coincidentally pretty consistent with the buffer you're mentioning. It seems like the Tessie app is showing total usable capacity while the BMS is reporting percentages with this 4.5% buffer hidden. Does this a reasonable interpretation to you?
Thanks!
ATPMSD
Active Member
0% SOC will not kill the battery due to the built in safety margin. If you run it down until it has zero power it is very difficult to get them to charge again, if it the will charge at all.
The imbalance is mainly caused by a difference in capacity between two cells
If you charge two cells, one small and one large to the exact same voltage then they are balanced at that voltage.
If you after this discharges the cells together(in series) the smaller one will drop in voltage faster and also get empty faster.
Then they are not in balance any more, as the larger cell has a higher charge left it also has a higher voltage.
As the same current flow through two cells in series, a cell with less capacity will change the voltage at a higher rate per energy passing through than a larger cell.
Balancing is solely adjusting the charging level to the same voltage (SOC).
Upper part of pic: shows the difference in capacity. This is how it will look if alleas charged from 0% togheter. The first one reaching 100% set the stop.
Lower part of picture, top balanced, charged to 100% and balanced so all is full.
Still some has lower capacity but this will be hidden in the buffer below 0%.
If we balance the battery in the middle, we will see the lowest imbalance at that SOC and increasing at low SOC and high SOC.
This is the normal thing for most Tesla users as we most often use the car in the middle SOC (doesnt matter if we charge to 50% or 80%, still higher imbalance at lower and higher SOC).
They have in general flatter voltage curve but this is not related to balance/imbalance.
When you charge to 100% the car will take some descent time after reaching 100% to (top) balance by burning of the high cells and adding more energy to the battery.
You do not need to do this in advance, as it will be done automatically when charging to 100%.
Doing one charge complete to 100% probably keep the cells balanced about at 100% so the next full charge (if close in time) need less balancing, I assume.
When I had charged 55% for one month, abd had 4mV as always, charging to 100% set the initial imbalance to 10-12mV.
Letting it balance reduced this but if driving asap, 10-12mV might be 5-6mV in average
5 mV is about 0.5% capacity.
And we wont ever see 0mV as the imbalance so the drop in range is neglible.
Just an FYI: The Tessie developer chimed in on this is his thread. My above hypothesis is correct vis a vis what "Tessie" reports. Ergo we "have" 78.7KW, but we're seeing much more like 74KW via range reporting.
’23 MYLR….in USA? Or Europe? Or?
( different chemistry and different capacity).
78.7kWh ”usable capacity” actually is total capacity as Tessie calculates total capacity and then call it usable capacity.
You should use the energy graph to make sure for you self what capacity the BMS edtimates.
”Average” x ”estimated range” x 100 / SOC = Wh. This is the BMS estimated battery capacity, total including the buffer.
It should be close to the Tesdie ”usable” if Tessie is not off.
We know that the number Tessie presents is the total capacity. Tessies vendor has described using the (from the car) reported added energy to calculate the capacity.
The thing is, the car overreports the added energy with the buffer size, so the real added energy is 0.955 x the reported.
This is True for my model S as well.
Your finding is correct.
To try to reduce the tremendeous amount of misunderstandings we should use the total battery capacity as the number we use. Can not be missunderstood.
For calculations, just use total capacity x 0.955.
Hi AAKEE... I was rereading your posts (I do that a lot
)....this graphic interest me...if you charge to 100% to balance the pack ...does that mean you will lose your buffer and will risk the car stopping when you reach zero ?...or will the car rebalance while driving and the push the difference in capacity back to the top to save the buffer ?But, either way you are only hiding the imbalance (taking it from the top and putting it in the bottom buffer) so the car might show an increased range...but the actual range (100% till the car stops) has not changed at all. Am I correct ?
Balancing means that cells or modules become equally charged, i.e. to the same cell voltage at 100% State of Charge. In effect it increases the total capacity.
Whether the Battery Management System stuffs the gained capacity into the buffer or actually displays and uses it is a different question.
Whether the Battery Management System stuffs the gained capacity into the buffer or actually displays and uses it is a different question.
No, a top balanced pack at 100% ensures all cells are fully charged.Hi AAKEE... I was rereading your posts (I do that a lot
This maximize the capacity between 100% and 0% displayed.
The cells with less capacity still will set the lower limit where car shuts down at minimum voltage.
A very well matched pack, like my okd M3P with the same CAC on all cells (CAC max/min/average was the same) would probably be able to deliver all 4.5% buffer, a pack with differences in CAC might only deliver 3.5% buffer before the lowest cells reach the minimum voltage and the car stops.
Nope, never.
One of the reasons to have a lower buffer is to allow for imbalance and differences in capacity between cell groups.
Rebalancing does not happen during a drive but if it did it would be by burning up energy to no use instead of using it to move the car. Would be a bad idea.
No. Not correct.
As I wrote first of all, top balancing ensures all cells are fully charged, making them deliver most possible energy.
If you did not balance at top, when the first cell reach 100% the lowest might be at 98% and the average might be 99% capacity.
If you would charge each cell separately to 100% then the energy is maximized (top balancing does just that, if ypu charge at the sane time so the end result is 100%)
Just a small additional piece of information: The average has no meaning here.
If one part of the battery goes to 98% during charging, while another part goes up to 100%, then, if you discharge the battery and the weaker part reaches 0%, then the stronger part would stand at 2%. At this point the Battery Management System has to stop discharging, because otherwise the weaker part would go below 0%, which would damage these cells. So 2% remain unusable, and this is exactly the motivation for rebalancing.
During the rebalancing the weaker cells are also charged to 100% without overcharging the better ones, so they all end up at 100%, and all of this capacity becomes usable.
Yes, it has. What I mean is that we do not have full energy when we have imbalance at 100% charge.
Not really.
Most probably the smaller cells reach 4.20V /100% first.
The smaller cells will lose voltage faster.
If we for example start with a pack in perfect balance at 50%, the cells with less capacity will reach 100% first, and if no balancing was done, at discharge they will also loose voltage faster and have about the same voltage/SOC again at 50% and below 50% they will have less SOC-less voltage than the larger cells.
Balancing to increase the cells with lower voltage at 100% (probably the larger cells) will increase the pack energy and these cells will deliver more energy due to the higher voltage in these cells until the smaller cells sets the stopfrom the lower limit.
Tesla do balance by burning of energy and this is a slow process so it will not be done when the car says ”charge finished”.
When the car sleeps it might balance the cells, and in this case it drains the battery slightly.
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