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Discussion Starter #1
Has anyone found documentation on Bolt battery cell (Re)balancing?
How does it work?
What are the benefits? increased capacity? less degradation?
What level should your Bolt be charged to for it to start? 85%? 90%? 100%?
How long should it be left at that charge level for cell balancing to complete?
How often should it be done?
Thanks!
 

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Cell rebalancing is a subject that has perplexed me since I took possession of my Bolt in May of 2017.
In a post, someone once opined that rebalancing occurs at 100%. Therefore, it is a good idea to charge/rebalance to 100% monthly.
It seems to make sense, so, I do.
 

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According to this article cell balancing is the process of equalizing the voltages and state of charge among the cells when they are at a full charge.
 

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IIRC, EVs from Hyundai / Kia have their manuals mention that cell management happens after a full charge and thus recommends topping off every month or so. I think it's a reasonable strategy as the BMS can get a good picture of the cell's voltage characteristics when they are all equally fully charged.
 

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This subject is still a mystery to me as well. Balancing is usually either done at bottom (discharging to equal low end voltages), or top balanced (charging to equal fully charged voltages). The Bolt does top balancing, but others have pointed out a charging taper that occurs on hilltop reserve. It has been speculated that the taper is due to cell balancing. Apparently there is no taper at that SoC when doing a full charge.

If I owned a Bolt, I'd probably use Hilltop Reserve most of the time and not worry about balancing. Maybe I'd end up doing a full charge once a month due to wanting the extra range, and it would for sure get balanced then.
 

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This subject is still a mystery to me as well. Balancing is usually either done at bottom (discharging to equal low end voltages), or top balanced (charging to equal fully charged voltages). The Bolt does top balancing, but others have pointed out a charging taper that occurs on hilltop reserve. It has been speculated that the taper is due to cell balancing. Apparently there is no taper at that SoC when doing a full charge.

If I owned a Bolt, I'd probably use Hilltop Reserve most of the time and not worry about balancing. Maybe I'd end up doing a full charge once a month due to wanting the extra range, and it would for sure get balanced then.
All lithium ion chargers that I have ever seen/used do constant current/constant voltage charging. This means the charger is programmed to maintain constant current for X amount of time, then switching to constant voltage. This automatically leads to taper at the end of the charge...regardless of what voltage/percent charge you are charging to. Car manufacturers sometimes modify this procedure when charging at 1C rates or higher, adding additional constant current steps, to prolong battery life.

We own a 2017, so can only see Torque Pro voltage behavior at 100%, and "hilltop." Hopefully someone will verify my assumption that the taper occurs at any SOC setting on newer Bolts.

Balancing is most effective when the cells are at top or bottom of charge because, for lithium ion cells, this is where you see large changes in voltage with small changes in charge state. While having a very flat voltage profile at most states of charge is an advantage for users of lithium ion batteries, it means balancing at anywhere other than the ends ls not as precise.

You understand, correctly, that balancing does not effect the health of cells, only your usable range.
 

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All lithium ion chargers that I have ever seen/used do constant current/constant voltage charging. This means the charger is programmed to maintain constant current for X amount of time, then switching to constant voltage. This automatically leads to taper at the end of the charge...regardless of what voltage/percent charge you are charging to. Car manufacturers sometimes modify this procedure when charging at 1C rates or higher, adding additional constant current steps, to prolong battery life.

We own a 2017, so can only see Torque Pro voltage behavior at 100%, and "hilltop." Hopefully someone will verify my assumption that the taper occurs at any SOC setting on newer Bolts.

Balancing is most effective when the cells are at top or bottom of charge because, for lithium ion cells, this is where you see large changes in voltage with small changes in charge state. While having a very flat voltage profile at most states of charge is an advantage for users of lithium ion batteries, it means balancing at anywhere other than the ends ls not as precise.

You understand, correctly, that balancing does not effect the health of cells, only your usable range.
I drive a 2019... I do not have Torque Pro, but when I get close to the target charge, I did notice it slows down. Basing my charge complete time on L2 6.6KW rate, it would notice it seem to take about 30 min longer to complete charging... so it seems that the car tapers from 6.6KW to 0KW during the final hour.

Just now, I set my charger to top up from 80% to 82% tonight, 2% would be about 1.2KWh of energy. I had the car set on Departure Charge and the charge rate for 120V 8A (so about 1KW). The car is set to begin charging 1 hour 45 min before departure time. If I take 120v X 8A X 85% efficiency, it should only take 1hr 15 min. So, another extra 30 min. Again, leads me to believe that the car tapers during the final hour of planned charging.
 
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Cell Balancing is the act of making all cell voltages equal.

It can happen at any voltage, but usually it’s done at top of charge. This puts the maximum amount of energy inside every cell group and will maximize the capacity of the battery.

I’m certain the Bolt balances in Hill Top Mode. I’ve seen the current taper well below 2kw.

Any time your car can sit plugged in for an extra hour after reaching its charge point, it probably balances.

To balance, the pack has to literally waste the energy for the cell group that has reached the target voltage. That’s because all the cell groups are in series and there’s no way to “turn off” any particular cell group.

Each cell group has a resistor in parallel that is turned on to stop charging for that cell group. The amount of energy going into the cells is simultaneously burned off to prevent overcharging.

Once all cells groups reach target voltage, balancing is complete and all cell groups are “full”.

Balancing is required because each cell group has a small variation compared to the next. They are not the same capacity because manufacturing tolerances can’t make them the same.

They might vary by up to 2-5% (just a guess).

I have yet to see an article or YouTube video that properly explains it and why it is needed.

I have yet to find someone else who truly understands it beyond buzzwords.
 

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I’m certain the Bolt balances in Hill Top Mode. I’ve seen the current taper well below 2kw.

Balancing is required because each cell group has a small variation compared to the next.

They might vary by up to 2-5% (just a guess).
If your Bolt was charging at a constant voltage at the end of charge, like every other lithium ion device, the current will taper to zero without any resistors being switched on.

No it is not required. There are hundreds of DIY vehicles running around with no BMS whatsoever. Many have gone many thousands of miles without problem.

Here is our Bolt at 100.0% SOC, 50.2% SOC, and 1.6% SOC. The voltage spread between our strongest and weakest cells, after balancing at 100% SOC is 0.6%, at 50.2% SOC it is 0.3%, and at 1.6% SOC it is 3.4%. If you never charged above 18 bars...~90%."hilltop" charge, and never went below one bar...~5% charge, your Bolt would never balance, and never need to.



12-29-18-1.jpg 12-29-18-2.jpg 12-29-18-3.jpg
 

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I doubt balancing is done for the fun of it. It’s a lot of complexity.

Constant voltage will over charge cells when 96 of them are in series. The current going through one cell goes through all cells. The lowest capacity cells will overcharge.
 

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I doubt balancing is done for the fun of it. It’s a lot of complexity.

Constant voltage will over charge cells when 96 of them are in series. The current going through one cell goes through all cells. The lowest capacity cells will overcharge.
Balancing is not done for the fun of it. Over many cycles the small difference in impedance between paralleled groups will gradually cause them to drift apart. When charging to 100%, during the fixed voltage portion of the charge, the BMS will be asking the charger for the 100% pack voltage...say 96 x 4.180 v = 401.28 v. This fixed voltage will automatically, with no interference required, cause the current to taper as the pack voltage approaches the charger voltage. The usual analogy is water flowing through a tube from one tank with a higher level to a tank with a lower level. The current flow slows automatically, as they approach the same level.

As the highest parallel cell group reaches 4.180 v, its bleeder resistor circuit turns on. It doesn't require a huge resistor, because, at this point the current level is already very low. The BMS has a maximum cutoff voltage for each cell group too...say 4.181 v. If a bleed resistor circuit failed for some reason, the BMS would signal the charger to shut off to prevent overcharging that cell group, and presumably throw a fault code. There is a minimum pack voltage cutoff, and an individual cell group minimum cutoff as well.

So, if you never charged to 100%, the parallel cell groups would slowly drift apart. And if you then drove your car down to empty, the lowest cell group would cause the car to shut down as always. But since the cells have now drifted further apart, you will have had access to less of the pack's potential capacity than it would if all the cells were perfectly top balanced.

If you only ever charged your car to 50%, during the constant voltage portion of the charge the BMS will be asking the charger for...say 351.0 volts. Again, as the pack approaches this fixed voltage, the current will automatically, with no interference required, taper until it reaches zero. The cells will not continue to charge. At some point, perhaps weeks later, the current drawn from the pack by the accessory battery charger, and the computers, and battery temperature conditioning in very cold or hot weather will cause the voltage to drop low enough for the BMS to call for the charger to be turned on....rise and repeat. This will never call for balancing, and never overcharge your cells, as they will all be well blow 4.180 volts.
 

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TI has a good article about passive and active cell balancing here: https://e2e.ti.com/blogs_/archives/...5/how-active-and-passive-cell-balancing-works
And a couple from Analog Devices:

The Bolt likely uses passive balancing with bleed resistors in parallel with each group of parallel cells, as many have already mentioned. This is less efficient, less complicated, and less expensive than active balancing. It is usually only done above about 90% charge because that's when the charge current tapers off to lower values and the power dissipation in the bleed resistors is lower. Passive balancing can theoretically be done at any time during the charge cycle, but that would be very inefficient for a large EV battery.

Note that the charge current doesn't taper to zero. If a cell is continuously charged with a large constant current the cell voltage will keep rising and damage the cell. This is why chargers use a high current until the cell reaches a safe peak voltage, and the charger keeps decreasing the charge current to prevent the cell voltage from rising. The charge cycle ends when the current drops to the termination current (usually specified by the cell manufacturer). I have no idea what the termination current is for the Bolt's battery, but for typical lower current 18650 cells it is usually in the 100-500mA range. The Bolt's cells have much higher current capability and there are three in parallel, so the overall battery's termination current is probably in the 1-5A range. That would put the power in the 0.5-2kW range when the charge ends.
 

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If the Bolt only balances when fully charged, then if the Bolt isn't occasionally fully charged will the capacity decrease over time as the cells become more imbalanced? If so I assume to an occasional full charge would restore capacity?
 

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As the cells become more imbalanced the usable capacity will decrease when charging without rebalancing because the pack is only as good as the weakest cell. The pack's physical capacity will not decrease, and a full charge with cell balancing should allow the usable capacity to equal the physical capacity of the battery.

It's possible that the Bolt engineers also programmed in time limits that reduce the state of charge at which a partial rebalancing is performed to keep things from getting way out of whack if someone rarely fully charges (>90% or so) the battery.
 

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Note that the charge current doesn't taper to zero. If a cell is continuously charged with a large constant current the cell voltage will keep rising and damage the cell. This is why chargers use a high current until the cell reaches a safe peak voltage, and the charger keeps decreasing the charge current to prevent the cell voltage from rising. The charge cycle ends when the current drops to the termination current (usually specified by the cell manufacturer). I have no idea what the termination current is for the Bolt's battery, but for typical lower current 18650 cells it is usually in the 100-500mA range. The Bolt's cells have much higher current capability and there are three in parallel, so the overall battery's termination current is probably in the 1-5A range. That would put the power in the 0.5-2kW range when the charge ends.
This paragraph is just wrong. You do not understand what you are saying. For a cell, or pack, to be charged at a constant current requires circuitry to measure the current, and keep increasing the charge voltage to stay above the cell, or pack, voltage to maintain that current. It does not happen spontaneously. During the constant voltage portion of the charge they simply set the output voltage of the charger to a set value, and the current drops spontaneously, as the cell, or pack, voltage rises to meet the charge voltage. When both voltages are the same, the current flow is zero. Balancing is only necessary if you are talking about a battery with more than one cell in series. In a perfect world, all the cells in the pack would be identical, and no balancing would be required.

Let's assume we have a pack with two cells in series. The designers have decided they will charge these cells to 4.150 volts each. They set the constant charging voltage to 8.300 volts. If both cells were identical they would both reach 4.150 volts at the same time, and now balancing would occur. In the real world, one gets there first, and because current is still flowing through the circuit, the high cell would go to 4.151, etc, except for the bleed off resistor, which keeps draining the excess until the second cell reaches 4.150 volts too.


If you have ever used a quality lithium ion battery charger you understand that the charger measures the pack voltage before starting to charge, and it then delivers voltage slightly higher than that. The current will overshoot the desired value slightly, and the circuit will adjust the voltage to hit the desired current, continuing to actively adjust the voltage to maintain that current..
 

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If the Bolt only balances when fully charged, then if the Bolt isn't occasionally fully charged will the capacity decrease over time as the cells become more imbalanced? If so I assume to an occasional full charge would restore capacity?
Lithium ion cells, particularly those made to high precision standards, are extremely stable. It takes a long time for them to become appreciably unbalanced. They do not suffer from the kinds of problems we all associate with lead acid cells.
 

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This paragraph is just wrong. You do not understand what you are saying. For a cell, or pack, to be charged at a constant current requires circuitry to measure the current, and keep increasing the charge voltage to stay above the cell, or pack, voltage to maintain that current. It does not happen spontaneously. During the constant voltage portion of the charge they simply set the output voltage of the charger to a set value, and the current drops spontaneously, as the cell, or pack, voltage rises to meet the charge voltage. When both voltages are the same, the current flow is zero.
So what was the rule to follow? Make sure the target rate is set where the charge time would be at least 2 hours and the last half hour gets stretched to 1 hour where the balancing naturally occurs, right? (so 2.5 hr total charge time)
 

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So what was the rule to follow? Make sure the target rate is set where the charge time would be at least 2 hours and the last half hour gets stretched to 1 hour where the balancing naturally occurs, right? (so 2.5 hr total charge time)
Sorry. You lost me. What, or who's, rule are we talking about? If you are asking how the engineers decide how long to make the constant current portion, before switching to constant voltage...I have no idea. That is way beyond my limited knowledge. but I would love to hear an answer from a charger/BMS designer.
 

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Sorry. You lost me. What, or who's, rule are we talking about?
To cause 2 things to happen. Regular charging and constant voltage charging.
 

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To cause 2 things to happen. Regular charging and constant voltage charging.
I wouldn't call constant current charging "regular" charging. It is just the first part of a two part process. In my experience, if the pack is already very close to the SOC you are asking for, the charger will start out constant current charging, but may almost immediately switch to constant voltage charging. I have no idea how they set the programming.
 
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