For those who charge their Bolt EV to 70-80% but never 100%, give it a try. It works. Yes, the video is for Tesla but it applies to the Bolt EV too.

I have been monitoring our Bolt's cells for 6 years. The same two cell groups were highest, and lowest for all that time. I have seen zero evidence of top balancing. Here are two shots from the same charge. It was more out of balance at 100%, than at 50%.

Here is our Bolt at 100% DIC SoC. 0.025 volts delta

Here is our Bot at 50.2% DIC SoC. 0.012 volts delta

 

I have been monitoring our Bolt's cells for 6 years. The same two cell groups were highest, and lowest for all that time. I have seen zero evidence of top balancing. Here are two shots from the same charge. It was more out of balance at 100%, than at 50%.

Here is our Bolt at 100% DIC SoC. 0.025 volts delta


Here is our Bot at 50.2% DIC SoC. 0.012 volts delta

I'm not an expert, but I would think that's normal though. If one cell is out of balance by, say, 0.5%, that fraction would be very little change in v along the big flat section of the voltage curve, and much more apparent heading up the peak approaching 100%

 

So what you say it’s that your car at 100% SOC didn’t do anything (charging wise) even if you let it sit for three hours @ 100%, right ? I saw on mine, but it’s true, I charged my Bolt EV on a 20A circuit (16A).

 

I never let a battery sit at 100%, or low, for that long. Up until the third BMS software update was done, the car pulled no current past 100%, unlike Tesla. With that new update, the car pulls current at any percent SoC I stop the charge, for four hours. This is the accessory battery charger keeping up with the cell voltage/temperature monitoring drain on the 12 volt battery.

When I do my one year capacity test drive, I will document the cell voltage delta at 100%, and let it sit on the charger to see if any balancing takes place.

 

Here is a video that explains a little what balancing cells means and how to do it.


For those who charge their Bolt EV to 70-80% but never 100%, give it a try. It works. Yes, the video is for Tesla but it applies to the Bolt EV too.

Oh, that's good to know, thanks for that! Have you tried this yourself?

 

Am I the only one in this thread who notices that you have to watch an 8:51 minute video of a nauseating Tesla screen to learn this "balancing" trick?!?

Step 1: Remove any limits on your default charging setting and charge to 100%

Step 2: Keep charging at 100% until your Bolt stops drawing power.

Step 3: Go for a drive to push your battery's capacity below your normal default charge setting.

Step 4: Reset your default charge setting, such as 90%, 80% etc. and then charge again.

 

@LarryO thanks for dummying this down and saving time for all!

 

This idea only works for rather small packs. In the case of the Bolt and Tesla, the BMS can only create very small adjustments. In the case of the Bolt, this amounts to about 0.4 Watts maximum load imbalance. This amounts to an adjustment rate of about .05% per hour, maximum.

For this reason, any balancing must occur over a rather long period of time. Any cell voltage adjustments are very small and almost impossible to see. The adjustment action works to help keep the cells in a range and to help keep them from drifting out of balance. It cannot really correct much of an imbalance that is already present.

The BMS can easily measure imbalance at any charge level. It really does not need to be done at a high level. The main criteria is that the battery is in a fairly static state.

 

I charge my car to "full" (with Hill Top Reserve) about every 1000 miles specifically so the top end cell balancing can take place. FWIW, I've never noticed a significant amount of cell inbalance before I did so.

 

Might mention what software is on these. It may be part of difference??

 

In the owner's manual of my Kia Soul EV, they specifically say to let the traction battery go below 20% and charge without stopping to 100%, once a month at a temp above 50 degrees F. This improves the GOM accuracy they say.

I believe that advice carries through to more recent Kia/Hyundai EV's. I believe charging to 100% once in a while is beneficial, as long as it doesn't stay there for long.

 

i have a 2023 Niro EV that also states i should charge the battery to 100% AT LEAST once a month to "keep the battery in optimal condition". same for the EV6 model.

 

The battery cell will discharge through a resistor as soon as its voltage goes above a certain voltage value ( Li-ion battery cell is above 4.18V, LiFePo4 battery cell is above 3.16V ). It will charge and discharge at the same time while the other cells which are lower than that voltage value charge only. This way, after the charging process, all cells of the battery pack get charged up to 4.2V and we got a balanced pack.

So, if the above is correct, only if the battery is charged to 100% and kept there for a while, the balancing of the pack happens. Right ?

 

Pretty much, yeah. There are two basic types of balancing - top and bottom. The type used by basically everyone is top balancing where you wait until the cells get close to the top of their voltage limit and then turn on a small load on any cells that are above the rest to try to bleed them down closer to the rest. Over long periods of time this helps to balance them at the top so that at 100% SOC they're all at the same voltage. They will pull apart in voltage a little bit throughout the cycle from 100% SOC to 0% SOC but they shouldn't a whole lot. You can't balance at 65% SOC because the voltage of the cells at that point is pretty much meaningless. You don't know if a cell at 3.6v is at 40% or 50% (all totally made up numbers. Don't quote me on what the voltage is expected to be at 40% SOC). So, people only balance at the extremes where the voltage is a much better indicator of the state of charge.

Bottom balancing is interesting. It's used by hobbyists. You balance all your cells at 0% SOC then let them be whatever the rest of the time. You stop charging when the first cell reaches the upper limit. Now you know that you can drive around safely because no cells are over the limit and when you fully discharge it they're all at the same low voltage so you don't damage them by accidentally draining one cell too much. This allows one to balance the cells hardly ever and still be reasonably safe. To my knowledge such a "hillbilly" style technique has never been used by an actual manufacturer.

 

I run a battery reconditioning and repacking business of my own and most if not all BMS I find in tools packs or ebikes or whatever, will do top balance. The thing is they do it by bleeding voltage through resistors and it is so low amperage (often around 50mA), it takes for ever. You can't really rely on balancing at that point. If you build the pack with almost identical cells specs (Capacity not more then a few mAh difference, overtime voltage drain) then rarely it needs to balance. When I build packs I will fast discharge and then fast charge and as soon as its done charging (before balancing kicks in) I check if each cells are at the same voltage, 4 digits precision, if so then I know the pack will be solid. For the Bolt I guess it is the same resistor method but would have hope that instead of passive balancing it would be active balancing (taking voltage from the highest and distribute to the lowest) more efficient and less heat loss. I also hope that the balance Amperage is way higher to be meaningful and not take ages.

 

would have hope that instead of passive balancing it would be active balancing (taking voltage from the highest and distribute to the lowest) more efficient and less heat loss. I also hope that the balance Amperage is way higher to be meaningful and not take ages.

Nope, and nope. There are no OEMs using active balancing and, at least in the Bolt, the resistors are supposed to be small. The BMS is the size of a PC keyboard, and sits on top of the battery pack, inside the battery enclosure, far from any coolant channels. You can see it below the rear guy's left arm.

I often notice, after charging, that battery section 5, the one stacked up under the driver side rear seat, is warmest, and battery module #6 is often the warmest. I assume this is do to the added heat from the BMS sitting on it.

Gad! Getting this post right took forever. I need more coffee.

 

By using these the energy is transferred and equalized from cell to cell via capacitors. Minimal waisted energy and minimal heat. Maybe future aftermarket EV packs will have these or an option to add these.

One thing up my head right now is that the down side of having this type of balancing is when a group of cell is defective, that would normally be weaker and lower V then others. it would be harder to detect because of the continuous active equalization through out the entire cycle. Maybe engineers could adapt the balancing to be activated only at the end of a charge cycle and catch weaker cell just before the balance step.

 

One thing up my head right now is that the down side of having this type of balancing is when a group of cell is defective, that would normally be weaker and lower V then others. it would be harder to detect.

Yes. A discussion I listened to between battery designers made just that point. A BMS that works well will hide a defective cell. This may be why the voltage distribution on our Bolt looks like this.

When the original pack finally threw a code, after three weeks of falling, and died in our garage, it looked like this.

 

OK, I've been reading this thread and I think I finally get it...
...
It's a good idea to balance your battery...
Although you don't need to do it...
However you should do it from time to time...
But it doesn't really make a significant difference...

Got it... ;-)

 

Short answer...

Batteries have, do, and always will suck.

 

GM acquires software startup to help maintain EV battery health

General Motors (GM) has acquired “substantially all the assets” of ALGOLiON Ltd, an Israel-based battery software startup, the company announced...

electrek.co

 

Video unavailable

 

Video unavailable

 

there is almost always a humming noise under the hood, and the coolant circulating (can see small ripples); seems like the BMS is doing something, balancing perhaps ? (active balancing via charge shuffling ?)

 

The humming is probably the electronics coolant loop pump. Put your hand on the reservoir. It is vibrating. The pump is about a foot below the reservoir. The electronics coolant pump is running because the accessory battery is being charged. That is happening because the computer and sensors are monitoring the battery for possible signs of a fire. This goes on for up to four hours every time you charge. This was part of the first fire fix software patch.

The BMS is silent, and under the rear passenger seat.

 

"Video unavailable"
"This video is private"

 

That's too bad, but instead of investing 9 minutes watching it, I summarized it earlier in this thread for the rest of you:

Here.

 

Silly question.... Wouldn't drawing heavily on the battery during highway driving for an hour or two balance the cells?

 

No. No two cells are identical. Any use, even just sitting, causes different amounts of discharge between cells. Image twins run a mile. They will not finish at the same time, and if they do, one will absolutely have used more energy.