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I’m certain the Bolt balances in Hill Top Mode. I’ve seen the current taper well below 2kw.
Yup. it looks like does, at least on a 2017 LT. Below is the power taper from a ChargePoint L2, which I assume is not driven by the same issues as a DCFC taper at high SOC. It spends more than 1 hour at under 2 kW. When it was all done [0 kW], the % user SOC was 88% instead of what I usually see for hilltop[87%]. OnStar pushes the 'charging complete" before it goes to 0 -- in this case the complete message was sent at 13:48, an hour before it was complete.
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The Bolt may well balance at levels other than 100%. This feature is now available on electric assist bicycles, so it wouldn't be surprising on a car. Balancing voltage anywhere outside the knee of the charge curve is less precise, but GM doesn't appear too concerned about keeping close balance even at 100% SoC.

That said, if you look at Torque Pro, at the end of a charge cycle, depending on temperature, the battery heater will come on. The battery heater pulls 2+ kW and then tapers.

Check out Mike's observations on battery conditioning from several years ago.

 

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Yup. it looks like does, at least on a 2017 LT. Below is the power taper from a ChargePoint L2, which I assume is not driven by the same issues as a DCFC taper at high SOC. It spends more than 1 hour at under 2 kW. When it was all done [0 kW], the % user SOC was 88% instead of what I usually see for hilltop[87%]. OnStar pushes the 'charging complete" before it goes to 0 -- in this case the complete message was sent at 13:48, an hour before it was complete.
This is the opposite of cell balancing. If what we believe is true, passive cell balancing just bleeds off excess power from the cells. So dropping down to under 2kW is not cell balancing, it's probably just running the battery heater.
 

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I get why they want the battery heater, and maybe the AC, to be able to run off the onboard charger only, in the cases you mentioned where the battery is outside specs. But in some situations, like preconditioning the cabin while the car is plugged into 240 volts in my garage, I have seen the onboard charger pulling 7.6 kW from the wall and outputting less, while the cabin heater is pulling 7500 watts, the AC is spooled way up, and the battery heater is pulling 2200 watts. In this case, the extra electron have to be coming out off the battery...correct?
But in the first case, they could simply close the battery contactors while having the rest of the HV system active. I don't see why that wouldn't allow DCFC to run the battery heater.

Also, yes, if you're pulling 7.6 from the wall and you have 7.5 for cabin and 2.2 for battery heater, then at least 2.1 is coming out from the battery.
 

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This is the opposite of cell balancing. If what we believe is true, passive cell balancing just bleeds off excess power from the cells. So dropping down to under 2kW is not cell balancing, it's probably just running the battery heater.
It wasn't cold enough [< ~0C] to run the battery heater - it's been a mild winter in Boston. And I see the same taper pattern, including the bump, every time I do it [once every 2 weeks or so]. L2 charging isn't subject to tapering to be kind to the battery when hilltop is on. So I'm not sure what that 1-hour reduced charge rate would be.
 

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I don't own a home charger so I DCFC all the time. Usually 1 or 2 times per week over the past 3 years. I don't always go to 100% but it is very important to do so once in a while. About once a month. The car loses track of where 100% is. This means your range on the guess-o-meter becomes less and less accurate.

We've all become accustomed to the charge curve and estimated charge times. Dare I say many of us have memorized it. But... on more than one occasion I have seen this:

The car, via ChargePoint and the MyChevy App displays 95%, 96%, 97%, 98%, 99%, 100% and keeps taking 20 kw charge rate from the DCFC. WTF????

Then there is a beep. A message "unable to charge" in the Bolt. The ChargePoint DCFC says "return plug to charger". I comply. Then I swipe my card and return the plug to the car.

I hit start. The car starts charging at 20 kw again, and the Car and ChargePoint Stand say 88%.

Now the car charges 88% to 100% again following the traditional taper we know so well.

I'd say that is a symptom of an unbalanced pack, as a result of my going 2 or 3 months with partial charges, never going to 100%. Since I only used DCFC I usually don't have the time or patience to sit there and let it trickle charge.
 

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I seldom (almost never) charge to 100%, and I drive a LOT in my Bolt EV (52,000 miles in 22 months)... and this became relevant this weekend at Fully Charged Live in Austin, TX when I spoke to the Bolt lead battery engineer. I asked him why I seem to have gotten the short end of the stick on charging taper points when using a CCS station. My 2017 Bolt does the first taper at 49%, the second one at 66%, and so on... he said that the taper is not triggered by SOC, or pack voltage, it is triggered by the highest individual cell voltage. In other words, my pack is poorly balanced, and this is what causes my early taper points! Unfortunately, it was the end of the day on Sunday, and I did not ask him about the cell balancing process and if charging to 100% was needed for proper balancing.

Will charging on a fairly regular basis to 100% re-balance my pack and allow me some of that sweet 49% to 55% 55 KW charging? I hope to find out soon!

He also said regarding cold weather charging on 2017-2019 Bolts that he "hopes" for a software update to be pushed to older Bolts to improve battery heater usage to optimize it for high powered CCS units, rather than shutting down when they reach the temperature where a 100 amp CCS can max out current flow. He did NOT promise this update, so take his "hope" with a grain of salt. He made no mention of shifting the previous year bolts to a gradual taper instead of a step taper charging scheme, so I don't expect that to ever happen.

He did explain why the max charge rate remained at 55 KW... the charge port, all the internal wiring for the charging system (breakers, fusible links, what have you) are all rated at 55 KW, and upgrading them would have cost significant amounts in R&D and recertification costs, as well as beefier components costing more in building the car. One of the other guys in the booth when I mentioned complaints about the lack of rear door unlock buttons said that it was a cost cutting measure to help mitigate the cost of the battery pack upgrade from 60 kWh to 66 kWh... so the whole "every penny counts" factor is why we didn't get faster charging on a minor mid cycle refresh.

Later,

Keith
 

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Discussion Starter #69
Is once a month charging to 100% (PA unheated garage L2 20A max @ 240V) leave it at 100% for 2 hours then drive until 75% the right thing to do?

Details: 2019 Bolt LT in SE Pennsylvania driven 2000 miles per month (50-70 miles per day). Charged every night via garage charging using an adjustable L2 EVSE set 20A max. Bolt set for 72% Departure Charging per heat while plugged in and leave 30 minutes after departure charging complete. Most evenings return to garage EVSE with 35%-50% Bolt indicated remaining charge.
 

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I did my 100% experiment on a L2 charger this morning, while monitoring the progress on torque pro.

Started at 75%. Current is a constant 30A all the way to 95%, before it started to taper off. Battery voltage was 398 to 399V. Average cell voltage was 4.151V. Seems like it is the last 10 to 20mV per cell it is trying to even out.

This lasted a good 20 minutes. Eventually charging stopped at 97 %. Battery voltage 397.5V, which was a bit lower than at 95%!

Questions for the torque pro power users. Anyway to pull the min / Max cell voltages? What PID to pull out battery capacity in kWh?

-TL

Sent from my SM-N960U using Tapatalk
 

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Questions for the torque pro power users. Anyway to pull the min / Max cell voltages? What PID to pull out battery capacity in kWh?
There is no PID for min or max. You have to observe the voltages of all 96 cells for a bit to determine which is your highest, and lowest cell. They will always be your highest and lowest cell. Reproduce the gauges for those two, and put the average cell voltage PID between them. If you have a new Bolt the battery capacity PID is no longer available. GM disabled it as part of their owners-are-mushrooms program.

Your finish voltage charge is low.

12-29-18-1.jpg
 

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Thanks. That's what I thought I would have to do. Anyway to download the scanned sensor data? A computer is better than my eyeballs to locate the min / Max.

As for the battery capacity in kWh, I'm going to improvise with soc * 60kwh for now.

-TL

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I'm goin to add this shot of my last charging session in the hope that it provides another data point, rather than detracts from the conversation. 2020 Bolt, Juicebox 32 Next Generation. The charge started at 32 amps, with a tiny taper across its entirety. Voltage (measured by the app) was constant at 242v. Charge limit was off, so 100%
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. Anyway to download the scanned sensor data? A computer is better than my eyeballs to locate the min / Max.
Yes. I am sure there is. I know folks have done it on here. I am not a computer guy. I am lucky I got it to work at all. Hopefully one of many IT guys on here will chime in with the answer.
 

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Ok I did it. It wasn't hard. Cell# 25 is the highest, followed by cell #94. Cell #65 is the lowest, followed cell #39. The spread is about 30mV, being the highest at 100%. As the battery is being discharged, the spread actually narrows.

-TL

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It wasn't cold enough [< ~0C] to run the battery heater - it's been a mild winter in Boston. And I see the same taper pattern, including the bump, every time I do it [once every 2 weeks or so]. L2 charging isn't subject to tapering to be kind to the battery when hilltop is on. So I'm not sure what that 1-hour reduced charge rate would be.
Is it over 60F? I figured even a mild winter was colder than that in Boston!

At the end of each charge cycle (hill top reserve, or full charge) the BMS conditions the battery temperature. If it is cold, it heats it until it reaches 60F... not sure what temperature it cools it down to in hot weather charging. This is one of the reasons to use departure schedule charging, if you charge up immediately and the charge ends 6 hours before you leave on your next trip the battery will have cooled off significantly by the time you leave and thus be less efficient.

Keith
 

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It wasn't cold enough [< ~0C] to run the battery heater - it's been a mild winter in Boston. And I see the same taper pattern, including the bump, every time I do it [once every 2 weeks or so]. L2 charging isn't subject to tapering to be kind to the battery when hilltop is on. So I'm not sure what that 1-hour reduced charge rate would be.
Stupid double post! I didn't hit "post reply" again until after it said "oops, try again later"...

Keith
 

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Ok I did it. It wasn't hard. Cell# 25 is the highest, followed by cell #94. Cell #65 is the lowest, followed cell #39. The spread is about 30mV, being the highest at 100%. As the battery is being discharged, the spread actually narrows.

-TL

Sent from my SM-N960U using Tapatalk
I actually did a log of all battery voltages on a charge from 7% to 72% and graphed out what my eyeball said was the high and low in Excel, and then I compared every other cell to those ones I picked out by eye. Turns out my eye was faulty on both the high and the low! What I found was that the one I though was the high actually was the high up until about 40% SOC, but on average, a different cell that was very slightly lower up to 40% was significantly higher after 40% SOC. Similar situation with the low. So, what is your high cell and low cell actually changes depending on your SOC. I went with the cells that "on average" had the highest and lowest voltages, rather than going with the highest and lowest peak voltage. For me the low is cell 69 closely followed by cell 1 and cell 51 with cell 16 being my high cell closely followed by cell 2 and cell 76. My max difference between low and high cells on that charge was 0.032501V just before the second step down, it seemed to be getting wider as the SOC got higher. I wonder what the difference is at 100%?

This particular charging session was the one that made me very sad about my charging rate step downs. It stepped down the first time at 47% and did the next step down at 64%. I have been doing full charges with departure time charging so I don't sit for hours at 100% for the past couple weeks, and I will be doing a CCS charge from low SOC to high SOC to see if I have improved my battery health and charging step points before doing the "complain about low temperature charging" to get the update to use the 2020 charging taper instead of the 2017 to 2019 charging step down curve. I may actually do a low SOC to 100% charge pre and post update to document the benefits of the new curve.

Keith
 
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