Battery Management

[boltage]

But would two 100 mile runs in a row be equal to a single discharge of two hundred miles?

Depends on how you charge. A 100 mile run would use about 42% of the battery if you got the EPA rated efficiency/range. If we assume (as discussed in another thread) that lithium ion battery degradation is worst at high state of charge (particularly when charging at high state of charge), and that each charging cycle causes battery degradation:

Two 100 mile charges, recharging to full (58% -> 100%) each time, is probably the worst for battery degradation.
Two 100 mile charges, recharging to hill reserve (48% -> 90%) each time, is better than the above.

One 200 mile charge, recharging to full (16% -> 100%) should be better than the above for battery life, due to half as many charge cycles and less time spent in high state of charge.
One 200 mile charge, recharging to hill reserve (6% -> 90%) should be even better, but you may not want to go down that low in case weather conditions, side trips, etc. cause you use up that last 6% before getting home.

Two 100 mile charges, recharging to 55% (13% -> 55%), may be somewhat better than one 200 mile charge, due to keeping the battery out of the high state of charge that increases risk of degradation. But setting up your charging to do that is less obvious, though it can be done. Obviously, when you do a longer trip, you want to reset your charging parameters to charge to a higher level than 55%.

 

[cyaopec]

Thank you very much. What you said makes sense. Although total miles driven is a factor, it seems number of charge cycles and keeping the battery in a high SOC is the more important variable. I could do the 13-55% charge each day. I'm guessing the 10-40% potential loss cited in the Bolt manual represents at the 40% level a worse case scenario that represents the most ignorant driver in the most extreme weather conditions such as parking the car outside in 120 degrees in Palm Springs six months of the year. Volt engineers have said end of life for the battery is when it degrades 20 percent, which they estimated would occur in 11 years in a hot environment and 15 years in a cold environment like Michigan. So I think with careful management you could only see degradation of perhaps 10-20% between 100-150k miles.

 

[oilerlord]

A good idea to charge to 100% on a weekly or monthly schedule, but for the second reason and not the first. The balancing of cells is usually done near 100%, or "top balanced". You might gain range by having the cells better balanced: how much depends on how the car balances cells.

I've adjusted my charging / SOC habits lower for the long term health of my battery. Though I could probably go all summer charging to 75% (approx 21 out of 36 kWh's), I also think it's a good idea to charge to 100% of usable capacity every so often so the cells are balanced on a regular basis. No idea if that should be done weekly, or monthly.

I'd assume that hilltop mode in a Bolt allows the car to properly balance the cells. The problem with my car, is that there is no setting for SOC maximum - I have to do that manually. For that reason, periodic cell balancing probably is a big deal.

Thoughts?

 

[peteblair]

A good idea to charge to 100% on a weekly or monthly schedule, but for the second reason and not the first. The balancing of cells is usually done near 100%, or "top balanced". You might gain range by having the cells better balanced: how much depends on how the car balances cells.

I've adjusted my charging / SOC habits lower for the long term health of my battery. Though I could probably go all summer charging to 75% (approx 21 out of 36 kWh's), I also think it's a good idea to charge to 100% of usable capacity every so often so the cells are balanced on a regular basis. No idea if that should be done weekly, or monthly.

I'd assume that hilltop mode in a Bolt allows the car to properly balance the cells. The problem with my car, is that there is no setting for SOC maximum - I have to do that manually. For that reason, periodic cell balancing probably is a big deal.

Thoughts?

I don't think with Hilltop set that the charger does any balancing of the cells. I say this because if I watch the car charging in hilltop when it hits 90% or close to it, the charge cycle just stops. However if I let if charge full there is a time period where the car reports as fully charged yet the car is still "charging" if you watch it and I believe this is when it is performing cell balancing or other battery maintenance. I don't know this for sure of course, this is just my observation so far.

I have also noticed that if I do hilltop charges for 2 weeks that the reported SOC seems to drift, so it will say it is done but only say it is 86% - 88% charged instead of 90%. However the hilltop charge immediately after the full charge will go to 90% when it is done. I am assuming this is due to the cells being re balanced / whatever it does for battery maintenance after a full charge.

So this isn't scientific of course as I wasn't doing any direct measurements, just watching what the car says it was doing.

 

[LeftieBiker]

I don't think with Hilltop set that the charger does any balancing of the cells. I say this because if I watch the car charging in hilltop when it hits 90% or close to it, the charge cycle just stops. However if I let if charge full there is a time period where the car reports as fully charged yet the car is still "charging" if you watch it and I believe this is when it is performing cell balancing or other battery maintenance. I don't know this for sure of course, this is just my observation so far.

My 2013 Leaf seems to do some balancing at 80% when set for that limit, but it still seems to benefit from an occasional 100% charge. So even if the Bolt did do balancing at...88%?... it would be worth doing a 100% charge occasionally.

 

[phil0909]

I have also noticed that if I do hilltop charges for 2 weeks that the reported SOC seems to drift, so it will say it is done but only say it is 86% - 88% charged instead of 90%. However the hilltop charge immediately after the full charge will go to 90% when it is done. I am assuming this is due to the cells being re balanced / whatever it does for battery maintenance after a full charge.

I don't think that assumption is right. My Bolt always charges to 87% on the hill top setting, including immediately following a 100% charge.

There is no indication in the Bolt Owner's Manual that the hill top charge should be 90%.

 

[peteblair]

I have also noticed that if I do hilltop charges for 2 weeks that the reported SOC seems to drift, so it will say it is done but only say it is 86% - 88% charged instead of 90%. However the hilltop charge immediately after the full charge will go to 90% when it is done. I am assuming this is due to the cells being re balanced / whatever it does for battery maintenance after a full charge.

I don't think that assumption is right. My Bolt always charges to 87% on the hill top setting, including immediately following a 100% charge.

There is no indication in the Bolt Owner's Manual that the hill top charge should be 90%.

Yes, it doesn't state the 90% figure in the manual. There are however reports from engineers who worked on the Bolt that it is supposed to charge to 90% with Hill Top reserve set.

http://www.motortrend.com/news/chevrolet-bolt-ev-engineers-reveal-11-cool-facts-car-year/

That is just one place i found it.

Also, i'm just stating what I have observed and the assumptions from that, I don't know if it is accurate. I would need to actually plug something into the car that would report out all the data needed to really know if my assumptions are correct.

In my Bolt it does charge to 90% each time after I do a 100% charge. I don't know if it will always do that, right now I have only done 4 100% charges. So it is certainly a small sample set so far. Every other hill top charge for me has ranged from 86% to 89%.

 

[LeftieBiker]

When my Leaf turned about one year old, it started to charge to 79% on the 80% setting. It has done so ever since. At first I was convinced it was about to lose a capacity bar, but almost three years later I still have 12 bars. Not all glitches are harmful...

 

[peteblair]

When my Leaf turned about one year old, it started to charge to 79% on the 80% setting. It has done so ever since. At first I was convinced it was about to lose a capacity bar, but almost three years later I still have 12 bars. Not all glitches are harmful...

Yeah, i'm not concerned with it. Just interesting observations to me really.

I'm mainly just curious about how it all works and I don't have the equipment to really dig into it. Plus with sharing my observations perhaps that will help those who have the time and equipment.

 

[redpoint5]

I thought about the design that went into this car when choosing on a routine for myself; should you charge to full? Probably not. How can I tell? The engineers thought it prudent to give us a mode that only charges to 90%.

TLDR entire thread-

Part of the confusion is what we users are calling 0% or 100%, and what the actual 0% and 100% charge levels are.

The car doesn't allow the battery to drain to actual 0% or to charge to actual 100%, and this is to preserve battery life. For this reason, it mostly doesn't matter what level to charge or discharge to. That said, it's better to keep the battery somewhere in the middle range. If there is a good chance you will need the "full" charge, then charge it to the user 100% level and don't worry about it. Otherwise, use the option to partially charge.

As far as balancing is concerned, I would think the only way to accomplish it is to top balance, since we never discharge fully (bottom balancing). Top balancing means charging to user 100%. Balancing only needs to be done occasionally, so as long as the car occasionally is charged to 100%, then it should be good enough. I just wouldn't go years between full charging.

The engineering recommendations should be made accessible to enthusiasts, but I understand why it isn't included in the user manual. Those things are thick enough as it is, and regular consumers don't want to think about the minor details of how they drive or take care of their cars. For people that cannot be bothered to learn about how to operate or maintain their vehicles, I would think EVs have a huge advantage over their gasoline counterparts. It's one of the perks of being a simpler machine.

 

[LeftieBiker]

As far as balancing is concerned, I would think the only way to accomplish it is to top balance, since we never discharge fully (bottom balancing).

Lithium packs are always top balanced. I think only NiMH packs can be bottom balanced. And I think the 87% Hilltop option is to prevent just this sort of agonizing over SOC. Although I wouldn't be surprised if GM started to "recommend" using it in hot locations once they start getting data on degradation.

 

[EldRick]

peteblair - I agree with your reasoning. Since I live near the top of a hill, I've been using Hilltop Reserve, and every week or so I give it a full charge for cell balancing.
I'm also guessing (with no evidence) that cell balancing might be more important/effective when the car is new, while the battery "breaks in" for a while.

 

[SeanNelson]

The engineering recommendations should be made accessible to enthusiasts, but I understand why it isn't included in the user manual. Those things are thick enough as it is, and regular consumers don't want to think about the minor details of how they drive or take care of their cars.

Not only that, but if GM put it down in writing then the Internet would light up with stupid accusations that "the 238-mile EPA range is a sham because you're not supposed to charge the battery that much".

 

[michael]

As far as balancing is concerned, I would think the only way to accomplish it is to top balance, since we never discharge fully (bottom balancing).

Lithium packs are always top balanced. I think only NiMH packs can be bottom balanced. And I think the 87% Hilltop option is to prevent just this sort of agonizing over SOC. Although I wouldn't be surprised if GM started to "recommend" using it in hot locations once they start getting data on degradation.

Lithium batteries are usually top balanced but they can be and occasionally are bottom balanced

 

[redpoint5]

I'm also guessing (with no evidence) that cell balancing might be more important/effective when the car is new, while the battery "breaks in" for a while.

New cells should be closely matched and balanced from the factory, so balancing wouldn't be as important initially. As the battery ages, small differences in construction and environment compound to make individual cell differences substantial. This makes cell balancing more important as these differences become greater over time.

Furthermore, the more deeply a battery is charged or discharged, the greater the need for balancing. If a battery remains near half capacity, the likelihood of over-charging or over-discharging individual cells is minimized.

 

[WetEV]

Lithium packs are always top balanced.

Lithium batteries are usually top balanced but they can be and occasionally are bottom balanced

Li-ion battery packs are usually top balanced, it is fairly easy. Bottom balance is somewhat rare, and makes the most sense if you often fully discharge the pack. There are also dynamic balancing schemes, that balance cells in the battery pack over the whole range of charge.

 

[gpsman]

I'd like to reiterate my earlier comments with data to support IT IS GOOD TO FULLY CHARGE at least once per month.

Recently I went 2 weeks without fully charging. I kept the car in the "comfortable" range for me of 35% to 85%.

Then I needed to take a long trip. I wanted 100%.

I went to a DCFC starting at about 75% (becuase it was free at a generous local school, I didn't care about the lower charge rate. Level 3 is faster than Level 2 all the way to about 95% SOC, at which point L3 becomes about the same, 6kw of charge rate).

STARTING AT 75% SOC I was shocked to watch the car charge all the way to 100% at 16kw then abruptly stop. Also, the number of miniutes it took to get from 75% to 100% was way shorter than expected. Only about 15-20 minutes.

I was suspisious because at the end, at 100% it did not say charge finished or finalizing. It abruptly stopped and said "unable to charge".

So I took the cord off. Returned it to the DCFC holster, reinitiated another charging sesssion, plugged the cord back into my car, and low and behold.... IT STARTED CHARGING AGAIN AT 24kw then tapered to 16 kw.

AND THE CHARGE GAUGE HAD INSTANTLY RESET TO 86%.

It took the expected amount of time to go the rest of the way from 86% to 100% and tapered off in the expected manner. 24kw / 16kw / 10kw / 6kw / 3kw

Charge rate never dropped lower than 3 kw.

Goes to show... the "guess-o-meter" guesses get worse with time if you never charge to 100%.

 

[stevewa]

I'd like to reiterate my earlier comments with data to support IT IS GOOD TO FULLY CHARGE at least once per month.

Recently I went 2 weeks without fully charging. I kept the car in the "comfortable" range for me of 35% to 85%.

Then I needed to take a long trip. I wanted 100%.

I went to a DCFC starting at about 75% (becuase it was free at a generous local school, I didn't care about the lower charge rate. Level 3 is faster than Level 2 all the way to about 95% SOC, at which point L3 becomes about the same, 6kw of charge rate).

STARTING AT 75% SOC I was shocked to watch the car charge all the way to 100% at 16kw then abruptly stop. Also, the number of miniutes it took to get from 75% to 100% was way shorter than expected. Only about 15-20 minutes.

I was suspisious because at the end, at 100% it did not say charge finished or finalizing. It abruptly stopped and said "unable to charge".

So I took the cord off. Returned it to the DCFC holster, reinitiated another charging sesssion, plugged the cord back into my car, and low and behold.... IT STARTED CHARGING AGAIN AT 24kw then tapered to 16 kw.

AND THE CHARGE GAUGE HAD INSTANTLY RESET TO 86%.

It took the expected amount of time to go the rest of the way from 86% to 100% and tapered off in the expected manner. 24kw / 16kw / 10kw / 6kw / 3kw

Charge rate never dropped lower than 3 kw.

Goes to show... the "guess-o-meter" guesses get worse with time if you never charge to 100%.

Very interesting. I had noticed the guess-o-meter becoming more and more optimistic over time (I have had hilltop reserve turned on for charging at home all this time) and after my Seattle trip the numbers are much lower than before. Guess I should give an occasional charge to 100% as well...

 

[SparkE]

Yes, it is a good idea to fully charge every so often. For me, that's about once every 4 weeks or so (but, since i have a SparkEV, that is only about every 8-900 miles or so). I make a vague effort to avoid charging the battery over 80% SoC and rarely charge over 90%.

I figure that if a full charge 'fatigues' the battery, I am only doing it at about 1/30th the rate of what most people used to do. Thirty times longer use out of the battery? Good enough for me . And when I DO fill it to 100%, I try really, really hard to time it so that I do so on a day when I will soon drive it over 15 miles, so I use enough energy to drop the SoC to below 85% almost immediately.

I also try to NOT charge the battery when it is hot - to any % charge at all, not just when I charge to full. I wait until it is cool outside during the summer instead of charging when it is 95 degrees outside.

Yes, I realize I am being a bit paranoidly anal about it (especially since it is a lease) - but I got the car to be green, and I'd like the next user (who may be me) to have a good, healthy battery. So far, not much appreciable (noticeable) battery fade.

Needless to say, my 'Guess-o-Meter' is usually between 30-85 miles (since it is about 105 when full - 'Sparky' is driven mostly in town).

(Oops, I just said it!)

 

[Anonymous]

Is there a way to limit the charge to a speficic percentage, ex. 80%.
Or, is it possible to reveice an alert indicating specific percentage of charge is reached, knowing that recharging time is longer after 80%.
Thanks