DSH1
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Joined: Sat Mar 11, 2017 3:16 pm

Should one fully charge the lithium battery in an i3?

Thu May 18, 2017 3:25 pm

I note that the i3 manual states "In order to operate the high voltage battery optimally, charge the vehicle regularly and completely in a compatible charger." However, a friend with an EV (not i3) says that his owners group claim that one should not regularly charge the lithium battery above 80% or damage may result.

Is there any truth to this latter claim? Just wondering.

I am, of course, following the i3 manual's instructions.

jadnashuanh
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Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Should one fully charge the lithium battery in an i3?

Thu May 18, 2017 9:06 pm

While many people consider all LiOn batteries the same...in reality, there are lots of different chemical formulations and construction schemes that can be called LiOn. Combine that with sophisticated heating/cooling that the i3 has of its battery pack and a pretty sophisticated charging logic, throw in that they don't let the battery be charged fully or depleted (based on temp and condition), I'd believe the BMW engineers...charge it.

If you read the free i3 e-book, it has some interesting discussion of this. One other thing to consider...recharging the car 10x from 90% (down 10%), is the same wear on the batteries as once from 0%. Now, take a typical cellphone, and this is not true for many reasons...different chemistry, different charging logic, lack of cooling/heating to get the batteries in their optimum state.

So, while it may be true for other brands...trying to use that same logic on a different system is not accurate...charge the i3 up, and don't worry about it. On others, follow the manufacturer's recommendations. Tesla, for example, uses thousands of commodity cells in their battery pack...BMW uses 96 cells. Yes their battery pack is larger, but if you scaled the BMW pack up to the same capacity, it would still be hundreds verses thousands on the Tesla. Even when manufactured on the same line, over thousands, there will be variations in the cells and trying to monitor their individual states to balance everything out is much more problematic verses monitoring 96. They are not the same animal.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

alohart
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Location: Honolulu, HI

Re: Should one fully charge the lithium battery in an i3?

Fri May 19, 2017 5:21 am

DSH1 wrote:I note that the i3 manual states "In order to operate the high voltage battery optimally, charge the vehicle regularly and completely in a compatible charger." However, a friend with an EV (not i3) says that his owners group claim that one should not regularly charge the lithium battery above 80% or damage may result.

It is undeniably true from an electrochemical perspective that any battery cell, including Li-ion of any chemical composition, is more prone to irreversible electrochemical side-reactions when it is at a high charge level which reduce the cell's capacity over time. However, the i3's battery management system (BMS), similar to the BMS's of most (all?) EV's other than Tesla's which allows the owner to charge to a very high charge level as an option, limits the high charge level to the low 90% range, thus minimizing, but not eliminating high charge level capacity degradation. So I don't typically charge to the highest level allowed unless I might need full range within a few hours.

However, reversible battery pack capacity degradation can occur if the charge levels of each cell begin to diverge over time, a natural occurrence as battery cells age and are charged/discharged. The BMS won't allow the cells with the lowest charge levels to be discharged below a safe charge level and those with the highest charge levels to be charged above a safe charge level, so the operating range of the entire battery pack would be reduced by cell charge level imbalance. The fix is to balance the charge levels of all cells. Electric Vehicles and the i3 (60Ah and 94Ah) states that cell balancing is done only when the battery pack has been charged to its maximum allowed charge level, so if one never charges fully, cell balancing would not occur which would result in a gradual reversible loss of battery pack capacity.

So I will be charging fully more often than I have done previously because cell balancing is crucial for good battery pack performance. Unfortunately, it's not obvious when cell imbalance is significant or how long a battery pack must remain at full charge for cell balancing to complete. Possibly to avoid concerning a typical BMW owner with such details, BMW recommends to always charge to full apparently confident that such advice is unlikely to cause battery pack capacity degradation more than the 30% allowed by the battery pack warranty. However, those who plan to keep their i3's past the expiration of the battery pack warranty might want to minimize capacity degradation by being as wise as possible with charging. It would be nice if BMW would assist with this goal, but I don't expect this to happen.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

alohart
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Re: Should one fully charge the lithium battery in an i3?

Fri May 19, 2017 7:20 am

jadnashuanh wrote:Tesla, for example, uses thousands of commodity cells in their battery pack...BMW uses 96 cells. Yes their battery pack is larger, but if you scaled the BMW pack up to the same capacity, it would still be hundreds verses thousands on the Tesla. Even when manufactured on the same line, over thousands, there will be variations in the cells and trying to monitor their individual states to balance everything out is much more problematic verses monitoring 96.

Tesla's battery cells are a standard size but they are not commodity cells in that their chemistry, construction, etc., are different from other cells of the same size, they are optimized for use in an EV, and they can't be purchased individually.

In BMW's 96-cell battery pack, all cells are connected in series, so a problem with any single cell affects the entire series of cells which is the battery pack itself. If the problem with one cell is bad enough (e.g., severe overheating, very high electrical resistance), the BMS might shut down the battery pack which would disable the car. In Tesla's multi-thousand cell battery packs, many series-connected strings of cells are connected in parallel, so a problem with any single cell affects mainly its string of cells. If the problem with one cell is bad enough, its string of cells could be bypassed so that the battery pack would continue to operate with a bit less capacity but with the same voltage. So many thousands of cells makes Tesla's battery packs more reliable than battery packs that use only 96 large cells all in series which is a common design of battery packs in most EV's. It also allows Tesla to build battery packs of various capacities using the same cells arranged in different numbers of parallel strings of series-connected cells. To build 22 kWh and 33 kWh battery packs requires BMW to use different cells, 60 Ah and 94 Ah, so BMW's pack design is much less flexible.

Tesla's BMS is much more complex than BMW's, and thus likely more expensive.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

jadnashuanh
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Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Should one fully charge the lithium battery in an i3?

Fri May 19, 2017 3:08 pm

One thing that I've noticed on charging my i3 is that near the end, the EVSE tends to cycle on/off a few times with a significant pause in between. I think that some of this is the logic in the charging trying to see what voltage level is reached after removal of the charging current. IN that manner, it can also evaluate how balanced the cells are and maximize their level without overcharging any individual cell.

As to the sophistication of any single company's charging logic, it's hard to say unless you have intimate knowledge of both the chemistry used and the parameters monitored as well as the construction. Parts of that can be pieced together, but it's hard to evaluate reliably. One factor I consider is that it's much more likely to have 96 cell interconnections all be reliable verses about the 7K required on the larger Tesla pack. Automated equipment is quite reliable, but the larger number, even if the same error rate, means it's more likely to suffer in the long run. Plus, it's fairly easy on the i3 to replace a module. On the Tesla, you're more likely to need to exchange the pack. Don't know the end cost once out of warranty, but would expect swapping a module in an i3 will be cheaper than swapping a battery pack in a Tesla, even with the differences in labor (the Tesla pack can be swapped much faster)..
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

alohart
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Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: Should one fully charge the lithium battery in an i3?

Sat May 20, 2017 2:11 am

jadnashuanh wrote:One thing that I've noticed on charging my i3 is that near the end, the EVSE tends to cycle on/off a few times with a significant pause in between. I think that some of this is the logic in the charging trying to see what voltage level is reached after removal of the charging current. IN that manner, it can also evaluate how balanced the cells are and maximize their level without overcharging any individual cell.

That's an interesting observation that is consistent with one apparently common cell-balancing implementation in which charging stops when at least one cell's voltage reaches the maximum allowed. If this voltage is significantly higher than that of other cells, those cells at the maximum voltage are discharged through a resistor until their voltages are similar to other cells whose voltage is slightly less than the maximum voltage. Then charging resumes until at least one cell's voltage reaches the maximum allowed. This balancing process is repeated until the voltages of all cells are approximately the same.

Because of this, I will be charging to full more often than I have in the past to ensure that the cells in our i3's battery pack are well-balanced.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

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