mux
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Re: Battery Upgrades are very possible

Sat Jun 20, 2020 11:45 am

coleafrado wrote:
Sat Jun 20, 2020 7:44 am
mux wrote:
Fri Jun 19, 2020 10:44 am
Ouch, wire bonded packs - those are notoriously unreliable. Every single cell has a very susceptible point of failure and even single cell failure tends to cause complete pack failure - which has a high likelihood with thousands of cells under the hood.
So you're not going to back this up with anything?
Oh you actually want this explained - I didn't understand that from the earlier responses.

This is not very obvious to many people because there is some misleading 'marketing' (or whatever you can call it) around highly-parallellized packs. The idea most people have about Tesla packs is that even if one cell fails, there are many in parallel that can take up the slack. This is an oversimplification and in general is simply not the case, because:

- The remaining parallel cells will have proportionally higher load on them, during discharge (generally fine) but also during charging. Even a few percent higher charging current will impact charging rates at high SOC quite significantly. Again, see the Leaf batteries for what happens when a pack anisotropically ages - you get disproportionally rapid degradation and imbalance
- The entire rest of the pack is reduced in capacity proportional to just that one failed cell.
- That cell didn't fail for no reason; there is almost never a single incidental cell or bonding failure, it's always a cluster affecting either multiple parallel cells or single adjacent series cells as well.
- You're introducing - in the case of a full Tesla pack for instance - roughly 10k-15k points of failure per battery pack.
- Cooling of cylindrical cells is already quite hard, but especially when there's a failed and possibly deformed or venting cell nearby.

Within the industry it's pretty well known what the Tesla battery failure rate is. You won't be surprised that especially near the beginning of manufacture, it's rare to find packs without bonding faults, even in otherwise healthy packs. Within the battery refurbishment industry which we rely on heavily, it's basically impossible to refurbish entire Tesla packs. There is always one or two S/X modules that have a serious issue in replaced packs that enter the waste stream, and most of this has to do with their packaging strategy.

Now, it's also pretty well known in the industry that all of this has become much, MUCH better over the years. They've got 7 years of wire bonding experience to perfect the tech, but that doesn't mean wire bonding as a general technology has progressed - it's just Tesla that has improved by and large. Since last year - i.e. before we started mass production of our extender batteries - we have been looking around for alternatives to the batteries we're using now (large prismatics in modules with BMS attached), and one of the cheaper options would have been to buy a wire bonding machine and enter a supply contract with Samsung SDI or LG for 18650s. But we're getting quoted - at best - 0.1% bonding faults. Even premade packs have these kinds of reliability ratings. This is why wirebonded packs are basically never sold for automotive uses.

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A second line of critique on using 18650s in a non-thermally managed pack is their poor performance. Intrinsically, 18650s have bad packing density, i.e. you spend a lot of volume and weight on packaging compared to active material when compared to prismatics. To compensate, they have to either increase their active-material-to-electrode volume or decrease their energy density. This means for most 18650s, their specific power is quite low, at least for commercial offerings you can consider for EVs.

An NCR18650B, the 'tesla cell', has a continuous discharge rating of 3.35A and a peak of 6.7A. You can fit, realistically, something like 40kWh worth of these cells inside a Leaf battery enclosure, or about 96S40P. That translates into a 140/280A continuous/peak rating, or about 50/100kW continuous/peak - at 25C. The 24kWh pack of yesteryear has a cont/peak rating of 30/80kW at freezing! So you're looking at a significantly lower performance, especially at low temperatures.

Tesla gets around this by - first - just making giant battery packs (more parallel solves all these problems), but also making sure the pack never operates at low temperatures to begin with and gets cooled down really aggressively at high performance. You need to remove roughly 10kW of heat when charging at (old) supercharger rates with a 40kWh pack made out of NCR18650Bs, or about 2.5kW at chademo rates. Again, you can compensate for a lot of bad design options by improving in other ways. Tesla chose a shit battery construction from a reliability, maintainability and performance point of view, but coupled it to absolutely excellent thermal management to make batteries that can do anything you'd want from an EV battery and more. Better than a lot of their competition even, which is a truly underappreciated accomplishment.

Additionally, 18650s have bad overloading and aging characteristics. Again, bit less of an issue with packs that are well-managed, but lithium ion cells expand not just when they get (over)charged, but when they age as well - building up pressure inside the mechanically constrained shell. At about 70-75% SOH, a cell expands 5-10%, and that's something you can account for in prismatics - but not in 18650s. They just fail - and if you have thousands in a pack, that's a lot of opportunities for failure again.

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All of that is why I am VERY critical of 18650-based rebuilt batteries. It is a good choice for a multi-billion dollar company whose core technology is these batteries. It is a bad choice for a 2-man startup without a rigorous background in battery engineering. There is a lot you can do wrong with battery design in general, and all that gets magnified if you use cells that can barely handle the load, and need specialized BMSes to survive for a decent amount of time - oh and there's tons of labor going into production and QA, and it's mighty attractive to skimp on that last bit to preserve your margins.

brunohill
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Re: Battery Upgrades are very possible

Sat Jun 20, 2020 8:55 pm

Can these prismatics handle 70'C inside a vehicle that is parked in the sun all day?

mux
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Re: Battery Upgrades are very possible

Sat Jun 20, 2020 11:09 pm

Just sitting still? maybe, for once or twice. Operating? No, they generally can't tolerate temperatures over 55C. 70C is also well over the storage temperature of most interior plastics and glues, so you're basically melting your car at that point.

DougWantsALeaf
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Re: Battery Upgrades are very possible

Sun Jun 21, 2020 7:07 am

Mux

It appears that the newer chemistries (Tesla, Leaf, etc..) all appear to be more heat tolerant than the 2010-2014 chemistry. Watching Bjorn videos, Tesla’s will bring their batteries up over 50C for charging.

For the newer Leaf, the heat doesn’t seem to accelerate degradation in the way it did with early packs.

I don't know enough about the Bolt temps.
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brunohill
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Re: Battery Upgrades are very possible

Sun Jun 21, 2020 7:14 am

mux wrote:
Sat Jun 20, 2020 11:09 pm
Just sitting still? maybe, for once or twice. Operating? No, they generally can't tolerate temperatures over 55C. 70C is also well over the storage temperature of most interior plastics and glues, so you're basically melting your car at that point.
Yes, the glue melts and panels warp. Extender packs in warm climates may have to go underneath the vehicles.
https://goodcalculators.com/inside-car- ... alculator/

lorenfb
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Re: Battery Upgrades are very possible

Sun Jun 21, 2020 9:42 am

Here’s an interesting technical research paper which should help provide some with a better understanding of what occurs during
the charge and discharge process of Li Ion batteries as it relates to battery heat:

http://cpb.iphy.ac.cn/article/2016/1806 ... 10509.html

Entropy and heat generation of lithium cells/batteries
Heat generation inside a battery It is important to understand how heat generated inside a battery. Heat is produced in batteries from two sources; electrochemical operation and Joule heating [5-7]. Reference [8] found a way to calculate the battery heat using a thermodynamic energy balance and cited frequently in the literature in its simplified form is shown in equation 3 below where the first term is the heat generation due to Joule heating and the second term is the heat generation due to entropy changes.
The most important factor that affects the energy losses of a cell is the polarizations. The total polarizations of a cell include: ( i ) Ohmic polarization, which causes the voltage drop during operation, and also consumes part of the useful energy as waste heat. The total ohmic polarization of a cell is the sum of the polarizations caused by the ionic resistance of the electrolyte, the electronic resistances of the electrodes, the current collectors and electrical tabs of both electrodes, and the contact resistance between the active materials and current collectors. The Ohmic polarization follows Ohm’s law, with a linear relationship between the current and the voltage drop. (ii) activation polarization, which drives the electrochemical reaction at the electrode/electrolyte interface, and (iii) concentration polarization, which appears due to the concentration differences between the reactants and the products at the electrode/electrolyte interface and the concentration differences in the bulks as a result of mass transferring.
All these polarizations cause consumption of Gibbs energy, which is given off as heat energy during the charge–discharge process.
Conclusion
The changes of entropy and heat generation of lithium cells are ineluctable. Investigations of them are a part of the research into lithium cells. Understanding the change of entropy and heat generation can benefit the study of the state and the safety of lithium cells. The quantification of the change of entropy and heat generation can improve the level of management and control for security of lithium cells/batteries.
Battery Heat Loss = I^2 * R + d (entropy)/dt, where R is the typically measured battery resistance
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mux
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Re: Battery Upgrades are very possible

Sun Jun 21, 2020 10:57 pm

DougWantsALeaf wrote:
Sun Jun 21, 2020 7:07 am
Mux

It appears that the newer chemistries (Tesla, Leaf, etc..) all appear to be more heat tolerant than the 2010-2014 chemistry. Watching Bjorn videos, Tesla’s will bring their batteries up over 50C for charging.

For the newer Leaf, the heat doesn’t seem to accelerate degradation in the way it did with early packs.

I don't know enough about the Bolt temps.
Yes, LMO and NCM do respond differently to high temperatures. But the actual temperature sensitivity is a much more complex issue. Keep in mind that for the same usage, newer Leaf packs experience lower C-rates during charging and discharging and less charge/discharge cycles. This has been discussed before in this thread; higher stress on the pack dominates battery life for small batteries.

The thermal management of Tesla has been discussed at length as well: Batteries have better performance at higher temperatures, but also much faster degradation. Tesla brings the packs up to 50C for DC QC sessions, but then rapidly cools them down afterwards to avoid degradation. Leaf packs sit hot and idle for many hours after use, and that's the reason they degrade so quickly. This is why we don't call thermal management 'battery cooling'. We're not cooling the batteries, we're getting them to the correct temperature for the job at all times. This sometimes involves removing heat, and sometimes involves adding heat. Most of the time it just involves circulating coolant to make sure all parts of the battery are at precisely the same temperature, avoiding anisotropic stress.

LeftieBiker
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Re: Battery Upgrades are very possible

Mon Jun 22, 2020 12:55 am

Yes, LMO and NCM do respond differently to high temperatures. But the actual temperature sensitivity is a much more complex issue. Keep in mind that for the same usage, newer Leaf packs experience lower C-rates during charging and discharging and less charge/discharge cycles. This has been discussed before in this thread; higher stress on the pack dominates battery life for small batteries.
The first generation "Canary" packs degraded in moderate to high ambient air temperatures, not just when heated internally. There are plenty of stories of people who babied the cars, drove gently, never or rarely quick charged, but who still experienced the same high rates of degradation. Only those with cars in climates that were actually on the cool side avoided the infamous rapid bar losses.
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mux
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Re: Battery Upgrades are very possible

Mon Jun 22, 2020 2:33 am

I've interpreted the previous question way too nuanced. Yes, pre-Lizard packs fared horribly in hot weather. And cold weather for that matter, they were shit packs.

That's not an LMO vs NCM issue, which is what I was answering.

jfr2006
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Re: Battery Upgrades are very possible

Mon Jun 22, 2020 5:30 am

Hi, mux:

I don't think you fully understood my previous questions, so i will rephrase them and add another one.

1 - About the LG cells inside the leaf cells can, i was talking about the cell used by Jesus from evbatteryupgrades. Just as in theLeaf that has 2 cells for "can", the process would be the same, but with LG cells. They don't seem that bad.

2 - When i asked you about the upgrade process of my battery to a Leaf 62kWh battery, the idea was the following: I have a 30kwh Leaf model that i would take to you and you would replace the battery with one of a 62kWh Leaf. The question was: is it easy to get such a battery pack?

3 - I think i remember seeing somewhere that you where working a adding CCS to Leaf. How is that process? Would it be replacing the Chademo connector, or would it keep the Chademo adapter and replace the J1772 connector with a type 2 connector and a the CCS pins, so we would keep the Chademo, Type 2 and CCS connectors?

Regards.

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