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Sad that you deleted the paragraph with links that show Sunking at his best. Below is the text from one of the posts in that thread that has some important and useful information in it. I don't usually like posting without attribution but looks like I have to.
Simon
I started to go through the OP, planning to knock down points one by one, but I just don't have the patience...not today.
The problem here Sunking, and with all the other guys over at DIY Electric Car who think they can bottom balance and not bother with any cell level monitoring, is a plethora of incorrect assumptions upon which is built a house of cards.
OEM's DO NOT balance plug-in vehicles at less than 100% SOC. The 20%-80% thing you describe applies only to HEV's...where accurate balancing is extremely difficult (and thus risky) at the extreme edges of charge, so it is generally (but not completely) avoided. Longer life is an ancillary benefit, and some have learned to manipulate this and take advantage of it, but this is unique to HEV's. PHEVs and full electrics live in a range-crazed world, and, AFAIK, every OEM out there takes their packs to 100% SOC during a complete plug-in cycle. I know for a fact that this is the case with GM (Chevy Spark), the Fisker Karma, and cars built by Chinese OEM SAIC. I don't know for certain what Nissan and Tesla do, but I would be very surprised to find them doing anything different. So, this is one of your cornerstone assumptions blown out of the water--it just isn't true.
Here's another...OEM's DO NOT spend a lot of time sorting cells. If they build the pack themselves, they may do a visual inspection of the cells and check OCV out of the box. They may run tests on a small sample of incoming cells, but they DO NOT hand-sort, match and select every cell for their packs. This would be an enormously expensive endeavor, one that OEM's expect the battery maker is already providing through their own quality test processes. So here is another fundamental assumption in your thinking that is just plain wrong.
You also dramatically overstate the detriment of taking a cell to 100% SOC. Keeping a cell at 100% SOC for 24 hours will destroy it? Nonsense. Let's talk about the mechanisms at work in LFP. If you take a cell over 3.60 Volts (RESTED), you will plate Li onto the cathodes, causing permanent capacity loss. This is not something that happens terribly rapidly...unless you were to do it habitually, you probably would not even notice anything at all, and even then it would take weeks, or more likely months, before the trend became visible. It's a bad thing to do, but nowhere near as severe abuse as driving a cell negative, which is indeed nearly an instant killer. If you don't exceed 3.60 Volts, the only stress you are subjecting the cell to is a slightly accelerated growth of the SEI layer. The rate of this is roughly proportional to SOC. If you want zero SEI layer growth (the main cause of Li cell degradation under normal, proper use), keep your cells at 0% SOC, or hold them below 50 degrees F at all times. These are obviously not practical in a battery you wish to use on short notice, which is why nobody does them. The SEI growth difference between 80% and 100% is not great, and 100% SOC should not be considered an abusive condition.
Speaking of what the OEM's do.....every one of them uses extensive BMS systems with cell-level voltage monitoring and balancing electronics. Why does this not play into your considerations, if you're trying to get to what they do? The fact is, without these things, you are flying blind. A cell defect or other anomalous condition can get past the safeguards you recommend if it is severe enough, or is allowed to fester long enough (and the bigger the pack, the more likely a user is to fall into that trap.) THE PRACTICES YOU AND OTHER ARE ADVOCATING HERE ARE EVENTUALLY GOING TO KILL SOMEBODY, IF THEY HAVEN'T ALREADY . I may seem like an ass when discussing this topic, but that is the bottom line right there. It might happen only in 1 car out of 100 or 1000, but that level of risk is still totally unacceptable and taking that kind of risk would land OEM personnel in jail. It probably seemed like hyperbole when I first stated that at DIY EC, but just a couple weeks later the justice dept. announced they were looking at criminal charges against GM personnel for the ignition switch issue...which has a FAR smaller risk than what you are advocating here.
A small pack on a bike (in plain sight at all times) or similar in the hands of a technically competent individual carries a small risk and still requires vigilant, regular babysitting. The larger a pack gets, the greater the energy potential, the greater the risk. A pack like this tucked away in a car (out of sight, out of mind) in the hands of a technically competent individual is riskier than it should be. In the hands of a lay person (who believes you when you say it's perfectly safe), it's a death trap. The practice is quackery, plain and simple. Do yourself a favor and stop before it's too late.
The problem here Sunking, and with all the other guys over at DIY Electric Car who think they can bottom balance and not bother with any cell level monitoring, is a plethora of incorrect assumptions upon which is built a house of cards.
OEM's DO NOT balance plug-in vehicles at less than 100% SOC. The 20%-80% thing you describe applies only to HEV's...where accurate balancing is extremely difficult (and thus risky) at the extreme edges of charge, so it is generally (but not completely) avoided. Longer life is an ancillary benefit, and some have learned to manipulate this and take advantage of it, but this is unique to HEV's. PHEVs and full electrics live in a range-crazed world, and, AFAIK, every OEM out there takes their packs to 100% SOC during a complete plug-in cycle. I know for a fact that this is the case with GM (Chevy Spark), the Fisker Karma, and cars built by Chinese OEM SAIC. I don't know for certain what Nissan and Tesla do, but I would be very surprised to find them doing anything different. So, this is one of your cornerstone assumptions blown out of the water--it just isn't true.
Here's another...OEM's DO NOT spend a lot of time sorting cells. If they build the pack themselves, they may do a visual inspection of the cells and check OCV out of the box. They may run tests on a small sample of incoming cells, but they DO NOT hand-sort, match and select every cell for their packs. This would be an enormously expensive endeavor, one that OEM's expect the battery maker is already providing through their own quality test processes. So here is another fundamental assumption in your thinking that is just plain wrong.
You also dramatically overstate the detriment of taking a cell to 100% SOC. Keeping a cell at 100% SOC for 24 hours will destroy it? Nonsense. Let's talk about the mechanisms at work in LFP. If you take a cell over 3.60 Volts (RESTED), you will plate Li onto the cathodes, causing permanent capacity loss. This is not something that happens terribly rapidly...unless you were to do it habitually, you probably would not even notice anything at all, and even then it would take weeks, or more likely months, before the trend became visible. It's a bad thing to do, but nowhere near as severe abuse as driving a cell negative, which is indeed nearly an instant killer. If you don't exceed 3.60 Volts, the only stress you are subjecting the cell to is a slightly accelerated growth of the SEI layer. The rate of this is roughly proportional to SOC. If you want zero SEI layer growth (the main cause of Li cell degradation under normal, proper use), keep your cells at 0% SOC, or hold them below 50 degrees F at all times. These are obviously not practical in a battery you wish to use on short notice, which is why nobody does them. The SEI growth difference between 80% and 100% is not great, and 100% SOC should not be considered an abusive condition.
Speaking of what the OEM's do.....every one of them uses extensive BMS systems with cell-level voltage monitoring and balancing electronics. Why does this not play into your considerations, if you're trying to get to what they do? The fact is, without these things, you are flying blind. A cell defect or other anomalous condition can get past the safeguards you recommend if it is severe enough, or is allowed to fester long enough (and the bigger the pack, the more likely a user is to fall into that trap.) THE PRACTICES YOU AND OTHER ARE ADVOCATING HERE ARE EVENTUALLY GOING TO KILL SOMEBODY, IF THEY HAVEN'T ALREADY . I may seem like an ass when discussing this topic, but that is the bottom line right there. It might happen only in 1 car out of 100 or 1000, but that level of risk is still totally unacceptable and taking that kind of risk would land OEM personnel in jail. It probably seemed like hyperbole when I first stated that at DIY EC, but just a couple weeks later the justice dept. announced they were looking at criminal charges against GM personnel for the ignition switch issue...which has a FAR smaller risk than what you are advocating here.
A small pack on a bike (in plain sight at all times) or similar in the hands of a technically competent individual carries a small risk and still requires vigilant, regular babysitting. The larger a pack gets, the greater the energy potential, the greater the risk. A pack like this tucked away in a car (out of sight, out of mind) in the hands of a technically competent individual is riskier than it should be. In the hands of a lay person (who believes you when you say it's perfectly safe), it's a death trap. The practice is quackery, plain and simple. Do yourself a favor and stop before it's too late.
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