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| Learning Path for buiding my own BMS |
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| Siwastaja:
--- Quote from: mc172 on July 24, 2019, 08:48:56 am ---Please can you tell us which ones were not any good in your experience? --- End quote --- OK: bq24610 charger and power path controller IC: * Overvoltage detection does not turn off the battery disconnect FET, which can be verified by taking a close look at the block diagram: they are missing the obviously needed connection there. Yet they claim there is an overvoltage protection. It's just... doing nothing, just preventing the buck from oscillating, which already works that way through the normal FB mechanism. * Implements a so-called overdischarged cell conditioning cycle, which cannot be disabled, with wrong parameters. Conditioning current cannot be practically adjusted to be small enough or disabled; the pin to control that is shared with the CV stopping current function. A proper IC should deny charging overdischarged cells completely; or possibly implement a proper conditioning cycle as per cell manufacturer specifications. bq24610, instead, runs a self-destruct program, increasing the risk of copper dendrites shorting the cell and causing thermal runaway. * There were some other issues I have already forgot about. * One can argue that a proper BMS should be able to protect against all the abuse bq24610 is going to put through. This is not a valid line of thinking, as the number of safety layers has been reduced to 1. bq76925PWR AFE: * Balancing doesn't have timeout or any other means to prevent it from killing a cell. Combined to a notoriously unreliable I2C communication, the risk that the BMS gets stuck into the cell discharging state during the lifetime * number of units on field is just way too high. This is a showstopper. The rest are just babbling: * Datasheet and appnotes disagree on the connections. You need to guess or do your own reverse engineering to verify proper circuit. * The current shunt amplifiers suck in their analog performance so much that you can't really use them for coulomb counting, which is usually what people want from such BMS analog front-ends. A separate $0.20 INAxxx chip from TI will do over 10x better job, enabling low-value shunts to be used, for easy thermal design and energy savings. * I'm sure there was something else I forgot about already. I ditched this before starting to write code for it! |
| redgear:
--- Quote from: Siwastaja on July 24, 2019, 08:17:28 am ---My personal learning path has been: * Looked at overly complex redistributive balancing schemes, built prototypes * Teardowns of failed commercial BMS units, recognized the usual issue: too much quiescent current, possibly because being stuck in the wrong state, killing the cells * Designed an overly simplified distributed BMS system. Still a valid design * Tried two TI BQ ICs in another product, trying to "reduce development time" since I had much more to do than just battery management. An utter failure. Removed all TI BQ ICs for production PCBs, redid from scratch. --- End quote --- Cool! Thanks! --- Quote ---What's the difference? --- End quote --- Sorry, I messed up while foramatting, That was supposed to be in brackets like this: Over Charge Protection(Overvoltage) Over Discharge Protection (Undervoltage) --- Quote ---I assume you mean temperature monitoring, and specifically the upper limit. Do not forget to add lower limit for charging. Limit charging currents below 25degC linearly, until you reach zero current at about -5 degC. (Traditionally a step function of no charging below 0 degC, full charging above 0 degC is used. I don't recommend it, as full charging currents at +1 degC can be damaging to the cell lifetime.) You can't stop thermal runaway if it happens, and it's almost never happening due to anything that can be prevented by BMS temperature monitoring. The only such scenario I can think about is totally failing the thermal design of the pack, or completely miscalculating the fuse rating. Don't forget the traditional fuse! So thermal runaways happen due to internal cell failures, physical damage (puncture/deformation), severe overcharge (which you have already covered by voltage monitoring). --- End quote --- Cool thanks! Yep i'm planning to add cell level fuses to the pack... --- Quote ---Remember you need very little balancing, and not much muscle to do it. I have seen systems designed to shunt currents in excess of 1A, and fail at thermal design with stuck-on balancer. Don't do that. My system balanced at 40mA, and was still fully able to keep a repurposed 300Ah pack with damaged cells with increased self-discharge in balance. --- End quote --- Thank you for educating, I looked at the BQ's datasheet and it provides only 27mA of balancing current. I did not know if that was enough, my next topic on the forums would have been 'How to select proper balancing currents for battery packs' --- Quote ---I haven't seen any formal, scientifically valid definition for this. I have a feeling it was a trendy buzzword in battery academic papers a decade ago. --- End quote --- Just a percentage factor of the cell again based on the Internal resistance? --- Quote ---Remember to add cell temperature dependency to this. Charging does less damage at higher temperatures. Somewhere around 30-40 degC is optimum. I have measured greatly decreased cycle lives when cycled in a refrigerator (at +6 degC) at the rated current. --- End quote --- Noting down this.I read about this in a research paper, I was considering to add external cooling fans when the temperature was high but did not give a thought about lower temperatures. --- Quote ---Cycle counting can be difficult, as the cycles are of different length, and with different starting points. A cycle between 60%->30%->60% is almost meaningless for the battery life. A cycle of 100%->70%->100% if a lot worse. A cycle of 100%->10%->100% is not that much worse anymore. --- End quote --- Can you explain this a bit more? I read it is preferred to discharge li-ion batteries in the 20-80% range but don't know why. |
| ogden:
--- Quote from: Siwastaja on July 24, 2019, 08:40:15 am --- --- Quote ---Leave BMS project alone and start "learning electronics" project now. --- End quote --- But I don't completely agree with this. People are different. For example, I'm not capable of working with a "learning project" approach. I need a real project, and often something far more complex and difficult than anyone would recommend to me. --- End quote --- Yes, exactly - people differ. Your approach of picking tasks are fine - because you shall be considered skilled and knowledgeable, you know how stuff works. For beginner ANY project will be far more complex than his non-existent projects of past. Speaking of complexity - my rare & occasional projects usually are about "blank areas" where no solution exists (yet), but when I was kid - I was building multi-vibrator blinker, not TV set. That blinker still were far more complex than anything I did before. You see what I mean? |
| Siwastaja:
--- Quote from: redgear on July 24, 2019, 09:11:02 am --- --- Quote ---Cycle counting can be difficult, as the cycles are of different length, and with different starting points. A cycle between 60%->30%->60% is almost meaningless for the battery life. A cycle of 100%->70%->100% if a lot worse. A cycle of 100%->10%->100% is not that much worse anymore. --- End quote --- Can you explain this a bit more? I read it is preferred to discharge li-ion batteries in the 20-80% range but don't know why. --- End quote --- Don't know all the exact mechanisms, but at least charging near full causes: * Expansion of the graphite anode, leading to cracking and reformation of the SEI layer, causing it to become thicker and blocking parts of the anode, increasing the resistance and decreasing capacity. * Plating of the metallic lithium at the anode. Avoiding large charge currents near the top, or even better, avoiding the top altogether, minimizes the effects. If you cycle continuously near the top, you are repeating the "worst" area of operating all the time, with no much benefit (only a slightly higher voltage) compared to cycling at a lower point. Practically all calendar fading similarly happens only near to top, between about 70-100%. Unintuitively, it seems that on some production cells, 80% may not be better at all, and even slightly worse than 100%! Below 70%, the real benefits start to show. Hence, store at 50% or less. At the bottom, a significant amount of extra hysteresis heating happens by an exothermic reaction that cannot be properly modeled as DC resistance (in addition to the higher DCR at the bottom). When charging near zero, it's an endothermic reaction, so the cells cool down quickly, internally (DCR-induced I^2R heating may make this harder to see). If the currents are high, the extra temperature cycling probably isn't good for the cell life, either. Hence we reach the recommendable region of interest between about 70% and 20%, where the cycle life can easily be like 5000-10000 cycles even if the cells are only specified to 500 cycles 100%-0%. |
| redgear:
--- Quote from: Siwastaja on July 24, 2019, 09:33:41 am --- --- Quote from: redgear on July 24, 2019, 09:11:02 am --- --- Quote ---Cycle counting can be difficult, as the cycles are of different length, and with different starting points. A cycle between 60%->30%->60% is almost meaningless for the battery life. A cycle of 100%->70%->100% if a lot worse. A cycle of 100%->10%->100% is not that much worse anymore. --- End quote --- Can you explain this a bit more? I read it is preferred to discharge li-ion batteries in the 20-80% range but don't know why. --- End quote --- Don't know all the exact mechanisms, but at least charging near full causes: * Expansion of the graphite anode, leading to cracking and reformation of the SEI layer, causing it to become thicker and blocking parts of the anode, increasing the resistance and decreasing capacity. * Plating of the metallic lithium at the anode. Avoiding large charge currents near the top, or even better, avoiding the top altogether, minimizes the effects. If you cycle continuously near the top, you are repeating the "worst" area of operating all the time, with no much benefit (only a slightly higher voltage) compared to cycling at a lower point. Practically all calendar fading similarly happens only near to top, between about 70-100%. Unintuitively, it seems that on some production cells, 80% may not be better at all, and even slightly worse than 100%! Below 70%, the real benefits start to show. Hence, store at 50% or less. At the bottom, a significant amount of extra hysteresis heating happens by an exothermic reaction that cannot be properly modeled as DC resistance (in addition to the higher DCR at the bottom). When charging near zero, it's an endothermic reaction, so the cells cool down quickly, internally (DCR-induced I^2R heating may make this harder to see). If the currents are high, the extra temperature cycling probably isn't good for the cell life, either. Hence we reach the recommendable region of interest between about 70% and 20%, where the cycle life can easily be like 5000-10000 cycles even if the cells are only specified to 500 cycles 100%-0%. --- End quote --- Thank You! :) |
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