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WI_Hedgehog

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  1. The resistor will try to dissipate charge current when cell voltage is above 4.20V. (Note how many cells are in Parallel doesn't matter unless there is more than one BMS, as one resistor R is used per cell group, regardless of the number of cells in the group. This is the problem with basing charger current on what the cells in parallel can handle instead of what the BMS can handle.) P = I x E Power is greatest at the Constant Currant -> Constant Voltage crossover point. If we take the charger to be 84V 2A (note 3A was used be the thread starter, but I'm using a more conservative 2A, and not implying his charging habits are the cause of the fire): E (84V) / 20 cells = 4.2V average, varies by cell imbalance I (2A) / 20 = 0.1A P(resistor) = I (0.1A) x E (4.2V) = 0.42W (at normal clamping voltage) However, with two dead cells: E (84V) / 18 cells = 4.67V average, varies by cell imbalance I (2A) / 18 = 0.11A Granted, the balance resistor is going to try to clamp the voltage at 4.2V, but as other cells reach full charge the E across all cells tries to approach 84V: P(resistor) = I (0.11A) x E (4.35V) = 0.48W (attempting 4.20V clamping voltage) Not the end of the world: 1 - 0.42W/0.48W = 12.5% overload. Except the Chinese are usually trying to cut costs and don't build in a 1.5x safety factor. If we assume the higher Gotway Low Voltage Cutoff of 3.30V (average): 3.30V x 20 cells = 66V, 66V / 4.2V/cell = 15.7 (meaning 4.3 cells can die and the wheel will still run) So the worst case scenario is 4 cells die and the wheel still works after being charged. We can therefore calculate the very real possibility 3 cell groups die: E (84V) / 17 cells = 4.94V average, varies by cell imbalance I (2A) / 18 = 0.117A P(r) = I (0.117A) x E (4.7V) = 0.55W (attempting 4.20V clamping voltage) 1 - 0.42W/0.55W = 25% overload this can burn out the resistor, which then charges the cell to cutoff. Granted, we hope the BMS charge shutdown circuit kicks in before 4.7V, and it almost always will kick in far below that. We do have to remember all parts have an acceptable tolerance range, which in the US is typically 5%, so if we assume China uses 10%: 4.27V shutdown with 10% acceptable variance = 4.62V (pretty close to 4.7V, isn't it?) However, on average that's 4.48V, so we might be a bit over-estimating. Then again, @houseofjob has taken me to task for thinking this actually happens in China. Even if we tighten the assumption to 5%, that means some wheels will have 4.48V cutoffs, if the cutoff circuit is reliable. While the numbers can be argued, what we do know though, is we're starting to see circuit boards -and wheels- burning up. (and remember, a 2A charger was used in these calculations, not a 5A Fast Charger)
  2. You may wish to read this series of posts on balancing, which appears to relate to your situation.
  3. HOW EUC FIRES ARE GOING TO BECOME MORE COMMON SUMMARY: To minimize the risk, use the Charging Best Practices. A new BMS thread popped up that relates to what we've been talking about with balance charging. The BMS got cooked. This post builds on how cells get out of balance, and state what happens next. This problem is compounded by using any/all of: recycled cells, charging to 80%, and fast-charging. To quickly recap, one of the dangers in using recycled cells is some cells will be near their End Of Life and start to go out of balance quickly, which is unexpected on a wheel less than two years old. Battery voltage reported in an app and on the wheel is an average across all cells and will read as somewhere between 25% and 50% battery remaining, but the weak cell/cell group voltage will be below 3.00V, and the BMS in EUCs is not monitoring for this condition. As the rider continues on, the strong cell/cell group voltages will continue dropping (say 3.5V), but the weak cell voltage will go below Critical Low Voltage (say 2.5V). This causes a permanent chemical change in the weak cell/cell group where the cell shorts, as if removing it from the circuit. The full charger voltage is applied to the remaining cells, as stated previously. Here's what happens next: Weak/used/older/recycled/aging and "cells in the middle of the pack that can't cool as fast as the outer cells" have less capacity than strong cells, therefore, they reach full capacity (4.20V) sooner. The charger applies power across the whole pack, so the weaker cells which are fully charged must somehow stop themselves from being charged; because they don't have this ability, the Battery Management System does this by employing a balance circuit for that cell/cell group. The balance circuit turns on and bleeds off current from the charger for that cell group. Normally this works well. Consider the following: Fast charging increases the current through the battery pack, and therefore through the cells. The balance circuit resistor must bleed off this extra current. When there are dead cells, the full charger voltage is applied to the remaining cells. This higher voltage is applied to the balance circuit resistor, which must now bleed off the extra current caused by the extra voltage. In both cases, the resistor must dissipate more power than it was designed to handle, and it's a very tiny resistor as seen in BMS pictures. The resister overloads, and the balance circuit can't dissipate the extra power. In the worst case scenario the resister burns out. Either way, the balance circuit can't dissipate the extra power and the cell group continues charging to the point of being over-charged, damaging the cell group. (This can quickly cause those spiky dendrites to form.) Hopefully the BMS stops the charge, preventing a fire. However, this leaves cells in an unbalanced state, worsening the battery condition (a circular problem). Because there is no alert, the user sees a shorter charge and shorter riding time. There is also more stress on what's left of the battery, especially on the under-charged cells. With the added stress, when cells go under-voltage they heat up quickly, potentially starting a fire. (There is a safety mechanism, hopefully it works. If not...) To minimize the risk, use the Charging Best Practices. Once the rider sees shortened charge and riding times, cells should be capacity tested and matched. Given the time & cost in the teardown, testing, and rebuild process, coupled with the expected lifespan of the used cells in an EUC application, it's quite possibly best to start with new cells and rebuild a pack with a new, unstressed, reliable BMS, depending on the condition and number of the surviving cells, plus taking new, longer-lasting cell technology into consideration.
  4. I think it's possibly more like top-tier sellers forcing battery exchanges, and I should have stated that more clearly. Gotway is probably not going to do more than they absolutely have to. If that is the case, it's potentially very short-sighted, and could cause a similar ban to the cheap-China-hoverboard ban where poor lithium battery management caused a fire in a plane's cargo hold leading to a complete ban on low-quality imports for a while. (I know someone who lost $150,000 because the ship was prohibited from unloading the hoverboard cargo. Of course, the Chinese were already paid, so what did they care? They didn't. And that ended a lot of sales in the short term, and led airlines to ban lithium batteries. Again, the Chinese didn't care. So in this case, it's possible we'll see more Gotway/used TESLA cell fires in the future. IF that's the case, I hope it's only a Gotway ban vs. a wheel ban.)
  5. Well, there's the other shoe. I'm going to guess the reputable sellers got the Panasonic 21700 information from Gotway. Gotway possibly bought refurbished cells, and is now recalling them because, quite frankly, using used cells not designed for this task is generally a bad idea. China does a lot of "recycling," that's how all those plastic straws ended up in the ocean. Now we have the evidence they imported TESLA batteries in bulk and are ripping them apart for the cells. I can understand third party sellers using them, figuring "they seem good and are cheap, they'll last six months." And as @houseofjob said, anything for a buck. If Gotway marketed the wheels as having TESLA cells, and they are actual TESLA cells, they're used cells, selling at more than 18650 prices. If that really is the case, I'd cancel my Veteran order knowing that will most likely be more of the same in a different wrapper. I read the top EUC racers build their own battery packs because factory "isn't great." Maybe this will be proof. Gotway quality was not good in other areas, I can understand it could be horrible here too. Lot of speculation, but enough proof for me to wait to see how InMotion's suspension fairs. Thanks guys, this really shed light on the rumors.
  6. So I learned something new, thank you @houseofjob !! Those sellers do indeed say Panasonic 21700 to this day! As far as I could find, Panasonic makes only one 21700 packaged cell, exclusively for TESLA. @Mike Sacristan seems to have gotten to the bottom of where the used cells are coming from. While there aren't any laser markings (my mistake), there are the three unique squiggles on the bottom of the cell as Mike mentions, though there is also the ultrasonic weld joint from the fuseable wire on the top: The seller adds various colored wrappers.
  7. I think you read my post incorrectly, as my position is since TESLA automotive cells are not available to anyone but TESLA, they are only used in-house, and TESLA doesn't have enough cells for their own use, the cells in Gotway wheels most likely aren't new TESLA cells. If TESLA/Panasonic was selling TESLA cells to third-parties, it would be big news. If for no other reason than real TESLA cells are very traceable, Gotway probably wouldn't buy stolen ("siphoned") cells to put in new EUCs. If Gotway was using TESLA cells, there should be evidence of it by now. Without any evidence at all, it is not proper to imply Gotway is being disreputable in that manner. I stated knockoff ("fake") cells are a reality, as are many other items. While a third-party could put knockoff or recycled cells in, so far nobody has reported finding cells marked "TESLA" in their Gotway (that I'm aware of). Real TESLA cells are not available anywhere but inside TESLA batteries. As I stated previously, "other cell manufacturers saw it as a good idea and copied the cell dimensions (the chemistry is specific to Tesla and otherwise unavailable)." Real TESLA cells do have the highest energy density, but are not as well suited to applications outside TESLA vehicles--other chemistries do better. Am I wrong? Has someone found cells marked "TESLA" in their Gotway??? I've only read that one seller was claiming Gotway's 21700 cells were sourced from salvaged TESLA batteries, which is why they were selling Gotways with 18650 cells. Members suspected the claim stemmed from them having a stock of Gotways with 18650 cells that they needed to sell. One seller in China making an unsupported claim. No wheels reported to have cells marked TESLA.
  8. @someguy152, congratulations on being inventive, it's quite an awesome feeling, isn't it? I'm guessing you have three sets of 8S balancing leads to connect to 20S, and are checking voltage when at home with an 8S cell monitor. 24AWG Silicone Wire resists fire and melting reasonably well. Building Battery Packs (in case you're interested) Shrink wrap keeps the mechanical connections from vibrating & breaking, so it is important on packs, not as much on a BMS. Reasonable quality Kapton tape doesn't out-gas or cause chemical reactions, it's what you'd want to use for securing wires in this case. (I'd suggest not using RTV and becoming the Gotway Glue Gun Guy.) Personally, I re-do a job until it's done extremely well (typically 3 to 4 times on something I'm new at). This way I'm not repairing my own stuff 2 years from now, asking what "genius" (sarcastic) came up with that idea, and what the heck was their goal? You're probably okay safety-wise, but if you don't feel good about it take it apart and do it again after practicing on something else so you don't lift the pads on the BMS from too much heat. Congratulations again, I commend your initiative!
  9. He has fairly non-typical riding positions. For stairs he leans back, keeps his shins straight, and tilts his feet forward/down. (I'd fall off.) Also of note, that's a non-suspension EUC in @mrelwood's video, though a suspension should help make stairs easier.
  10. I [by mistake] live in a totalitarian community (in the U.S.) where everything the city government dislikes is considered illegal, and most other things they want you to purchase a permit (permission) for. This has prompted me to plan moving elsewhere in the future. Otherwise it is a very beautiful area. Despite this (and a flat-out ban on Personal Electric Vehicles) I ride hoverboards everywhere (and once I'm better will ride EUCs in public)--this is not without planning and consideration. Being very respectful goes a long, long way. Smiling, waving, and always letting other pedestrians have the right of way de-escilates most situations before you even know of them. Giving police a smile/nod/and wave lets them know you respect their job and are not intending to cause trouble. Dressing respectably most likely helps. Offering to take pictures of those trying to take selfies helps; people hundreds of yards (meters) away have commented how nice it was to do that for other people. I'm guessing most police-being people themselves-are supportive of genuinely nice people. Driving responsibly and at respectable speeds helps (it also minimizes insurance risks for property owners). There are a few places I've been turned away, I just come back later. Generally it's to take pictures of a beautiful area, and pictures harm nobody. If you're a person people want to be around, the PEV is mostly overlooked. I hope it goes as well for you @John Montpetit.
  11. (not directed at any one person) It is unethical to keep spreading the Tesla car battery rumor without evidence. Tesla prompted the development of larger 21700 cell for its vehicles, other cell manufacturers saw it as a good idea and copied the cell dimensions (the chemistry is specific to Tesla and otherwise unavailable). Some Chinese manufacturers make very low quality cells that are put in wrappers to look like name brand cells--this is not exclusive to 21700 cells, some companies copy everything from workout DVDs to clothing (the fashion industry has been plagued by knockoffs for decades). (Note: Tesla cells have no plastic sleeve.) My understanding is Tesla car batteries are laser etched to track product geneology (not like the below cell which is an empty showpiece, and to address volume considerations it may be Lot Number only), so opening any first generation Gotway 21700 pack would address the rumor. I think the rumor is probably false given Tesla cars themselves are only recently available in China, so there are no old vehicles to salvage cells from, and new "budget" cells are really, really cheap there. Also, Tesla re-purposes their own batteries, so those cells are not available for third-parties to salvage. (Some third-party reseller may have gotten hold of some used Tesla cells, but Gotway probably would not use them for several logistical reasons.)
  12. @ShanesPlanet He's pretty crash-proof.
  13. EUC World app and a Bluetooth earbud. Doesn't annoy the world and lets you know what's going on at all times. (one earbud, so you can also hear the world around you)
  14. In that last picture it looks like a sea mine. Or terminator robot... 18650's have 10% more capacity per unit-volume than 21700 cells, so they take up less space for the same amount of total power output. Also, the amp output is higher when putting more cells in parallel.* *EDIT: Someone on this forum did the math with the latest cells, though I can't find the post. Looking around it seems it depends on the cells, 18650 vs 21700 varies...depending... So @mike_bike_kite might be right (see the next post).
  15. I believe the concern is uneven braking. Yes. During heavy braking with the Extreme firmware the acceleration/deceleration is stronger than factory. In my experience, stopping very quickly is rarely done in a perfectly straight, balanced manner. Example: You are going "fast" (3x the speed of walking=10MPH=16kph), looking around and enjoying the ride. Also watching your path, you spot a hazard which is now close in front of you, mostly on the right. Lean back and heavily brake. At the same time, steer left to try to avoid the hazard. Most of your weight/down-force is applied to the left (closer, slower moving) wheel. The right wheel, having less down-force on it but almost the same braking force, skids under extreme braking, and loses traction (skidding is less traction that not skidding). The right wheel skidding and left wheel still having traction turns the miniPRO left, so the steering bar presses on your right leg, which is the input for the miniPRO to steer left. The miniPRO steers left, causing the right wheel to slip out from under you. You must put your right leg down to catch yourself from falling, so your body slows. The miniPRO keeps going, so your left foot loses contact, losing control of the miniPRO. The miniPRO keeps going at whatever speed you fell off it at, and direction you fell off at (left), possibly eventually crashing into something, or running out into traffic and being hit by a car. Your shoulders and back are sore from the stress of the jolt they just took, even though you did not fall down. Note the example is at a comfortable riding speed, not the fastest speed, which is 1.5x faster yet (much faster). Small things that cannot be easily seen in the distance can appear very quickly given you are actually moving very quickly. (I once came upon a service panel in the sidewalk that was new and did not have filler around it, leaving an 8"=20cm "hole" around it. Because of the plastic it looked normal at a distance, but not up close. Sometimes even a sidewalk that is pushed up 1.75"=4.5cm looks normal.) Braking hard in a straight line requires much skill, and equal wheel traction (no sand, gravel, dirt, or bumps). This happens sometimes, but most often something complicates the situation. Both tires must be the same (sometimes they are a different shape, even if they are the originals that came with the miniPRO), and definitely must be at the same inflation pressure. There are solutions I have found: Wear safety gear. Maybe to some it looks a little funny, but you will hopefully concentrate on braking instead of how to not get hurt. This aides in you not getting hurt. Tether your miniPRO to your gear, enough to slow it down. Not so much if it is hit by a truck you are dragged with it. Drive slower. Just because you can go fast does not mean you should go fast. Always drive within your limits. We like to say "ride" but it is driving. If you are tired or distracted, dehydrated, cold, etc., drive slower. Think about what can go wrong and drive defensively. Many drivers have not yet been in an accident. An accident can be your fault, or someone else's. If you hover a lot you probably will be in a bad situation, how bad it turns out depends on how smart you are before it happens.
  16. Free-spin speed is one thing, but the torque curve is another. Gotway goes fast, but the torque falls off and cannot necessarily support the rider if they over-lean or hit a bump (depending on many factors). I'd like to see a torque curve before pushing a wheel to its limits, rather than to discover torque limits by exceeding them. @RockyTop said many things, though to focus on this one: I would not want to fall off the back of a pickup truck at 60MPH=100kph. Even as a stunt person this is very dangerous, though with this you hopefully choose when to jump, and how to jump. You would not jump when you could be hit by traffic, or slide into a building or guard rail, or over a cliff. (Maybe you would do that, though probably only once...) I also would not want to fall off an EUC at 60MPH=100kph. I think even if we do not consider the driver, the EUC is probably not able to be restrained from being a projectile and causing serious damage. It is at these speeds as @RockyTop sort-of mentions we should probably be looking at electric motorcycles with a front wheel, turn signals, brakes, etc.
  17. A lot of excellent information has been provided in this thread, to add a bit more: Cell balancing isn't overly important because battery packs have uneven wear due to uneven heating. Cells on the inside of the pack cannot shed heat during discharge and charge, and heat ages cells. (Cells generate heat, and are surrounded by cells generating heat, with no real cooling ability. This is partially why Tesla car company flows coolant through the battery packs.) Passive balancing works well, but not with fast chargers unless the BMS is designed to handle fast charging (they are not). While cost is a driving factor in supplying low-current chargers, so are parallel charging considerations. (Parallel charging adds many non-obvious risks; EUCs usually have many cells in parallel.) Passive balancing does not monitor for dead (0V) cells. 0V cells pass the full current and voltage to remaining cells, which is one reason why BMSs shut down charging if any cell is over-voltage. Because there is no alert, this can lead to prompt battery failure as cell balancing is not occurring, worsening the problem. It is up to the owner to determine the EUC charge cycle is "shorter than normal" and range is degraded. This is not stated in any EUC manual I'm aware of, making the risk to the owner much higher than necessary. Charging batteries to only 80% does not activate balancing circuits, which is dangerous and does not prolong battery life in EUCs. 80% charge is done in R/C with Smart Chargers that balance cells at the set voltage--they are designed for this, EUC BMSs are not. As such, usual practice should probably be to charge batteries to 75% for storage, 100% for usage.
  18. @mrelwood, I think @Jason McNeil doesn't know. The forum has a wide knowledge base from engineers, extensive R/C designers*, multi-PEV owners, and members who are super smart and researched the internals of their wheels and shared their knowledge here. An EUC design company doesn't even have that broad expertise base. EUCs are new to the market and tended to be obsolete before having battery issues. (We learned "what didn't work" from the hoverboard fires.) Newly introduced "fast" wheels are only recently being used as commuter vehicles putting on lots of miles. Batteries used to be small, so standard chargers were fine, it's only recently fast fast-chargers with 80% charge capability were introduced. This is all new! Just a few years ago top speed was 12 miles per hour with limited range, now EUCs have gone from "toy" to serious investment! (Who ran their Luffy this hard? ) --- *A bit of history: R/C guys used to run NiCd, when the hobby switched to LiPo they went through this leaning curve. At first there weren't a lot of people running lithium, so there wasn't a lot of risk. As more people ran lithium they had more ideas, not all of them good, and the LiPo fires became a huge topic, as it still is. EUCs are now getting into massive batteries and very-parallel with huge current draws as R/C did, so charge time shortcuts are inevitable, as are lithium fires. The experienced R/C guys warn about parallel charging risks, and use high-end parallel charging boards where every every cell is fused, on smart chargers that hook to laptops and record everything. If anything looks wrong it alarms and shuts down, the packs are removed from parallel charging and are single-charged to see where the problem lies. Smart BMSs would certainly help the EUC situation. --- Note we don't know the cause of the fire, and don't want to imply charging was/wasn't part of it (we have no way of knowing that). This thread should probably be split into a second battery charging thread. @Unventor @Chriull @meepmeepmayer
  19. Really glad to see you suffered only minimal damage! A few scrapes and bruises and being stiff for a few days is excellent for splatting at that speed! That says a lot about your gear choices and skills! Also glad your custom wheel took the crash without being smashed! It's unfortunate this happened, though the good thing is the damage is limited to minor cosmetics for you and the wheel. I ride in the city for transportation because it's faster than a car during high traffic hours. Yesterday I slowed way down due to fatigue and not feeling sure of my skills while tired, even stopping to rest a few times. What happens if we're riding alone and crash? If the PEV is damaged? If we get a flat? If the battery overheats? You have a reliable wheel and gear--smart, very smart.
  20. Inside Veteran EUC: I told you no use Tesla battery! Look! Wheel on fire! Smoke everywhere! We get bad name before make first sale! Gotway: Same problem, different day. King Song: This why our wheels so slow--more safe. InMotion: (hope they no steal our suspension idea)
  21. This is the first wheel I've been really excited about. I want a fast wheel that is well-built, especially because the faster you go, the better the build should be; going fast in/on a "parts bucket" is kind of like the pod races in Star Wars...which is not great if you're the one crashing. From the looks of it, the V11 answers that call. Plus, it has batteries in the hub! [edit: almost in the hub] It's very controllable due to a great design! I was looking at the Gotway torque curves this weekend (@Chriull's post) , and am not so impressed--not to say anything against Gotway for this reason, they do allow disabling the warnings and don't force safeties on the rider, so that's all good. It is more to say that InMotion's speed limit of 35MPH=56kph is understandable. I'd still like more speed (say 40MPH=65kph) with the understanding this is beyond the torque curve's safe zone and riders should be very careful, knowing they should not push the wheel hard. I'd also like for InMotion to publish their torque curve so we know what to expect when it comes to riding fast. As @Unventor says, this [first] release may not solve all the world's problems, but it is a huge improvement, and many people will be very happy. In the real world, not on a pretend specifications sheet, it appears to be a very, very good wheel, and I look forward to its release.
  22. Thank you. I'm not convinced this fire is a charging or battery specific issue either. Having followed lots of Gotway threads I would personally want to open a newly ordered wheel and check everything over first. Gotway's Quality Control department seems to be the Glue Gun Guy: "If I can't glue it, screw it--it's a warranty issue and we have none."
  23. Summary: Always Balance Charge EUC Batteries It is true, charging to 4.0/4.1V maximum doubles the cell life. Discharging to 3.30/3.35V minimum doubles the cell life. (exact voltage depends on the cell chemistry) The problem is we cannot currently measure cell voltage, only pack voltage. So we assume 4.0V = 80% capacity & 3.3V = 30% capacity for the pack applies to each cell, but it does not. As @Archee Jan Bloch correctly points out, when the battery, which is the whole pack of cells, reaches "low voltage," it is for all cells in series. We do not usually know what the voltage of each cell is. If one cell is weak the remaining cells in parallel will support it. If the battery is balance charged, more time will be spent recharging the weak cell group to bring it up to the level of the rest of the cells. The one cell will always be weak, but balancing brings things back into balance. @Archee Jan Bloch continues saying that if the battery is charged but not balanced, the weak cell/cell group will not be fully charged, and during the next use be under more stress to perform. The weak cell(s) will get hotter, the internal resistance will go up, and the weak cell(s) will have to work even harder to supply energy, depleting the weak cell(s) even further. From observation he is correct: when I discharge a balanced pack from 4.20V average to 3.00V average, the weak cell has usually started out at rest at 4.19V, maybe 4.18V. It has to work harder than the cells at 4.20V. And honestly, this is most of the cells in the battery--only a few cells will be at 4.20V. At the end of the ride, the strong cells are at 3.10V, but the rest at 3.0V with some at 3.29V & 3.28V. When I recharge without balancing, the strong cells are at 4.01V, the rest at 4.00V, 3.99V, 3.88V, and usually one or two stragglers at 3.97V. These stragglers are of concern--a normal balance charge with a smart BMS would put them at 3.99V, but they are at 3.97V. On the next use, the voltage spreads out: 3.10V, 3.0V, 2.99V, 2.98V, 2.97V, 2.96V, 2.95V. Here's where it gets interesting: recharging the second time, some cells will be at 4.1V, but the low cells of interest-the stragglers-will be at 3.93 & 3.92V. If I take the unit out for a third ride the voltage will be spread out from 3.2V down to 2.85 & 2.82V. So by not balance-charging for two charges, damage is being done to 1 or 2 cells. This problem does not develop so fast with cells in parallel, and when discharging to 3.30V=30% instead of 3.00V=0%, but over 10 charges it can still happen. The cells that reached 2.82V will always be damaged, so if I balance charge and then keep charging to 80%, those cells or cell groups in parallel are going to continue to degrade and die. As @Archee Jan Bloch points out, once a cell group is dead, more strain is put on the rest of the battery both during use and during charge. Because there are so many cells we do not usually notice unless checking voltage as @Chriull alludes to. This is where the real problem comes in as @Archee Jan Bloch points out, because when the second cell group dies the remaining cells (18S in a 20S pack, or 22S in a 24S pack) get the full charger voltage, and the bleed-off circuits cannot keep up with the power supplied by the charger, especially a fast-charger! Some cells are at 0V, some cells are 4.35V, and the BMS circuitry is ready to burn up from dissipating so much energy--the charger is supplying: 4V x 20V/cell (normally) = 80V 80V over 18cells (2 dead) = 4.44V/cell The next balance charge at 100% voltage is: 84V over 18cells (2 dead) = 4.66V/cell! If you charge 18 cells to 4.66V/cell, then take it out and start off from a dead stop, which is very high amp draw.... The wheel has all these overcharged cells that built up spikey dendrites in them, making them effectively "little bombs." (perhaps a bit dramatic, but you should see an Remote Control (R/C) race car, heli, or plane go up in flames at 80+ MPH! THAT's dramatic!) This is why the racers that cheat by overcharging to 4.35V/cell or 4.38V/cell or heaven forbid 4.40V/cell dispose of their batteries after 2 races. Charging to 4.5V/cell would certainly be taking risks for a LiPo fire. The 4.66 volts per cell an EUC charger would put on a battery is crazy. It is for these reasons R/C people strongly warn about charging in parallel, where as EUC people do that every charge. (R/C says do not charge battery packs in parallel so you know the health of every single cell and can minimize the risk of catastrophic failure.) R/C does not use a BMS, but rather a smart charger and known discharge cycle--the smart charger is the key, which is why a Smart BMS is important in EUC use. Not knowing individual cell/cell group voltages is why charging EUC batteries to 100% every time to balance the cells is so important, and will extend the life of the battery for EUCs. If you had a smart charger like in R/C that knew the state of charge of every cell and balanced the cells to 4.00V/cell, then it would be okay to charge to 80%. Not knowing individual cell/cell group voltages is why EUC batteries should not be run below 3.3V/cell for safe use. It keeps the cells that have lower voltages from discharging too low, heating up too much, and aging faster than necessary. (I note the Segway/Ninebot miniPROs that I own have Smart Battery Management Systems that are always active and charges the batteries to 80%=4.00V/cell. If I charge to 100% full the BMS has charged and balanced the cells to 4.00V/cell=80%. Since the Smart BMS is always active, even when the unit is off, the batteries should not become unsafe. I should not be able to use the board so that any cell is discharged below 2.87V (and this is what happens as far as I can tell). I do not know about their EUCs.)
  24. My understanding from the members here is all* major EUC manufacturers use top-balancing circuits. This is smart, as it is cheaper, smaller, lighter, and allows the most power into the cells the fastest. *The exception may be Ninebot/Segway, though I don't know. Both miniPROs I have use active balancing circuits that are always running and monitor each cell, even when the batteries are removed from the units and just sitting around. Because of per-cell monitoring and reporting, it is easy to see how fast a cell can start going out of balance and by how much. The miniPRO charges to 4.00V (80%) and throttles at 30%, encouraging the owner to use only 50% of the batteries total capacity, and by doing so extending battery life from 500 charges to 2000 charges (assuming battery life means the cells can hold only 80% of their rated capacity). Because it appears in general EUCs do not monitor individual cells, nor cell groups during discharge, best practice seems to be (and I belive @Chriull mentioned this in the battery charging threads): Let the battery cool before charging (reduces risk, extends life). Do not charge until going riding. (The further the charge is above 50% = 3.85V the more the batteries wear out, even during periods of non-use). Charge fully (100% = 4.20V, engages the balance circuits), Check the voltage at full charge (there are devices made to do so if the EUC app doesn't report voltage). Voltage should be 100% of rated voltage = 4.20V x # cells in series. (4.2V x 20 = 84.0V, 4.2V x 24 = 100.8V). If the voltage starts to fall below normal, there's a problem developing--get the problem fixed immediately. Plan your ride so the battery is not discharged below 30% = ~3.30V. (Gotway's 0% remaining is 3.3V, so riders are forced to observe this. King Song 0% is 3.0V, so the rider can self-manage their usage.) Wheels need a certain amount of power to move you. Power is: Voltage (volts) x Current (amps) = Power (watts) so as voltage drops current goes up, because the amount of power needed to do something doesn't change. Current causes heat, heat cooks the enamal on motor windings, MOFSETs, and battery cells, therefore the less heat that is generated the longer things last, meaning the more voltage a battery has the fewer amps that are needed to do the same amount of work, and the cooler things are. This is part of why using the last 30% of battery capacity is so hard on everything: not as much voltage and more current. This is also why speed is limited as battery capacity drops. Think of it this way: Voltage is like strength. Current is like the amount of motion. If you have three hours to move a large pile of gravel from the road where it was dropped off to the back of your yard, it is better to take a wheelbarrow full of gravel and use your strength, than it is to use a child's sand pail and run back and forth very fast--you will soon be hot and exhausted. You will need more time to recover. If your wife does not let you sleep (recharge) fully, and if you get a new load of gravel every day for a month, by the end of the month you may be worn out and not able to continue. However, if you used the wheelbarrow each day (strength) and were allowed to sleep (recharge) fully, you would be much healthier--not like new, but much better (maybe you would not like your wife for making such a large project, but you would still love her and continue on with her). : Your battery will work better and last longer if you charge it fully, and do not exhaust it completely. Several members have said it is important for the owner to be able to monitor cell voltage. I think it is not so important IF you follow the usage guidelines above and expect to buy a new wheel every two years or so, or at least replace the batteries when they start to age (which is dependant on how they are used, the quality of the cells, and time). That is an IF. As wheels are made with more batteries in parallel and owners hope to use them for more than two years (or sell them to someone who will), it does become more important to monitor the cells/cell groups more closely. @Chriull and @RagingGrandpa, @meepmeepmayer and others have very smartly detailed in many posts why Smart BMS are [becoming] important (their posts are very informative and well-thought). tldr: Charge the battery 100% using the charger that came with the wheel and you'll be fine. When the wheel starts to wear out and/or the green "100% charge complete" light no longer illuminates, buy a new wheel.
  25. Note of caution: Charging to 90% doesn't engage the balancing circuitry in the top-balancing boards I've seen. I understand Gotway uses top-balancing boards. In theory, this could allow a cell to get out of balance and drop below critical voltage. At such a depressed state a cell can undergo a chemical change and permanently die, or overheat during use. The death of one cell affects those in parallel with it, amplifying the problem and increasing the risk. --- Also of note is cell design. In highly simplified terms, the more lithium there is in the anode the more powerful the cell. However, when charging, lithium ions reaching the anode and don’t form a smooth layer, but instead form spikes called dendrites, which can grow large enough to pierce the insulator and create a short circuit between anode and cathode, possibly resulting in a fire, not necessarily during charging, as heat (thermal expansion) / vibration could cause the dendrites to piece the insulator. --- Oddly, the two can work together. Dendrites don't easily break up, which lessons the cell's charge capacity, causing the rest of the cell to work harder and heat up during discharge and thermally expand. With a lowered charge capacity the cell also charges up faster, possibly forming more dendrites. Also of note is battery life can generally be doubled by not charging above 80% capacity, or not running the battery below 30%. So if you have to pick one or the other, it's better to charge the battery fully than it is to discharge the battery fully. An added benefit is charging the battery fully engages the balance circuits, and also doesn't cause a decrease in speed due to lowered battery voltage while riding like would more easily happen when charging to only 80%.
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