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WI_Hedgehog

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About WI_Hedgehog

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    Milwaukee, Wisconsin, U.S.A.
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    miniPro, KS-14SMD; EUC World Supporter

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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.
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