| Electronics > Projects, Designs, and Technical Stuff |
| Panasonic NKY467B2 36V 15AH 540Wh. Ebike Li ion upgrade, burning my father'ass? |
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| Zucca:
--- Quote from: Siwastaja on October 11, 2018, 06:16:29 pm ---I built this in 2014-2015 --- End quote --- That's impressive, how many cells in parallel? Wow... |
| Siwastaja:
--- Quote from: zucca on October 11, 2018, 10:22:15 pm ---That's impressive, how many cells in parallel? Wow... --- End quote --- That one in the video, IIRC it was a 7s46p pack, a 3.2kWh module. Twelve such modules total 39 kWh. |
| Zucca:
Let's see if I can do some homework. 7s46p= 322 cells Nominal voltage per module = 3,6V*46 = 165,6V Charged voltage per module = 4,2V*46 = 193,3V Low voltage per module = 3,3V*46 = 151,8V Nominal Capacity per module = 2,78 * 7 = 19,46 Ah 3,2Kwh / 322 = about 10Wh/cell 10Wh / 3,6V = about 2,78 Ah EDIT wrong of course Massive, I would be scared to touch that beast... I am managing that 540Wh bike battery with carefull triple checked slowly baby steps, I can't imagine the safety you are dealing with those monsters! Respect. Good bathroom reading: https://batteryuniversity.com/learn/article/safety_concerns_with_li_ion PS: Starting to design a storage engergy for my home in my spare time... let's see. If I will not burn that bike I will not burn my home neither. :popcorn: |
| Siwastaja:
--- Quote from: zucca on October 12, 2018, 01:23:29 pm ---7s46p= 322 cells Nominal voltage per module = 3,6V*46 = 165,6V Nominal Capacity per module = 2,78 * 7 = 19,46 Ah --- End quote --- Other way around. 46 in parallel, 7 in series. So only 25.2V nominal. But put 12 of such modules in series, and it's 300V. --- Quote ---3,2Kwh / 322 = about 10W/cell 10W / 3,6V = about 2,78 Ah --- End quote --- Wh, not W :). Yeah, Samsung INR18650-29E, nominal capacity 2.85Ah, nominal energy capacity 10.3Wh. This was and still is quite a good cell. At 220Wh/kg, it's not the newest or most energy-dense hi-tech anymore, but still available widely for a good price. Best commercial cells are around 280Wh/kg now. The trick is how to construct the cells into modules with as little weight, size and especially price overhead as possible. BMS in a part of this equation. Over a decade ago, it was OK to be expensive when the cells were even more expensive. This has changed. Cells for a 30kWh pack, enough for a small passenger EV, would only cost around $4000-5000 for a manufacturer, I guesstimate. For Tesla, probably even less! A traditionally overdesigned BMS would easily cost some $300-500 on the top of it. The EV battery system design really being a cost-limited process now, this money is directly away from putting more cells in (say, about 2kWh extra for saving $400 by using minimalized BMS). This would translate to 10km extra range for an EV, something that isn't meaningless! The same is true for grid/home/solar/etc. energy storage, except it's even more cost-driven, because the weight and size factors are less important than in a car. And, every customer wants as much capacity as possibly, limited only or mostly by their budget. The question always asked is, how much storage do I get with $100, or with $10k, or with $1M. Cell prices being what they are, that means you optimize on labor (and material) to assemble them into packs, and this includes the BMS cost (BOM, installation, and service). While, of course, not compromising safety or other pack-level features the customer needs. Now, the raw cells costing at bit over $200/kWh for us small players, Chinese packs that use these cells cost around $400/kWh, and Western packs with the exact same cells start from about $1000/kWh, if not more, and even then, are quite some special snowflakes. Or as nctnico said: there are almost no decent li-ion packs available on the market! --- Quote ---Massive, I would be scared to touch that beast... I am managing that 540Wh bike battery with carefull triple checked slowly baby steps, I can't imagine the safety you are dealing with those monsters! --- End quote --- Welding the first side is easy-peasy since you won't short anything... But after turning the module around, you need to be very careful when aligning and installing the copper sheets, and have some measures against the welder robot hitting between two adjacent copper plates by mistake, shorting them out. I use plastic covers so that one segment is accessible at once. Adds a bit of manual work to the welding, however, moving the plastic cover 6 times during the process. After this, the modules use fixed plastic covers so that only the ends are exposed. A good physical design doesn't let the copper sheet ends touch each other. In these modules, they were at the opposite ends, and opposite sides as well. Cardboard covers tightly taped over the copper contacts until the modules are ready to be installed are still paramount; someone could still put the module on a metallic table! 7s is enough voltage to cause some serious arcing. Accidentally shorting 1 cell is not such a big deal (of course, the chances are, you are doing some hidden damage to the cells with this act. If nothing else, if the short is long enough so the PTC trips in the cells, it will have slightly higher DC resistance afterwards.) I do have a large tub full of water* to push the full battery into in case of a fire (and a big garage door right next to this, so it can be pushed outside). Although, such an incident is very unlikely, but being prepared is still a must. We have tried to induce something like that small scale, with no success. In addition to basic overcharging (30V) and short circuit tests, we have tested (both on purpose, and by accident) what happens when you accidentally apply approx. 100 times of too much TIG welding energy to the cell, so it zaps through the cell case, causing a massive hole directly to the insides and charring the electrode roll in the process. No fire, no explosion, no smoke. Li-ion cells have quite some advanced separator materials (and possibly other tricks) nowadays. (But don't count on these features; if you do, you lose one important safety layer! Remember that the underlying chemistry is very volatile and dangerous, and the advanced safety mechanisms are not supposed to be put "on test" in normal operation / unintentionally. They still need to produce these safety features as cheaply as possible!) *) Note that despite some "Battery University" style myths, water - a lot of it, quickly, everywhere, submerged - is the preferred way to deal with li-ion fires. There is no metallic lithium present; or anything else which would react violently with water. Water has the greatest cooling effect, and has the best chances of removing energy quickly enough to prevent or stop thermal runaway.) Working with large packs really requires displicine, lack of disturbances (put your phone away, don't have chatty coworkers, don't "show off" your lab), and short working terms (preferably no more than half an hour of mechanical, repetative work at once). Add insulating tape or heat shrink tubing to all metallic tools. Use insulating temporary covers everywhere - a standard bath towel is great if you have random shapes with a lot of exposed contacts everywhere. When done, be a bit too OCD and perfectionist on adding different types of tape, glue, plastic covers etc. I often add pieces of both Kapton and then fiberglass tape on 18650 positive ends so that any sharp edge won't cut through the thin insulation the 18650 cell comes with - especially at the edges and corners where the copper edge resides. Such "tape donuts" are used by some laptop battery pack manufacturers (but not all!) as well, since some incidents of shorted cells have been reported. IMHO this is an issue which should be solved by the cell manufacturers, but it isn't. Oh well... |
| nctnico:
--- Quote from: Siwastaja on October 11, 2018, 06:16:29 pm --- I built this in 2014-2015: . It's still in use, I now use it for building battery packs for mobile robots in a related startup I now design for... Not using nickel strip but direct copper interfaces is both cost and performance optimization. --- End quote --- It is not clear from the video but I don't see a slot in the copper plate to force the weld current through the top of the battery. Nickel strips always have these. When welding batteries you are doing a series spot weld which makes two weld in one go. Without the slot the current can go directly from one electrode to the other electrode of the welding machine making the welds vary in quality. I'm also not sure whether welding copper to nickel is a very good idea due to the metals being different. |
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