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| User Info | America's Scam, Part 65023; entered at 2017-11-26 11:24:07 | |||
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Tickerguy Posts: 204032 Registered: 2007-06-26 
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Quote:Actually, it's bad, but not quite as bad as your numbers reflect. Batteries are rated in kilowatt-hours (kWh) which is a measure of energy, rather than kW which is a measure of the rate at which they can deliver power. But doing the calculation based on energy still makes Tesla a moron. That was what I was accounting for. There are allegedly eight 100kwH batteries in that truck cab. Therefore if you wish to charge them you need 800kwH + losses. Losses are pretty easy; it's about 80-85% efficiency to charge a Li-chemistry battery during the bulk phase (which is up to ~80% charge state) then it starts to drop off in efficiency. The electronics loss is pretty simple too -- 90% is excellent (and I will assume "excellent".) Losses are multiplicative (so the total loss is 85% * 90%, so ~76.5% efficient.) This means to get 800kwH into the battery you must put in 800kwH/.765, or 1,045kwH (just over 1 Megawatt-hour.) Now the problem is that you want to do it in 30 minutes, which is 1/2 hour. So you must multiply the power delivery rate by TWO. Thus you must deliver 2 Megawatts to the vehicle for 30 minutes, which is 1 Megawatt/hour. 800kwH of that will get stored, and ~200kwH will be dissipated by the pack and electronics as heat, and since we're doing it in 1/2 hour the pack and electronics have to be able to sink the heat at a nearly 500kwH rate without catching on fire. If the battery was just in use at high rate (to move the truck) then it is already carrying some amount of heat in its thermal mass over ambient to begin with. Exactly how you manage to dissipate heat at a 500kwH rate to the atmosphere during this operation without setting things on fire is an interesting engineering problem all on its own. Note that if you are forced to rate-limit the charge due to heating then there is no way you'll achieve that 30 minute charge cycle. May I point out that if you actually take a Model S and try to drive it on a REAL track (not a 1/4 mile dragstrip) you'll find out all about this because when you start drawing energy out of that pack at the sort of rate the systems in the vehicle will limit the withdrawal rate from the battery so you don't turn the car into a flaming torch. At that point my little puny Mazda 6 spanks you. I will assume they intend to run 480VDC for the charging to keep conductor sizes reasonable. That is still a delivery rate of over 4,000 Amps! If there are eight charging sockets we're still talking about 500 Amps (!!) each. For your next exercise figure out the (stranded, please!) conductor size required to carry that at an acceptable heating rate and voltage drop and then determine how you're going to maneuver those pigtails (all 8 of them) to charge said truck. You need 600V insulation, a protective ground jacket over the outside casing (to detect incipient cable failure before someone gets turned into a cooked hot dog if the insulation is compromised) and a 90C insulating rating. When you get all this figured out you will realize that you DO intend to have mechanical assistance for those cables, plugs and the mating/unmating operation. Oh, and do the engineering right on the connectors too; 500 Amps will rock your world if the connection goes high-resistance on you or (God forbid) arcs, and it's utterly necessary that you make very sure they're NEVER separated or mated hot under any circumstances (because if you do you're replacing both ends as the arcing will destroy them instantly never mind splatter damage from it, especially with DC where there's no zero-crossing quench.) Yeah, this is all going to work out just fine... NOT! Last modified: 2017-11-26 13:16:12 by tickerguy
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