Let me play devils advocate. If it was so hard, then why does my cell phone/radio/satellite work? They pickup microvolts out of the air and work pretty well. I'm communicating my email to a tower 10 miles away. I'm doing itwith 2 watts! That's pretty amazing.
the bypass resistor can do 0.5-1 amp. That's a non trivial signal to look for. Now put it at a particular freq and give itenough cycles and look for it with an FFT in software and I'd guess it's going to be detectable. Now addCRC for errors and viola. And if occasionally 1 out of 100000 times you get a bad reading...who cares. It's a BMS. It's not going to explode.Just make some annoying beep. I can live with that. The advantage is no wires. No fire hazard. Decent stats on the battery. I'd say the wins outweigh the disadvantages. Don't let the perfect be the enemy of the good. Things change relatively slow in the battery world and a fewbad readings are probably acceptable. In a master/slave system if the BMS doesn't respond to a query, thentreat it as a t fault. Turn off the charger, or let the user know by beeping or blinking or something. I think that the proof will be in someone doing it. Perhaps I'm opotomistic, but I have confidence that it would work. On Tuesday, April 28, 2020, 6:48:38 PM PDT, Lee Hart via EV <ev@lists.evdl.org> wrote: Bill Dube via EV wrote: > I have thought about doing this for perhaps 20 years. It may well be > possible to communicate via the traction conductors. It is probably > worth the effort to do so because it would allow you to incorporate the > BMS in the cell. Sealing the BMS inside each cell could be very useful, > especially from a warranty/liability angle. The cell manufacturer would > love to have a log of the SOC history of the cell. It is certainly possible. The problem is whether it is practical. If you are an automaker, with control over every aspect of the vehicle, the situation may be manageable. You can pick a part of the RF spectrum for your BMS communications where you know (or create) a "hole" in the noise from the other parts of the vehicle. You can also route your wiring so as not to create any "dead spots". When you don't have a controlled impedance (known capacitance and inductance in the wiring), RF systems will have peaks and nulls that can prevent certain locations from communicating, where moving it a foot down the wire either way works. But I think the situation is nearly hopeless in an open-source hobby EV. It would boil down to trial and error, where the installer doesn't know what noise the pieces are producing, and can't do anything to change them, and can't change the RF spectrum that the BMS is trying to use. That's why providing a separate communication channel is almost universal. It might be wired, or optical, or RF (not relying on the traction wiring to carry the signal). You have a far better chance of it working. > Communicating _to_ the BMS is simple. You have two VERY large > transmitters, the charger and the inverter. Simply frequency modulate > the pwm of the inverter and/or the charger and put an FM detector in > each BMS on the cell level. You turn the "noise" source into the > communication transmitter. Done. That works if you designed the charger and inverter and BMS specifically to work together to do this. > Communication _from_ the cells is not quite as simple, but doable. Use > the by-pass circuit to talk to the outside world. Put a capacitor in > parallel with the by-pass resistor so that when you switch on the > resistor, you get a spike. Perhaps; but the batteries themselves still have a huge equivalent capacitance. The charger and controller are also likely to have huge low-ESR filter capacitors across them, which try to short out any RF signals present. It sounds easy; but put a spectrum analyzer on your battery leads to see what's *really* there. I think you'd be shocked at the noise level. > There are a few clever tricks you can employ. During charging, you can > have the charger pause for a regular "moment of silence" in which the > BMS can communicate quickly and in the clear without having to "shout" > over the charger PWM. Perhaps the same thing could occur to a lesser > extent with the inverter. Same as above. Are you going to design a special charger that must be used with your BMS? There are lots of solutions that work *some* of the time. There are a few that work *most* of the time. But it gets damnably difficult to find schemes that work *all* of the time. The problem is that a BMS is a safety system that you want to work *all* of the time. Lee Hart -- If happiness is on your mind, here's a daily list to find: - something to do - something to look forward to - someone to love - someone to take good care of - and misbehave, just a little -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub ARCHIVE: http://www.evdl.org/archive/index.html INFO: http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20200429/257cddb8/attachment.html> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub ARCHIVE: http://www.evdl.org/archive/index.html INFO: http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
