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
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