Indeed, you would have to design the entire system, inverter, charger,
etc. to accommodate this communication scheme. Not an "add-on" type BMS
my any means.
All of the "noise" comes from the components, and the designers have
control over each and every component in that environment. A regularly
spaced, "moment of silence" is not difficult to coordinate if you talk
to every component with the CAN bus. It can be so short that the user
will not be able to sense that it has happened.
You can also pick your communication band without a care, because you
are on a private "network". Perhaps have the cell BMS sweep to find a
band that works best during commissioning.
The input caps might block the cell BMS RF signal from _entering_ the
inverter (or charger). However, the signal will still be present and
detectable as a _current_ in the traction wiring, rather than as a
voltage on the input terminals of the inverter (or charger.)
Bill D.
On 4/29/2020 1:48 PM, Lee Hart via EV 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
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