See below.
------ Original Message ------
From: "Lee Hart" <leeah...@earthlink.net>
To: "Peri Hartman" <pe...@kotatko.com>; "Electric Vehicle Discussion
List" <ev@lists.evdl.org>
Sent: 28-Apr-20 9:10:37 PM
Subject: Re: [EVDL] Communicating over "mains" (Was: Minibms?)
Peri Hartman via EV wrote:
The BMS for each cell would normally be quiet and only send something when
prompted by a central BMS component.
I think it is important to consider just what you want a BMS to do.
- Is it just there to provide feedback on what each cell is doing?
(battery MONITORING system)
- Is it there to actively balance cells?
(battery BALANCING system)
- Is it a safety system, there to prevent damage and fires?
(battery SAFETY system)
Doesn't matter. It can still be handled by a poll from a master. The
point I was trying to make is to find a way to not have a bunch of data
collisions and keep the comm part of the system simple. Since the comm
is fast, it would be easy to round-robin monitor all cells 100s if not
1000s of times a second.
By "simple" extension of the protocol, one could add instructions back
to the cells. As long as the master is in control, there won't be
collisions.
You could have an interrupt based system where cells try to send to the
master when an emergency condition happens. But I don't think there's
any need for that. If the master is working, it will poll the cell soon
enough to respond to the emergency. If the master is not working, it
won't respond to the interrupt, anyway.
I think the real issue is how to handle a failure of the comm system or
the master. The cells need to have some sort of built-in timer that
shuts them down if the master fails to poll them. Or maybe there are
some other, better ways to do that.
In regard to your three example flavors, it seems that for a robust BMS
each individual cell monitor must do all three. I'm out of my domain
here, but can't balancing be handled by checking cell voltage ? That is,
when charging, slow the charge current (shunt the excess current) as the
voltage reaches the cut-off (and opposite for discharging). What needs
to be done for safety ? I presume that's primarily keeping current
within limits and watching the temperature.
Peri
These are three very different devices, with completely different design
requirements.
When you talk about systems with a computer on every cell, and CAN bus
networking with TCP/IP, you're talking about a Monitoring system. It's supposed
to provide accurate real-time data on things like cell voltages, currents, and
temperatures. It probably needs a fancy display (blinkinlights), and data
logging (pretty charts and graphs).
Such a system is necessarily complicated, with correspondingly high cost and
low reliability. It will need hundreds, perhaps thousands of parts, and so have
many failure modes. Given the heavy dependence on software, it will be nearly
impossible to predict what happens when it fails.
And, it's going to be the exact opposite of a safety system.
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, http://www.sunrise-ev.com
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