Peri Hartman via EV wrote:
- 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.
For this to work, both the master and all the slaves have to be working.
No failures. In a bus system (USB, CANbus, etc.) any failed device takes
out the entire system.
In regard to your three example flavors, it seems that for a robust BMS
each individual cell monitor must do all three.
Yes. Most likely, there will be separate circuits for each, so one
failure doesn't take them all out.
I'm out of my domain here, but can't balancing be handled by checking
cell voltage?
Measuring cell voltage only tells you that there might be a problem. It
doesn't correct the problem, or even tell you what to do about it.
The fashionable way to balance is by switching a resistor (or some
equivalent) across the cells if they get high while charging. There are
issues with this, but it's generally the cheap way to do it. This is the
"balancing" part of the three. Common failures here are a) the load
didn't switch on when it should have, b) it didn't turn off when it
should have, c) the charger didn't cut back, so it overwhelms the
resistor and the cell overcharges anyway, d) the cells are too far out
of balance for a small resistor to correct, e) insuring that the
resistors aren't overheating due to excessive on-time.
What needs to be done for safety?
The safety circuits need to detect these failures, stop the charger (and
driving, if regen could occur), and alter the driver that there's a
problem that must be fixed.
Besides these, it needs to watch for other failure modes. Things like
excessively high or low cell temperatures, excessively high or low
voltages, loose terminals (such as by high terminal temperatures), and
ground faults.
The safety circuits also need to be fail-safe. If something goes wrong
with them, they *shut down* the vehicle rather than let you keep driving
it unprotected.
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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