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Some alternative ideas
Hello everybody,
harlequin2, when I came across your BMS design, I really loved the daisy chain
idea - full isolation and two way communication possible while being
really cheap - excellent thing.
While reading this thread I came across three ideas - maybe we could
discuss them?
1. Use only transistor in a TO220 package for shunting
This would mean that the module would have to be calibrated (the MCU would have
to output the right voltage for the MOSFET to pass for example 1A), but we
could screw the transistor to the positive battery terminal and have it dump
the heat into the battery.
Advantages:
- higher shunting current possible
- it could double as a cell heater for winter charging (but the currents of all
the shunts in the system would have to match closely) when fed from the
charger
- less component count
Disadvantages:
- current calibration step required
- possible thermal coefficients issues (the Vgs / Rdson curve)
- DAC or PWM (and a low pass filter) required from the MCU
- TO220 case component possibly more expensive than SMT
The calibration could be a feature of the control unit.
2. Use optical link in the daisy chain setup
Your project used transoptors connected with cables.
How about replacing the cables with a ray of light?
Example:
tefnet.pl tmp filip optical_daisy_chain.png
(please replace spaces with slashes)
Batteries from GBS like GBS-LFMP60AH
have a cool feature - they have bumps and corresponding holes on sides of their
casings so that when you make a pack out of them then they are nicely aligned.
This means that if we put the pack together then we could just have diodes and
phototransistors on adjacent modules' sides and just shine from one module
to another. It should be quite reliabe on such a short distance, especially
with short tubes mounted on photo elements (to narrow down the angles).
We would have to have mounting points for both phototransistor and
diode on each side so that we could link them disregarding orientation.
The only problem is joining ends of battery strings - we could go
back to wire there or use something like toslink (2 or 3 pieces
should not drive the cost up very much).
Your battery pack would not gain much from this setup
(parallel-series combination), but I plan
to have two or three long strings of batteries in series and
this could work out nicely.
Advantages:
- less wiring
- less connectors
- vibration/time wear resistant
- easy "loss of link" debugging - just look at your
pack with a camera - IR will show up as white
- better EMI resistance
Disadvantages:
- possibility of interferences between links
- better battery alignment required
- slightly higher power usage (IR diode vs transoptor diode)
- communication might be slower
- IR diode and photo transistor combo might be slightly more expensive
than a transoptor
- better electrical isolation
3. Use optical link in a star configuration
We could put an IR diode and an integrated IR receiver
on each module. Let's say we'd emit 26kHz with the diode and receive 40kHz with the receiver
to reduce interferences between sending and receiving sides.
Then we could have one additional module in the battery box which would emit
40kHz with an IR diode and receive 26kHz with a receiver.
IR protocols like RC5 seem very robust (more than 10 meters with the daylight),
so communicating over 1m in a closed box should be trivial.
Advantages:
- no wiring
- no connectors
- vibration/time wear resistant
- better EMI resistance
- module connectivity does not affect other modules
- easy "loss of link" debugging
- asynchronous alarms from the modules with a good IR protocol
- perfect electrical isolation
Disadvantages:
- needs a bit of space over batteries for the light to pass
- higher cost of an IR diode and a receiver (compared to a transoptor)
So, what do you think of these?
Also a question - the battery terminals visible in post #43
look like they were reinforced with something like tubular rivets.
Can you share some details on this?
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