Low cost BMS

[harlequin2]

Originally Posted by filip

We start with assigning a one or two byte integer to each board as an address and then query them one by one in a loop (M - battery module, C - main controller):

Yes, that "assigning" might be the difficult part. You might be able to do it by having a jumper on each board to control the power - the controller would assign the next vacant address to the next module to respond and you would control that by powering them up via the jumper, one at a time. You'd need to make a map. The modules would need to store their own addresses in eeprom.
With the wired system, the connection order determines the address. There is no need to store any addresses and all modules are identical, its just the order in the daisy chain that determines the address.

Anyway, keep on thinking; good, new ideas are always welcome!

[filip]

Originally Posted by harlequin2

Yes, that "assigning" might be the difficult part. You might be able to do it by having a jumper on each board to control the power - the controller would assign the next vacant address to the next module to respond and you would control that by powering them up via the jumper, one at a time.

I'd rather avoid such delicate mechanical connectors if possible - maybe an 'address assign mode' jumper would be better - it wouldn't fail when using the car, because it wouldn't be there.

Couldn't we just use the ISP connector to set up the addresses?

Another possibility:

When programming the cell modules the primary controller unit IR diode is covered with something similar to a pen cap (or disabled by software) and another one is hooked up with a wire. The second IR diode works with much less current (let's say 2% of the current of the main diode) and is enclosed in a tube so that you can put this tube over any cell module's IR receiver and talk just to this one.

[harlequin2]

Jumpers are not "delicate" at all and can work very well in this sort of application.
I've done a lot of work with networking systems for various applications and addressing has always been a bugbear. If you have a few hundred nodes and they all have to be set up individually, it becomes a real PITA. Hence my daisy chain system where its all taken care of automatically! I can't think of a better way to do it and I look forward to the day that someone shows me such a thing.

[filip]

Originally Posted by harlequin2

I've done a lot of work with networking systems for various applications and addressing has always been a bugbear. If you have a few hundred nodes and they all have to be set up individually, it becomes a real PITA. Hence my daisy chain system where its all taken care of automatically! I can't think of a better way to do it and I look forward to the day that someone shows me such a thing.

I will try to play with the IR broadcasting solution and provide some feedback on advantages/disadvantages.

Each module needs a bench test after assembly and it seems to me that setting the address after verifying the MCU operation won't be that troublesome.

[kovgab3]

Originally Posted by harlequin2

First off, you can't use a fet as a load without some form of stabilisation - simply applying some voltage to the gate results in a completely unknown and quite unstable current flowing. "Calibration" as you suggested does not work, it is simply too unstable to be even thought about.

Many "commercial" BMS cell modules are using FETs for dissipating.
They don't measure the dissipating current, but they are monitoring the temperature of the FET.
And they use PWM to keep the FET living

for example:

[kovgab3]

doesn't let me attach a link...

[kovgab3]

maybe after 2 more posts...

[kovgab3]

last empty post... sorry for that

[kovgab3]

maybe now:
EV-Power | RT-BMS System

[harlequin2]

Yes, that could work. You'd need a choke in series with the fet to limit the rate of rise of current - you'd effectively be making a switch-mode power supply! Also, a temperature sensor on the fet. Is that simpler than a current sensor? And why use a fet? A bipolar transistor might be better as its easier to drive from a logic level.
So there you go: A pwm drive output from the micro, a bipolar power transistor, a choke and a small resistor in the emitter lead of the transistor to act as a current sensor feeding into a spare adc input on the micro. The transistor is turned on and the current monitored - transistor turned off when I reaches the desired level. Oh, you'd need a Schottky diode across it as well to absorb the voltage spike when the transistor turns off.
All this to shift the heat dissipation from a resistor to a transistor.
If anyone can think of a clever way of sending this power to the cell with the lowest voltage, you might have something.