Low cost BMS

[harlequin2]

Soon!

[harlequin2]

OK, I have the thing working reliably now so I'll send the files to sawickem to create a wiki thing.

[sawickm]

Originally Posted by harlequin2

OK, I have the thing working reliably now so I'll send the files to sawickem to create a wiki thing.

I'll get those files uploaded into the wiki tomorrow !!!

-Mark

[Ladogaboy]

Wiki!!! I'm going to put building one of these on my to-do list. Thanks!

[harlequin2]

For anyone interested in building this, the relevant files should appear on the wiki soon. Its fairly straightforward, but does use all smd parts, so if you haven't worked with these before, bite the bullet and get into it! I reckon smd's are easier to assemble than thru-hole parts, once you get the hang of it.
I do have pcbs available and also can supply them with the micros mounted and programmed if you wish. Send me a PM if interested, although all the info needed to make your own is included.
A quick overview:
Each cell has its own module which bolts directly to its terminals. These modules use a PIC micro to measure the cell voltage and temperature, and the have a 6R8 shunt which can be turned on or off for balancing. The controller talks to all of the modules via a "daisy-chain" comms link and collects voltage and temperature data. It displays the max and min values on a LCD and also the total battery voltage. It also controls cell balancing via the shunts and by controlling the charger.
Have at it!

[sawickm]

Originally Posted by harlequin2

For anyone interested in building this, the relevant files should appear on the wiki soon.

I just finished uploading Harlequin2's BMS files into the wiki for those interested.Open ReVolt/PCB Schematics - EcoModder

Let me know if anyone has problems downloading his BMS files.

Regards,

-Mark

[Anaerin]

Here's my thinking on a BMS. Each cell should be able to be switched out completely. So, for instance, there are 2 MOSFETS on each cell, one to enable output (Connect from + to out) and the other to disable output and short the cell (Connect from - to out). This will ensure that, upon error or failure, the entire pack doesn't die, the voltage just drops by one cell's worth. This can continue until the entire pack (or at least the majority of it) has dropped out, or a low voltage condition forces a stop. When charging, this means the cell is charged to full and dropped out completely (a signal could be issued to the charger to drop input voltage by one cell's worth) to continue charging the rest of the cells, so every single cell is charged to completely "full".

This would give both top and bottom balancing. It would have the entire pack's entire capacity available for use, and would give a better "ratcheting down" of available power to let you know you're nearing the end of your range, rather than a sudden drop off a cliff.

However, it would mean that series strings would need to be widely paralleled, as the MOSFETs would severely limit the available current. Alternatively, small relays could be used. But no matter the switching system, it's that switch that would be the limiting factor, unfortunately.

[sawickm]

Originally Posted by sawickm

I just finished uploading Harlequin2's BMS files into the wiki for those interested.Open ReVolt/PCB Schematics - EcoModder

Let me know if anyone has problems downloading his BMS files.

Update:

Harlequin2 just sent me some updated files for his BMS. I will posting them soon.

-Mark

[harlequin2]

Originally Posted by Anaerin

Here's my thinking on a BMS. Each cell should be able to be switched out completely. So, for instance, there are 2 MOSFETS on each cell, one to enable output (Connect from + to out) and the other to disable output and short the cell (Connect from - to out). This will ensure that, upon error or failure, the entire pack doesn't die, the voltage just drops by one cell's worth. This can continue until the entire pack (or at least the majority of it) has dropped out, or a low voltage condition forces a stop. When charging, this means the cell is charged to full and dropped out completely (a signal could be issued to the charger to drop input voltage by one cell's worth) to continue charging the rest of the cells, so every single cell is charged to completely "full".

This would give both top and bottom balancing. It would have the entire pack's entire capacity available for use, and would give a better "ratcheting down" of available power to let you know you're nearing the end of your range, rather than a sudden drop off a cliff.

However, it would mean that series strings would need to be widely paralleled, as the MOSFETs would severely limit the available current. Alternatively, small relays could be used. But no matter the switching system, it's that switch that would be the limiting factor, unfortunately.

Yes, "unfortunately" is the word.
My car draws a peak current of just over 600A and has 45 cells, so to do what you suggest would require 90 switches each capable of carrying 600A. Preferably with zero voltage drop!
Might be a bit difficult......
It is certainly a possibility for charging though, as the current is more like 15 or 20 A. But still, 90 x 20A relays at about $10 each?

[sawickm]

Originally Posted by sawickm

Update:

Harlequin2 just sent me some updated files for his BMS. I will posting them soon.

-Mark

Files are now updated: Open ReVolt/PCB Schematics - EcoModder

-Mark