Another BMS project using single cell modules

[cwah]

Definitely interested if i can track cell balance on screen.

You can track cell balance on the leds too.
You can track cell balance on PC, using simple hardware adapter which is not yet published on my site (it will be published soon).
You can track cell balance on central module with the display, or even (with bluetooth adapter and simple interface) on smart phone... Problem is that I am not skilled in programming Android applications and don't have time for developing such applications. However, I'm seeking for partner who can do this job, as well as I'll need somebody to make Linux application too...
If you are not so much interested which cells are charging slower and if information about balance status of whole battery is good enough (three statuses: no cell bleeding, some cells bleeding and all cell bleeding) then simplest central module is enough. Simplest central module is good enough for complete protection of the battery...

Doesn't need app or anything, not even trackinh while charging.

The best indicator of cells healthyness are "sag" discrepancy on discharge. If a small set of cells are sagging much more than the rest, its an indicator of future problem and the cells need to be changed.

My ncr18650pf cells haven't been balanced after a year of use. But I carefully check cells sagginess on discharge and had to remove 1 string of cell to keep the pack safe.

 

[Peter.bp]

Single cell voltage sag is easy to detect through LVC alarm of cell module, which is sent automatically if the module is not polled.
In short, the central module is not needed even to poll the cells when discharging as it will receive the alarm message when particular cell reaches low voltage level.
Such central module can be supplied from last cell or from motor controller 5V supply.
Easiest way is to make open drain binary output (enough bits to address the cell in LV alarm) from such central module. Every open drain output can then drive the led through serial resistor from external 3-15V source.
In total, you need max 8 leds for up to 255S configuration.
If outputs are open drain and central module doesn't have to send anything to cell modules, it's supply current consumption is low enough that it can be supplied from most negative cell (less than 10uA).
E.g. for 24-31S you need only 5 leds in order to decode which central module triggered LV alarm.
It's easy to decode the cell with sag problem...

 

[cwah]

Yup. There are 2 key things to be aware of:
- voltage sag higher than usual
- voltage variation between cells

If it can be plugged permanently without making the pack unbalanced... then that's all I need

 

[Peter.bp]

10uA quiescent current means ~82mAh per year imbalance charge per year...
82mAh charge balancing can be sorted within couple hours even with cheapest balancer...
Think that after couple years you'll have much bigger problem with difference in self discharging currents between cells...
I receivedt yesterday one battery (16S1P Headway 38120S) for repairs and found BMS as the problem. The BMS was been designed to cut off charging circuit as soon as any cell reaches 3.6V (according to tech. specs)...
After only two years I found cells out of balance...
Now I'm checking the threshold points of balancer on the BMS...
First result of tests: the BMS cuts off charge as soon as as the battery reaches 53.6V.
My idea (if the BMS passes balancer checks) is just to add ~50Ohm resistor between charge and discharge terminal in order to let the balancer to do its' job...

 

[Peter.bp]

Here's 20S BMS:

The 6.2R/2W resistors are mounted on bottom of Al bar (50*5*250mm). The BMS has been tested with 16S/8A charger and 200W 24VDC halogen light bulb (inrush current >30A), no problems found even in communication.
As this BMS is to deliver away, everything is additional isolated with plastic spray.
Al bar helps mounting BMS close to battery.
BMS is connected to the battery through DB25M (on BMS side) connector and has Teflon insulated wires from cell modules to the battery.
On side of PCB has been added small universal PCB for serial communication with the BMS and block drawing is shown here: http://bps.biz.hr/BMS/20S_connection.pdf.
NB the U2 doesn't have anti parallel diode to its' input as used interface was FTD based USB/RS232 adapter.

Same BMS has been calibrated to +/-2mV at 3.65V cell voltage and the result in excel is given here: http://bps.biz.hr/BMS/20S.xls (with formulas).
HTML version of calibration table: http://bps.biz.hr/BMS/20S.html.
The table shows deviation of reference voltage from datasheet or from average, not residual error after calibration.
The voltage measurements are done using Fluke 8840A bench voltmeter.

The photo of the BMS connected to laptop:

Sorry for the mess on the table.
On laptop screen is visible running calibration software, written in MS Basic 7.0. It is not quite modern (neither me, ha ha!), but it does the job.

Measured quiescent current at ~3.4V cell voltage is under 3uA. 8)
It's quite difficult to measure the current as it needs to be integrated (as it jumps up to couple mA during wake up of processor).

Next to do is to finish the software on the PC (removing all assembly language subroutines) as that program will be available to public, together with source code. under DWYW (eq. Do What You Want) licence.