Low cost BMS

[harlequin2]

OK, if you want to use a balancing current of maybe 5 A, then you have 18 watts to dissipate and a fet bolted to a heatsink might be a reasonable way to do it. In my car, with 45 cells, that would mean 810 watts to get rid of. Not very efficient is it?
Practically, if you want to use a balancing current significantly greater than 1/2 amp or so, then you really have to move to an active balancing system where power from a "high" cell is shunted to a "low" one. There are some quite good designs out there for such things, but they all cost a great deal more than the simple, lossy resistor.
By far the simplest and most cost effective way to do it is to balance all your cells before installing them into your battery and then keep them in balance with the little 1/2 amp shunt which is all it takes once you have done the initial balance.

[kovgab3]

You are totally right.
However, if you have to replace 1-2 cells in your pack, then you might think other way about the necessity of the 5A dissipating modules

An other "reason" can be if your charger can not decrease the current on an outer signal. If you have to switch it off when the first (old) cell reaches the limit, then the new ones will never get charged to their capacity.

[kovgab3]

Hello Harlequin,

I attached 3 images, please check them.

I was thinking about the wires between the cell modules.

On the first image I have rearranged your CellTop module schematic, and for the better visibility, I have removed everything that I don't need for the "experiment"

On the second image I put two of this circuits behind each other, and I added a shape representing the copper connector between the two battery cells, and I have added simple 2 lines for the twisted pair wire also.

Nothing special, just to show that the VCC of the first cell, and the GND of the next cell is connected.
And because of this, I can reposition the resistor R2, as shown on the third image.
This way we don't need the twisted pair, only 1 wire (just as they did: Battery Management System (BMS)).
However, this way the optocoupler becomes unnecessary also, am I right?

The BMS on the above link really doesn't use optos, and they can handle cells up to 255. They use twisted pairs only at the first and at the last cell, to connect them to the master unit. (and in one more case: if the battery pack is divided into multiple banks, and the banks are quite far from each other. Only the last module of Bank0 and the first module of Bank1 are connected this way.)

What is your opinion, can this make the design even simpler? (by the cost of the opto. Maybe a transistor needed to invert the signal, or it even can be handled inside the PIC?)

[harlequin2]

Yes, it can be connected that way and I realised that way back when I started this project. However, I wasn't sure how much interference the comms would suffer if one of the lines was also carrying the motor current with its high level of emi and so I decided the twisted pair would give the best protection. For that reason also I only run the data at 9600 bps which is quite enough to do the job and less prone to interference than higher Baud rates.
I haven't looked at doing away with the opto, but I guess that would be possible too.
I see the one described in the link you included runs at 56 kbps and uses a single wire, so maybe I was being too cautious. I have my doubts though, especially when I see a 2R7, 5 watt resistor being asked to carry 1.5 A of balancing current and thus dissipating 6 watts! In my experience, a 5 watt wire wound resistor can dissipate only about 2 watts in free air before catching fire.

[kovgab3]

I understand, thanks.

The e-motus BMS also uses PWM (you can configure the top limit for the dissipating current), and they also measure the temperature. However, the sensor is on the board, so it measures not the cell, but the PCB and the resistor together.
Hopefully they decrease the duty cycle, if the temp goes up

[harlequin2]

I'm sure they do that, but its not right to claim a balance current of 1.5 A, is it?
Also, I found that when balancing is turned on, there is a significant voltage drop along the connecting traces/wires and this affects the cell voltage reading unless you take care to separate the tracks carrying that current from the voltage sensing path. Their connecting wire to the +ve terminal shares both and will result in erroneous readings - unless they only measure the cell voltage while the pwm is in the off state.

[kovgab3]

these guys dissipate with 10A:
EV-Power | RT-BMS 2.0 - Cell Ballancing Unit (terminal) - High Power 10A
I'm sure, they have to deal with this problem.

[harlequin2]

Yah, they all seem to lack a basic understanding!
I re-iterate: If you balance your cells before installing them into your battery, a 1/2 A balancer is adequate to keep them that way, certainly for all the sizes commonly used by the home EV converter.
I balanced all mine with an individual cell charger made from an old 5 V/ 20 A computer power supply modified to put out 3.6 V instead of 5 V. Might take a while if you have a few hundred cells!
Also, weber and coulomb on the AEVA balanced theirs with only 1/2 A just by leaving the charger in balancing mode for a few days - they have about 220 cells.

[Astro]

@harlequin2 have you considered listing the cell top modules and master modules on eBay?
I would be interested in either a PCB only or if available a kit where you get the PCB and components and then build it yourself. Even selling pre-programmed pic chips would probably help a lot of the less technical EVers that would like to use you system.
This looks like a very nice BMS, very elegant. I like that it is simple enough to be easily understood yet complex enough to do the task. Well done.

[Astro]

Just had a thought that i would like to run past you all.
When top balancing a battery pack, if i understand it correctly. The charger puts out full charge current until the first cell reaches a set voltage. Then the charger goes to low current mode and the BMS modules switch a load across each cell as they reach the top balancing set voltage. This load effectively keeps them in a holding pattern until all the other cells catch up.
My thought was, why not have the BMS module switch a load across the cell with the highest voltage right at the start of the charging process. That way the charger could stay in high current mode. The cell with the highest voltage effectively gets charging current minus whatever the BMS module load absorbs.
As the other cells catch up another cell may become the highest voltage and so its BMS load is activated. So you are constantly reducing the charge rate of the cells with the highest state of charge.

Basically the cells are all in a race and you want them all to reach the finish line at the same time. So instead of letting the fast cells get to the finish line and then have to wait for the others. Simply reduce the fastest cells charge current allowing the slower cells to catch up mid race.
This way the charger stays in high current mode the whole time and you avoid the long wait at the end of the race as the slowest cells are slowly brought to the finish line with the reduced charger current.

I am new to this BMS stuff but i am slowly learning so please be gentle if this question is a total noob question.