Cheap Bluetooth BMS

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karrak

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Sep 4, 2017
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I have purchased and installed two ofthese 13S cheap (US$46) BMS units on my and my partner's electric pushbikes and thought they may be of use for people building their own powerwalls.

As far as I can see they work well and I haven't managed to blow anything up.

They are based around the Texas Instruments BQ76940 Li-ion battery monitor chip with an Atmel Mega32P microcontroller providing the smarts and coms.

The big plus with these units is that the can be connected either via the USB dongle provided to a computer and programmed and calibrated via software provided. There is also a bluetooth dongle which allows the BMS to be connected to an Android app on an Android phone.

The BMS has a 30A MOSFET switch to disconnect the battery from the outside world in the event of overvoltage/undervoltage, over current and temperature out of range.

If the 30A is not enough there are 100A units or you could tap into the separate Charge Disable and Discharge Disable signals that drive the charge and discharge switching FETS. I have done a really rough sketch of the FET driving circuitry.

image_jvprao.jpg


If anyone is interested I might look at writing some code in my BMS software to link in with this unit which would allow one to log the data and display it on the Internet.

Simon
 
I'm also looking for a cheap BMS and i came across the one suggested by you but i was skepticalin order it since i could not get a lot of information.
Do you know if is working if you add onlu 3S? My plan is to start with 3s and add more in the future.
Is it possible to configure top and bottom voltage?
 
cmg_george said:
Do you know if is working if you add onlu 3S? My plan is to start with 3s and add more in the future.
Is it possible to configure top and bottom voltage?
Click to expand...

I will try an experiment and see if I can get the BMS to work with 4S and maybe 3S in the next few days.

As far as I can see you can configure just about everything, below is the configuration screen of the software provided

image_vwoepn.jpg
 
Thank you a lot! Looking for your feedback regarding 4S :)
It may be a stupid question (still a noob) but having the same port for charge/discharge means it can not be charged and discharge at the same time?
 
I did some test and It looks like it will probably work with 8S but not 4S:-(

Current can only flow in one direction at a particular point in time. So if we have a load of 1A, and a charge current of 2A we will get 1A going into the battery from the charger and 1A going strait from the charger to the load. Another example is a load of 2A, and a charge current of 1A results in a battery discharge current of 1A.

Simon
 
Thank you for the update. For the moment i think i will play with a simple BMS (no administration).
 
Hi Simon,

Have you made any progress with writing code for these BMS units? They seem like they have great potential.

Dave
 
If there is enough interest I will write the interface code for my BMS software to use the output from these BMS boards instead of the hardware that my BMS currently uses.

So far I have worked out the command string you have to send via the USB/serial link to get the BMS board to dump the data. It looks like it is set up like some sort of pseudo CANBUS protocol. I don't have any experience with CANBUS so am not sure if this is the case.

The command sequence to send in HEX format as a binary string is
DD A5 03 00 FF FD 77 DD A5 04 00 FF FC 77 DD A5 05 00 FF FB 77

The reply packet is a binary string in HEX format is. I have added the EOLs for clarity
DD A5 00 1B 13 78 00 00 00 00 03 E8 00 00 22 C7 00 00 00 00 00 00 19 00 03 0C 02 0B 64 0B 5F FC 83 77
DD A5 00 18 10 39 10 3A 10 38 10 3A 10 3C 10 39 10 37 10 39 10 3B 10 3F 10 36 10 3A FC 74 77
DD A5 00 14 4C 48 2D 53 50 31 35 53 30 30 31 2D 50 31 33 53 2D 33 30 41 FB 39 77

The second line appear to be the individual cell voltages. Each voltage is a 16 bit number so 0x1039 gives a voltage of 4153mV
10 39 10 3A 10 38 10 3A 10 3C 10 39 10 37 10 39 10 3B 10 3F 10 36 10 3A
These converted to decimal in mV are
4153 4155 4152 4155 4157 4153 4151 4153 4156 4159 4150 4155

I am sure the battery current is in there somewhere but as yet haven't done any further decoding.
 
I just order 7S 60A active bms equivalent (since i need for the moment a 7S monitor/balance). I'm also interested in the interface code for it so i will be able to help after i will receive it.
 
The advantage on the version with charge and a discharge port is that you can disconect one or the other. On the BMS with both then you will disconnect charge and discharge at same time.

I did a video of it earlier:

So far i must say the version i got have some serious issue meassuring correctly.
 
daromer, thanks for taking the time to make and post the video!

I haven't had any problems with the measurement accuracy with the two units I have.

Only issues I have had is if you unplug any of the wires to the individual cells while it is on it displays a 'ramdom voltage' rather than displaying zero volts. It has also crashed a few times while I have been connecting and disconnecting things and fiddling around. Only way to fix this was to pull out the main connector. This fault has not occured while the BMS boards are in use.

The BMS has actually saved the battery on one of our Ebikes on one occasion when one pair of cells dropped too low and it worked as it should and disconnected the battery from the motor.

I am using the 13S units with a 12S battery.

To get this to work properly I had to connect wires 4 and 5 on the main connector together and connect the pair to the + of the 3rd battery.

I have just about finished writing the software interface between my BMS software and the BMS boards. Should have it finished in the next few days.
 
Yeah i have managed to crash it a little bit to easy :D
 
cmg_george said:
I just order 7S 60A active bms equivalent (since i need for the moment a 7S monitor/balance). I'm also interested in the interface code for it so i will be able to help after i will receive it.
Click to expand...

Can you give us a link to the device you bought
 
cmg_george no you need to change more than that. Like the transistors. and if having bigger balance current you also need to cool it down or move it out.

possible = yes.
Worth it = Perhaps not
 
That BMS board looks like it should have the same interface as the BMS boards that I have and should work with my BMS code when I have finished the interface, nearly there.

It may be possible by using larger resistors off the board. I would think heat dissipation would be the limiting factor for the surface mount resistors on the main board. You would have to make sure that the driver transistors could also handle the extra current. Larger balance current might not be necessary if you make sure your pack is balanced to start with.

Simon
 
I have written the software interface between my battery monitoring software and the Bluetooth BMS. Click on the link in my signature if you are interested.

As yet the interface is pretty basic but it works just fine.

image_lcegrp.jpg


Any questions, feedback or suggestions are welcome

Simon
 
I finally found the link to the post showing how to wire up the generic 10S to 15S bluetooth BMS pictured below to work correctly with different number of cells. You will get incorrect voltage readings if you don't follow these instructions.


image_txxtts.jpg


Simon
 
karrak said:
I have written the software interface between my battery monitoring software and the Bluetooth BMS. Click on the link in my signature if you are interested.

As yet the interface is pretty basic but it works just fine.

image_lcegrp.jpg


Any questions, feedback or suggestions are welcome

Simon
Click to expand...

How is this working for you? It looks like what I have been wanting to do. I'd love to integrate this information from the bms into a grafana dashboard.

I'm building a portable 2kwh battery with 1kw inverter and solar/grid charging inputs. I have a raspberrypi with 7" touchscreen already that I was going to use for a display of battery state and function.
 
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