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Cheap Bluetooth BMS
#1
I have purchased and installed two of these 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.


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
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller
2 pushbikes using Bafang BBS-02 electric motor kits, 6 x 4s 5Ah (10Ah 12s2p) LiPo batteries, bluetooth BMS
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#2
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?
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#3
(10-30-2017, 07:15 AM)cmg_george Wrote: 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?

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
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller
2 pushbikes using Bafang BBS-02 electric motor kits, 6 x 4s 5Ah (10Ah 12s2p) LiPo batteries, bluetooth BMS
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#4
Thank you a lot! Looking for your feedback regarding 4S Smile
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?
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#5
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
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller
2 pushbikes using Bafang BBS-02 electric motor kits, 6 x 4s 5Ah (10Ah 12s2p) LiPo batteries, bluetooth BMS
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#6
Thank you for the update. For the moment i think i will play with a simple BMS (no administration).
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#7
Hi Simon,

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

Dave
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#8
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.
Oderus420 and Korishan like this post
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor/wiki
Latronics 4kW Inverter, homemade MPPT controller
2 pushbikes using Bafang BBS-02 electric motor kits, 6 x 4s 5Ah (10Ah 12s2p) LiPo batteries, bluetooth BMS
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#9
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.
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#10
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.
Oderus420 likes this post
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