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Diy Active balancer
#1
Hello guys,

Together with someone else i am working on a DIY bms,  with webpage interface and active balancing using a flying cell/capacitor.
It is a modular design making it possible to use it for indefinit(well almost)  amount of cells in series.

Currently the webpage that is running on the ESP32 motherboard is seen at this link
https://bms.klumponline.nl/

Here you can see the finished pcb in EasyEda.
I have yet to order the first batch, but the total costs of 1 balancer will be around 15 dollars.


The specs are as following.

Up to 8 cells in series per balancer board
Isolated Comport for communication
Using a STM32 Bluepill as the brain
Measures capacity that goes in and out of each Pack, making it possible to monitor wich pack is weaker then the rest.
All settings are configurable through the webpage (Yet to be written, but its on the todo list)
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#2
And the board is completely designed for the JLC pcb assembly service.
Production cost is 74 dollars for 5 pcbs fully assembled
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#3
Impressive work. Looks to be a well thought out board design.

Do the ESP32 master and STM32 slaves share the same PCB design? So the assembled board would come with pin headers and you drop in your module of choice?

Do additional cascaded BMS boards balance between boards or only between the 8 cells that a board manages? I can't see a connection for this but it could be just me.

What are your balancing currents?
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#4
you should put the project on Github.
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#5
(04-21-2020, 11:46 PM)BaronVonChickenPants Wrote: Impressive work. Looks to be a well thought out board design.

Do the ESP32 master and STM32 slaves share the same PCB design? So the assembled board would come with pin headers and you drop in your module of choice?

Do additional cascaded BMS boards balance between boards or only between the 8 cells that a board manages? I can't see a connection for this but it could be just me.

What are your balancing currents?

The ESP32 board is a seperate board that looks nothing like the balancing modules. 
so for lets say 14S  you need 1 motherboard and 2 balancer boards.

it only balances between the 8 cells it manages.
I havent ordered the first batch yet, so i can not say what currents will be reached.
But i am going to do all the testing soon.
I can measure current with the balancer module, so should be easily done.
Max balance current will be the same current that flows from 2 cells. 1 at 4.2V 1 at 3.0V

Patalhetas likes this post
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#6
If you dont have any interconnect how do you manage issues with self discharging?

Because without taking energy or transferring energy to the whole pack or bleeding energy you will in the end have one of the modules with higher voltage than the others.

This is one of the most common issues on diy powerwalls with 2nd hand cells.
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#7
What you mean with that ?
The balancer modules are aiming for a average voltage for each 8 cells they are monitoring.
And in future the motherboard will tell the balancers at what voltage they need to balance.
So the energy of a higher cell will be transferred to a lower cell
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#8
(04-22-2020, 04:10 PM)daromer Wrote: If you dont have any interconnect how do you manage issues with self discharging?

Because without taking energy or transferring energy to the whole pack or bleeding energy you will in the end have one of the modules with higher voltage than the others.

This is one of the most common issues on diy powerwalls with 2nd hand cells.
Can you elaborate ?  I dont understand your question
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#9
If you have an average of 3.76V on bank 1 and 2. Lets say you then have 1 cell in Bank 1 that starts to discharge.

Suddenly bank 1 will have 3.5v and bank 2 still 3.76V. How do you intend to sort that issue?

In my world you have some scenarios to sort this
1. You can interconnect the slave boards so they can transfer energy in between each other. Bidi connection like others use
2. The slave boards can transfer energy to or from the main conection voltage pack. Then you have the issue with voltage range. Going above 65VDC is tricky
3. You add a passive dissipating resistor on each slave so if 1 slave gets to high that slave can dissipate it into heat. Though sounds wrong when going active.
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#10
(04-22-2020, 05:52 PM)daromer Wrote: If you have an average of 3.76V on bank 1 and 2. Lets say you then have 1 cell in Bank 1 that starts to discharge.

Suddenly bank 1 will have 3.5v and bank 2 still 3.76V. How do you intend to sort that issue?

In my world you have some scenarios to sort this
1. You can interconnect the slave boards so they can transfer energy in between each other. Bidi connection like others use
2. The slave boards can transfer energy to or from the main conection voltage pack. Then you have the issue with voltage range. Going above 65VDC is tricky
3. You add a passive dissipating resistor on each slave so if 1 slave gets to high that slave can dissipate it into heat. Though sounds wrong when going active.

So you are talking about a scenario where one of the packs will self discharge ?
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