Build your own BMS

[T1 Terry]

Originally Posted by AKA-None

Hmmm so why wouldn’t a mesh system work where you could combine individual cells rather than treat a bunch of cells as a “battery “?

With the cells in parallel, if you suspect a cell in that group is failing, simply remove the busbar link on either the negative or positive side and attach a cable to link the cells either side of the one you suspect. Test the cell voltage at disconnection and again 12 hrs later, if the voltage is still roughly the same, that cell is not the problem. If removing that cell from the group seemed to have sorted the problem but the cell voltage is still the same as when you disconnected it, put a lightbulb load across the suspect cell, if after 15 mins the voltage is still about the same, the problem was most likely dirty or loose connections, make a time in the service schedule to clean all the terminals and cell link plates with a scotchbrite disc on a drill and coat all the surfaces with Alminox and the problems should go away.

T1 Terry

[s/v Jedi]

The diagram. Note how a Victron BMV and a Blue Sea Systems RBS are used in addition to the BMS.

[goboatingnow]

Many ways of building your own BMS

[rgleason]

Originally Posted by s/v Jedi

The diagram. Note how a Victron BMV and a Blue Sea Systems RBS are used in addition to the BMS.

Interesting. Thanks.


Nick could you advise further regarding the parts?
8xINA226 module
12C Bus
What is the black box above the BMV 712? a balancer?
How does it work/description?
What you might call the specs?

Also is any programming needed?

[s/v Jedi]

Originally Posted by rgleason

Interesting. Thanks.


Nick could you advise further regarding the parts?
8xINA226 module
12C Bus
What is the black box above the BMV 712? a balancer?
How does it work/description?
What you might call the specs?

Also is any programming needed?

The black box is a small 1” diagonal OLED display.

The INA226 module is a precision battery monitor that we only use the voltage measurement of. Many (incl. me) have used the INA219 modules for this but these are only suitable for 12V batteries, not 24V. The rest they are the same. The INA226 goes up to something like 36V.

I2C bus or IIC bus is short for Inter-IC bus, an invention from Philips. It is a communication bus to be used between chips. The Arduinos have this bus aboard as well. To the INA226 modules, the OLED display as well as the Arduino communicate with each other over this bus.

Galvanic isolation is always the problem with BMS analog front ends. You can buy specialized chips but they all want to do so much more than measure cell voltages. I don’t need to replace the battery monitor and battery switch… I rather have something that works together with it.

So with the INA modules you take the brute force approach: the first module measures between battery negative and the plus of the first cell, like all systems do. The difference starts with cell 2: the INA module measures between the plus of cell 2 and the battery negative, not the negative of cell 2. This means you don’t need galvanic isolation. It also means this INA measures 6.4V instead of 3.2V, you you substract the reading from cell1.

For cell 3 you measure 9.6V and subtract the measurement from cell 2 which produces the voltage of cell 3. And so on for all cells. This is why the INA modules must be able to measure all the way up to battery voltage.

You have to program the Arduino. The programming environment is free and public domain. All the support libraries for the modules are free and public domain. The Arduinos are used high school so the young ones all know how to program these. It’s not hard if one has a little programming experience.

I will be posting source code as well. Relays for controlling high/low voltage warnings must be added as well but that is even easier.

The only delay for now is that I’m midway another project, also Arduino based. You can buy Arduinos that can handle this job for $3. But if you spend $20 then you get Bluetooth, wifi and even run webservers etc. on them.

I used the display to show voltages etc. but may leave it out now and just use a smartphone to see them.

[s/v Jedi]

Originally Posted by rgleason

Interesting. Thanks.


Nick could you advise further regarding the parts?
8xINA226 module
12C Bus
What is the black box above the BMV 712? a balancer?
How does it work/description?
What you might call the specs?

Also is any programming needed?

I forgot the BMV. Most boats have this battery monitor but most do not use it to it’s full potential. There are two important features that can be used with LFP batteries:

1. Programmable relay. It’s built in, ready for use. A good example is to connect the relay contacts to the remote on/off terminals of the solar controllers. Now you can program the BMV to shutdown the solar when the battery is at 70% SOC or something. I think you can even tell it to enable it again at 50% SOC. This would be perfect for a storage mode.

This is why I have a dedicated BMV for every LFP battery.

2. AUX input. The shunt has two inputs, the first goes to battery positive.it’s used for its own power requirements but also to measure the voltage. The aux port is often used to measure the voltage of the start battery or a temperature sensor is connected. Instead of that, connect it to the halfway point of the battery: the positive of cell2 for 12V or cell4 for 24V. Then in settings program the BMV to do midpoint monitoring. It will compare thw voltage of the first 2/4 cells with the voltage of the other 2/4 cells and show the difference in mV. You can also set alarms on too high difference (which means unbalanced cells) and I think you can even program the relay on this, which you could use to enable a cell balancer for example (but the BMS Arduino can do that easy as well).

[rgleason]

Very interesting. Thanks for the good explanations.

It appears that it would act as a BMS which protects the battery.
One issue for me is that it does not really provide advance signal and dry contacts for controlling an alternator or other charge devices.

[s/v Jedi]

Originally Posted by rgleason

Very interesting. Thanks for the good explanations.

It appears that it would act as a BMS which protects the battery.
One issue for me is that it does not really provide advance signal and dry contacts for controlling an alternator or other charge devices.

Yes, when you zoom in on the Arduino, you see the pins with a light blue number. Most of those that aren’t connected yet can be used to drive a relay. There’s a lot of them. When you look on amazon for Arduino relay module you will find ones with 1, 2, 4, 8 relais etc. You can add a 4 relay module and now you have plenty contacts to program.

The RBS uses 3 of those pins: two for output and one for input. The input pin connects to the LED negative signal from the RBS. It gets connected to ground when the solenoid is ON.
The other two pins are output but the RBS wants 12V or 24V depending on which version, while the Arduino only outputs 5V or even 3.3V. Here I use a small transistor to lift the voltage and add some power. A very simple circuit.

In software, you check the input port to find out the state of the solenoid. If you want to change it, you set the ON or OFF output to a high, wait until the state changes, then set that output to low again, because the solenoid has latched and stays in the new state. This goes very quick in real time so it is a pulse but very controlled to make sure the solenoid changes state.

The picture here is from a simple dimmer channel board that I prototyped. There’s a big MOSFET but also on the left is a small transistor and a resistor. This is my goto circuit for this; it’s used on this board to drive the 12V LED inside a switch, which is also dimmable. The big MOSFET is in a 24V circuit to drive the light.
This is part of the project I’m currently working on.

[s/v Jedi]

Here’s a good video showing the INA219 module in action:

https://www.youtube.com/watch?v=FXQoZbvM3Kw

[rgleason]

Could the ardino monitor cell temp and SOC?

[s/v Jedi]

Originally Posted by rgleason

Could the ardino monitor cell temp and SOC?

Temperature is very easy; the diagram has a couple temperature sensors in it.

SOC can be done as well: all the commercial BMS’s do and they all use microcontrollers. The question is how accurate are they and I find that nothing beats the BMV so I use that instead. Coulomb counting accurately isn’t easy.

[T1 Terry]

Have we added pre charge circuits and inverter switch on delays? .... I don't have to go back and read the last 6 pages do I :lol:

T1 Terry

[s/v Jedi]

Originally Posted by T1 Terry

Have we added pre charge circuits and inverter switch on delays? .... I don't have to go back and read the last 6 pages do I :lol:

T1 Terry

This can’t be done right from the BMS… it is something that needs to be in the circuit between inverter/charger and busbars, not affecting anything else.

This is something that I will address though; I also believe others already did this but charge a big price for the product so it will be great to have some free diy options.

Inverter switch-on delays means expensive contactors instead of regular manual switches. The simplest option is a little manual pre-charge circuit that works in combination with a manual switch which should already be in the circuit…

[T1 Terry]

Originally Posted by s/v Jedi

This can’t be done right from the BMS… it is something that needs to be in the circuit between inverter/charger and busbars, not affecting anything else.

This is something that I will address though; I also believe others already did this but charge a big price for the product so it will be great to have some free diy options.

Inverter switch-on delays means expensive contactors instead of regular manual switches. The simplest option is a little manual pre-charge circuit that works in combination with a manual switch which should already be in the circuit…

That works fine for those that actually remember to turn the inverter's off before resetting the battery isolator. With the inverter still switched on, the precharge circuit eventually overheats and fails, so both need to be in series and work together.
There is an open source precharge unit available from https://www.zeva.com.au/ He decided he'd had enough of designing and building EV stuff so open sourced the designs. All his stuff was good quality, so worth a look ....

T1 Terry

[goboatingnow]

Originally Posted by T1 Terry

That works fine for those that actually remember to turn the inverter's off before resetting the battery isolator. With the inverter still switched on, the precharge circuit eventually overheats and fails, so both need to be in series and work together.

There is an open source precharge unit available from https://www.zeva.com.au/ He decided he'd had enough of designing and building EV stuff so open sourced the designs. All his stuff was good quality, so worth a look ....



T1 Terry

Excellent resource Terry thanks