Basics of BMS implementation

Hey all,

First post here. I've looked through a lot of the threads and been looking for some good stickies for new users but haven't been able to find what I'm looking for.

I'm wanting to build a power wall for off-grid implementation on a new cabin I'm planning out. I have found a lot of awesome information about different batteries, construction implementation, and wiring. But I'm still confused by the implementation of the BMS system and what will work and what wont.

I read the review on the 48v 14s ChineseBMS which seemed like a possibility for my system. I also saw BatriumBMS system and browsed theirwebsite but I am still a bit confused on how it works and how it connects to the various possible configurations of a powerwall.

Say I build a 14s100p wall. Obviously, with each of these types of BMS I think it's unlikely that you can monitor the entire 1,400 cells and compare them all to each other.

So from what I've gathered, there are two primary ways to build these packs. I could build 14 packs of 100cells all attached in parallel. This seems to significantly simplify the bus bar wiring which seems advantageous on the construction side of things and I think I can understand better how that BMS might work. Basically, it would be the same implementation as if it was just on a 14s arrangement but I've traded out each battery for a pack of 100 batteries.

In this configuration, the BMS would be reading the entire 100 batteries as a single battery and making sure each "pack" got to 4.2v during charge. Does this work because the batteries are in parallel so physically they all have to reach the same voltage at the same time? This would make sure pack 1-14 all charge equally but Are there still issues with batteries of different capacities and conditions (particularly if using recycled cells) within the pack of parallel cells? is there anything that needs to happen to balance those? I know these batteries are fused to the bus bars so is that the only "management" that is implemented within the pack?

With the second possible build configuration, each pack would have series and parallelconnections inside the same holder. I've seen a lot of batteries built on youtube this way. So each pack would be 14s and say 10P or 140cells in each pack. these packs would have 14 cells negative down, then 14 more positive down and so forth. Now it seems that each of these would then be paralleled again so it would be like a 10p14s10p i guess. This configuration really makes no sense to me how you would possibly implement a BMS.

Can someone tell me if I'm on the right track with the first configuration and what things I would need to be careful of when designing the 14 different packs? I assume I need to balance the total capacity of each pack as closely as possible so that the BMS system doesn't have to work as hard to balance things out. What other considerations need to be taken within the pack?

One downside to configuration1 that I see as an advantage to #2 would be future expansion. If I wanted to add another 100 cells I couldn't build another 100 cell pack and add it in series or it would push the voltage to high. I'm not sure if you could just tie it in parallel to one of the existing packs or if that could cause some balance issues. ex. pack 7 has two 100cell packs in parallel.

With the second configuration, it would be easy to add another 48v pack in parallel to increase capacity.


I know this post had a lot of questions. Feel free to point me towards other posts or resources. I tried to search around a lot but still feel like there are some gaps in my understanding.

Thanks,
Matt

You pretty much got it. You listed the pros/cons of each style of arrangement. The #2 one is sometimes also called "system voltage pack".

Both methods can be expanded upon fairly easily. The biggest difference is the amount of cells required for #1 style of build to be expanded upon. So you'd need over 1000 cells, for example, to add another string. Whereas with the second method, you could add another pack that's only 140 cells (14s10p as an example)

Really, each style is up to the end user how they want to implement them. I personally plan on system voltage packs, but I also am designing my own bms to handle this.

Korishan said:

You pretty much got it. You listed the pros/cons of each style of arrangement. The #2 one is sometimes also called "system voltage pack".

Both methods can be expanded upon fairly easily. The biggest difference is the amount of cells required for #1 style of build to be expanded upon. So you'd need over 1000 cells, for example, to add another string. Whereas with the second method, you could add another pack that's only 140 cells (14s10p as an example)
}
Really, each style is up to the end user how they want to implement them. I personally plan on system voltage packs, but I also am designing my own bms to handle this.

Click to expand...

So this is where I'm stuck. How do you implement the BMS on the voltage pack arrangement? you can have one BMS for each pack but the expense of that seems to add up quickly. I like the expandability of the second arrangement because I'd like to add to the wall as I am able. However, this is my main sticking point to going this direction.


Another question I thought of, If I find an issue with a large pack of say 100 cells in the first arrangement. How do you go about finding the various cells in the pack that need to be replaced? Just take it apart and re-process all of them to weed out the weak cells?

You take it apart yes.

This is how you can have 1 bms om multiple packs.

lurch632 said:

Korishan said:

You pretty much got it. You listed the pros/cons of each style of arrangement. The #2 one is sometimes also called "system voltage pack".

Both methods can be expanded upon fairly easily. The biggest difference is the amount of cells required for #1 style of build to be expanded upon. So you'd need over 1000 cells, for example, to add another string. Whereas with the second method, you could add another pack that's only 140 cells (14s10p as an example)
}
Really, each style is up to the end user how they want to implement them. I personally plan on system voltage packs, but I also am designing my own bms to handle this.

Click to expand...

So this is where I'm stuck. How do you implement the BMS on the voltage pack arrangement? you can have one BMS for each pack but the expense of that seems to add up quickly. I like the expandability of the second arrangement because I'd like to add to the wall as I am able. However, this is my main sticking point to going this direction.

Click to expand...

By building my own BMS solution, the costs will be a lot lower. My path I am working towards is that there will be a master controller, which will in turn talk to the various slave modules on each pack. These slave modules will be self sufficient if the master isn't talking to them, and they will maintain the balances. The system I'm working on is based on the TI BQ769x0 series battery monitoring APU"s. Mikethezipper has done some extensive designing on this chip as well: https://secondlifestorage.com/t-TI-BQ769X0-Based-Monitors-Project-thread

Each pack will have an ESP32 or ESP8266 for the MCU, and each pack will also have the BQ chip, plus some other sensors and balancing boards. Then all these will talk with each other using BT most likely, to keep the WIFI bands clear for internet. Then the master will be an ESP32 or an STM32 with a BT module.

Korishan urs Will not be cheaper. Count all hours you put into Them.

I believe the future will hold larger cells and distributed BMS systems... I currently have 50Ah cells to work with. Much easier than 2Ah. Managing becomes less of a problem, individual cell problems are negated. I couldnt imagine 2800 connections being reliable MTBF. I could do that in much less. I cna do what is in (100) 18650 cells in five ( of the cells that I have.. ). 250Ah.

Research a distributed BMS vs a non distributed.

lurch632 said:

Korishan said:

You pretty much got it. You listed the pros/cons of each style of arrangement. The #2 one is sometimes also called "system voltage pack".

Both methods can be expanded upon fairly easily. The biggest difference is the amount of cells required for #1 style of build to be expanded upon. So you'd need over 1000 cells, for example, to add another string. Whereas with the second method, you could add another pack that's only 140 cells (14s10p as an example)
}
Really, each style is up to the end user how they want to implement them. I personally plan on system voltage packs, but I also am designing my own bms to handle this.

Click to expand...

So this is where I'm stuck. How do you implement the BMS on the voltage pack arrangement? you can have one BMS for each pack but the expense of that seems to add up quickly. I like the expandability of the second arrangement because I'd like to add to the wall as I am able. However, this is my main sticking point to going this direction.


Another question I thought of, If I find an issue with a large pack of say 100 cells in the first arrangement. How do you go about finding the various cells in the pack that need to be replaced? Just take it apart and re-process all of them to weed out the weak cells?

Click to expand...

Lurch, you can see in my old threads that i am as well favorite the second option you dsecribe, several independant full voltage packs which are parallelized and can easily withdrawn.

And the disadvantage for that is the need of a separate BMS for each, totally correct. Only solution is to go to cheaper ones, where you barely find something apalling for the vbalancing function.
( Remember - the term "BMS" is for several independatnt features like Voltage protecting, balancing et al. See the FAQ for nice basics).
At moment i do a compromise - i have increased my "Module" size from 6s8p to 10s20P, and these get a separate BMS with programmable balancing. (Bluetooth version from this board on Alipress)
As a second i still have a project of doing a KISS active Balancer but that must wait until i have my system up in the conventional way.