What size BMS do I need for a multiple BMS system?

[TommyHolly]

Hey everyone,
I searched through all the other posts and couldn’t find an exact answer for this.

I need to know:
1. What size BMSs do I need?
2. If I buy four of them, do they connect to each other in some way?
3. What other equipment/wiring/monitoring system is needed?
4. The cost of everything?

I have a 48-cell system I’m rebuilding. Each cell is 3.2v 100Ah. I plan on building four large 12-cell 3P 4S, so each battery would be 12v 300Ah. And the entire battery bank would be 1,200Ah 12v. I was told I would need a separate BMS for each of my four 300Ah 12v batteries for my system.

I have three Outback Solar Controllers charging the system. A Flexmax 60A, a Flexmax 80A and a final Flexmax 80A.

I will also be using this with a 3,000w Xantrex Charger/Inverter. (6,000w max surge)

I’m guessing I would need at least four 200A BMSs?
Here is my math and reasoning, not sure if this makes sense??? 6,000w max surge on the Xantrex at 12v would be 500A. Four batteries in parallel with each having its own 200A BMS would be 800A. Even if one battery/BMS dropped out, I think I’d have enough so the system would continue to run.

Better yet, four 250A BMS for a total of 1,000A. If let’s say two of the batteries/BMS dropped out, I’d still have the minimum 500A remaining to run the system. Does that make sense?

I currently have one 500A DALY 4S LiFePo4 Smart BMS and was hoping I could buy three more and they would work with each other. I don’t know if multiple BMS needs to work with each other? I also don’t know if the four batteries need to be balanced in between one another???

 

[John Frum]

3000 ac watts / .85 conversion factor / 12 volts low cutoff = 294.117647059 service amps.

overkill solar has a 4s bms rated for 120 amps.
I would put 1 on each battery.
Each battery including BMS is independent but the aggregate current is shared.
Special care must be taken to ensure the circuit resistance for each battery is as close as possible to ensure that they share the charge and load as equitably as possible.
See section 3.3 of this document https://diysolarforum.com/resources/wiring-unlimited.2/

 

[TommyHolly]

3000 ac watts / .85 conversion factor / 12 volts low cutoff = 294.117647059 service amps.

overkill solar has a 4s bms rated for 120 amps.
I would put 1 on each battery.
Each battery including BMS is independent but the aggregate current is shared.
Special care must be taken to ensure the circuit resistance for each battery is as close as possible to ensure that they share the charge and load as equitably as possible.
See section 3.3 of this document https://diysolarforum.com/resources/wiring-unlimited.2/

I contacted Overkill Solar a few weeks ago and he responded a few days ago saying BMS are on back order.

Since I already have a 500A DALY 4S BMS, couldn’t I just order 3 more of them? And do you really think only a 125A BMS is needed?

I’ll read the document now.

 

[John Frum]

I contacted Overkill Solar a few weeks ago and he responded a few days ago saying BMS are on back order.

Since I already have a 500A DALY 4S BMS, couldn’t I just order 3 more of them? And do you really think only a 125A BMS is needed?

Do you have any significant pure dc loads.
I've done the math for the inverter so if that is the entire load then 4x overkill BMSs will be adequate for the task.
4x 500A daly BMSs will be overkill. ?
Which is fine.
Just out of curiosity what guage of wire and what insulation rating does Daly put on a 500A BMS????

 

[TommyHolly]

Do you have any significant pure dc loads.
I've done the math for the inverter so if that is the entire load then 4x overkill BMSs will be adequate for the task.
4x 500A daly BMSs will be overkill. ?
Which is fine.
Just out of curiosity what guage of wire and what insulation rating does Daly put on a 500A BMS????

I can’t wait for the Overkill BMS. It takes 1-2 weeks just for them to respond to an email. And they said they were on back order last time I spoke to them about 12 days ago.

I will be using a combination of 1/4” solid copper bars for the parallel connections. And 1/0 Guage welding wire (very flexible) with 3/8 tinned copper ring connectors.

The DALY wire is pretty thick. I think they have two 1 Guage Wires. The Will Prose DIY Solar video he was bragging about it.

If I can, I’d like to use that 500A 4S DALY BMS I mistakenly bought with three more DALY BMSs. I was guessing that 250A would work but if you think I need less, maybe that’s fine too? (The cost isn’t that much different)

Do I need anything special to connect 4 BMSs together? Thanks.

 

[TommyHolly]

Do you have any significant pure dc loads.
I've done the math for the inverter so if that is the entire load then 4x overkill BMSs will be adequate for the task.
4x 500A daly BMSs will be overkill. ?
Which is fine.
Just out of curiosity what guage of wire and what insulation rating does Daly put on a 500A BMS????

Ooops I forgot to respond to the DC loads.

I have 2 small fridges that run off the 12v DC (the size people use in college), and sometimes my Navigation gear. Small LED lights and fans run off the 12v system and you can plug in your iPhone into the 12v chargers. The biggest would be the 12v 24A watermaker.

Really though, on the main panel, all of these things running at the same time, I’ve only seen a maximum of 12V 22A being loaded and it’s usually around 10A most of the day.

 

[Tecnodave]

One BMS for each battery and they do not need to communicate with each other in any way. Each battery with its BMS is a separate system.

I just bought two JBD BMS’s exact same unit as the Overkill from Current_Connect, minor issue with the order on pre shipping, JBD shipped the wrong cell sense wire with the BMS so shipping was delayed a bit. Current_Connect was very informative and fixed the issue, response to calls was fast and informative. I am happy with the way they deal with customers.

My system is 24 volt with 2 seperate banks of 2P8S LFP pouch cells @ 200 amps per bank so you will be using a different BMS but very similar.

Post a pic of your boat, I only got sail my Brown for a few years

 

[TommyHolly]

One BMS for each battery and they do not need to communicate with each other in any way. Each battery with its BMS is a separate system.

I just bought two JBD BMS’s exact same unit as the Overkill from Current_Connect, minor issue with the order on pre shipping, JBD shipped the wrong cell sense wire with the BMS so shipping was delayed a bit. Current_Connect was very informative and fixed the issue, response to calls was fast and informative. I am happy with the way they deal with customers.

My system is 24 volt with 2 seperate banks of 2P8S LFP pouch cells @ 200 amps per bank so you will be using a different BMS but very similar.

Post a pic of your boat, I only got sail my Brown for a few years

Thanks for the straight forward answer!
Ok, one BMS per battery and they don’t need to connect in any way.

Do you think 250A BMS per battery is too much, too little or just right? Since I already have a 500A DALY BMS still new in the box, I plan on using that one and ordering three 250A BMSs for a total of 4 BMSs.

My boat is a 2012 Lagoon 450F Catamaran. I’ve been working on it for 4 months and spending insane amounts of money thanks to the previous owner’s poor installation jobs and repairs on most things. This lithium battery system has been a nightmare for me trying to rip everything out and get it designed correctly. Here are some pics. The last one is the way the battery bank initially looked when I got it.

 

[John Frum]

If I can, I’d like to use that 500A 4S DALY BMS I mistakenly bought with three more DALY BMSs. I was guessing that 250A would work but if you think I need less, maybe that’s fine too? (The cost isn’t that much different)

Having 1x 500amp BMS and 3x 250 amp BMSs is likely going to result in different resistance through the high capacity BMS.
Asymmetric resistance means asymmetric current flow which is to be avoided.

 

[TommyHolly]

Having 1x 500amp BMS and 3x 250 amp BMSs is likely going to result in different resistance through the high capacity BMS.
Asymmetric resistance means asymmetric current flow which is to be avoided.

Aaargh, I was afraid of that. Thanks for the warning. Ok, back to the most important question…
What is the Amp value of the four BMS I need??

 

[John Frum]

Aaargh, I was afraid of that. Thanks for the warning. Ok, back to the most important question…
What is the Amp value of the four BMS I need??

I would not go lower than the 120 continuous rating of the overkill BMS.
Also the ratings on the cheap ebay or ali specials need to be taken with extreme skepticism.

 

[TommyHolly]

I would not go lower than the 120 continuous rating of the overkill BMS.
Also the ratings on the cheap ebay or ali specials need to be taken with extreme skepticism.

Ah ok. So the MINIMUM BMS I need would be 120A correct?

Ok I emailed Overkill Solar again. Let’s hope they respond faster than 2 weeks this time.

120A is the largest that Overkill Solar makes. Is there any other brands which are decent and easy to use for a NOOB like me?

 

[John Frum]

Ah ok. So the MINIMUM BMS I need would be 120A correct?

Yes

Ok I emailed Overkill Solar again. Let’s hope they respond faster than 2 weeks this time.

120A is the largest that Overkill Solar makes. Is there any other brands which are decent and easy to use for a NOOB like me?

The only commodity fet based bms that I can recommend is JBD/Overkill.
I don't even recommend Daly although some folks here report good results.
Their are more expensive options like Rec and Batrium which are more expensive but scale better.
Those are a little more involved to properly integrate and I have no personal experience with them.
Personally, I have an 8s overkill bms.

 

[TommyHolly]

Yes

The only commodity fet based bms that I can recommend is JBD/Overkill.
I don't even recommend Daly although some folks here report good results.
Their are more expensive options like Rec and Batrium which are more expensive but scale better.
Those are a little more involved to properly integrate and I have no personal experience with them.
Personally, I have an 8s overkill bms.

Ok cool.
I’ve written Overkill again since they don’t have a phone number. I also like the fact that I can watch Will’s video for an explanation of some of the features and setup.

So 4 BMS x 120A each = 480A correct?
If one of these batteries gets shut down, does the system’s Amp handling capacity go down by 120A as well??

And what if for some reason I hit my surge max of 6,000w / 12v = 500A?? Am in underpowered? (I don’t understand how multiple BMS work.)

 

[John Frum]

Ok cool.
I’ve written Overkill again since they don’t have a phone number. I also like the fact that I can watch Will’s video for an explanation of some of the features and setup.

So 4 BMS x 120A each = 480A correct?

480A in a perfect world.
In reality it will be less.
How much less depends on the asymmetry of the draw from the batteries.
If you are really meticulous with the setup lets guess at a de-rate to 80 percent.
480amps * .8 de-rate = 384 amps

If one of these batteries gets shut down, does the system’s Amp handling capacity go down by 120A as well??

384 amps * .75 = 288 amps.
Just slightly less than enough to run the inverter at full load.

And what if for some reason I hit my surge max of 6,000w / 12v = 500A?? Am in underpowered? (I don’t understand how multiple BMS work.)

The flagship Xantrex inverter/charger can only deliver 6000 watts for ~5 seconds so its not really significant to the math.

The batteries including bms are configured in parallel which means they share the load more or less equally.

 

[TommyHolly]

480A in a perfect world.
In reality it will be less.
How much less depends on the asymmetry of the draw from the batteries.
If you are really meticulous with the setup lets guess at a de-rate to 80 percent.
480amps * .8 de-rate = 384 amps

What is the “De-Rate” exactly? I’ve never heard of it so I haven’t been taking it into account.

384 amps * .75 = 288 amps.
Just slightly less than enough to run the inverter at full load.

The inverter at Full Load of 3,000w without a surge is 250A I thought? The most if ever be using anyway is about 2,400w if I had the ice maker and Air Conditionimg running simultaneously. Anything more and I’d simply turn on the 5kw generator.

The flagship Xantrex inverter/charger can only deliver 6000 watts for ~5 seconds so its not really significant to the math.

The batteries including bms are configured in parallel which means they share the load more or less equally.

 

[John Frum]

What is the “De-Rate” exactly? I’ve never heard of it so I haven’t been taking it into account.

Derating - Wikipedia

en.wikipedia.org

The inverter at Full Load of 3,000w without a surge is 250A I thought?

3000 ac watts / .85 conversion factor / 12 volts low cutoff = 294.117647059 service amps.
294.117647059 service amps / .8 fuse headroom = 367.647058824 fault amps.
You need your aggregate bms continous rating with a de-rate factor be greater than service amps.

The most if ever be using anyway is about 2,400w if I had the ice maker and Air Conditionimg running simultaneously. Anything more and I’d simply turn on the 5kw generator.

Its my policy to design for the full continuous rating of the BMS.

 

[John Frum]

I see I've repeated myself regarding the math.

 

[TommyHolly]

Derating - Wikipedia

en.wikipedia.org

3000 ac watts / .85 conversion factor / 12 volts low cutoff = 294.117647059 service amps.
294.117647059 service amps / .8 fuse headroom = 367.647058824 fault amps.
You need your aggregate bms continous rating with a de-rate factor be greater than service amps.


Its my policy to design for the full continuous rating of the BMS.

Hey thanks! I’m learning a lot.

Ok, should I just buy four DALY 200A or even 250A BMS’s?

That way im more than covered since Overkill only goes to 120A each. They haven’t written me back anyway.

 

[John Frum]

Hey thanks! I’m learning a lot.

Ok, should I just buy four DALY 200A or even 250A BMS’s?

That way im more than covered since Overkill only goes to 120A each. They haven’t written me back anyway.

As I've said before in this thread, I can't recommend Daly.
I'm glad to answer questions.
Not glad to repeat myself.