BMS sizing questions

[Stepandwolf]

The multiplus has substantial surge capability. Consult the fuse size and wire size listed in the manual for an idea of the input current they design for (it will be larger than you expect, Victron is a conservative company), this will give you a ballpark idea of what size BMS may be appropriate. Since a fuse is based on max continuous current but also has some safety margin built in.

The simple math I use to figure out rough max DC current (for the multi 3k):

based on continuous rating and 90% efficiency
2400W / 24V / ~0.9 = ~110A

based on 30 min surge 85% efficiency
3000W / 24V / ~0.85 = ~150A

based on max rated surge 85% efficiency
6000W / 24V / ~0.85 = ~300A

Not sure how accurate the efficiency numbers are but from the one Victron efficiency curve I've seen peak efficiency is at the full continuous rating.

That is the kind of information I was looking for. This decision is two part. Given the Victron Multiplus 24v 3000/70, can I use those numbers to determine the BMS output that is needed? All I could find on the Victron manual is that they want a 300A fuse and 50mm cable. In my ignorance, that tells me the inverter won't be taking more than 300A at 24V. It would then be safe to assume the BMS wouldn't need to have greater than 300A output? But isn't the fuse typically rated higher than the draw, i.e. a 300a fuse for a 250a draw? Not knowing any better, I was looking at this model of Daly: Smart BMS Lifepo4 8S 24V 300A UART/485 FAN Should it have active balance, but doesn't? I have been watching a Heltec thread and that seems to be a viable candidate as well?

 

[Dzl]

All I could find on the Victron manual is that they want a 300A fuse and 50mm cable.

50mm equals 1/0 AWG (i think)

In my ignorance, that tells me the inverter won't be taking more than 300A at 24V.

In my ignorance, I have the same takeaway

It would then be safe to assume the BMS wouldn't need to have greater than 300A output? But isn't the fuse typically rated higher than the draw, i.e. a 300a fuse for a 250a draw?

I don't know how inverter fuses are sized, I do know a common rule of thumb for overccurent protection is: minimum fuse size of current x 1.25 (but that is not universally applicable).
Victron is a reputable and conservative company, I would trust there fuse size to be properly sized.

The fuse size might be a good proxy for BMS size.

I think a 300A BMS for a 3000W / 24v BMS is a healthy safety margin. Its large enough that even at peak surge power its technically within the BMS limits, and at a more realistic continuous or moderate surge its got a healthy safety margin.

I was looking at this model of Daly...Should it have active balance, but doesn't?

Not necessarily. Almost no BMS popular here has active balancing.

The names are 'active' and 'passive' can be misleading. The (practical) difference is that a passive balancer uses resistors to 'burn off' energy from high cells, while an active balancer transfers energy from one cell to another. The other practical difference is active balancers tend to have higher max current.

But this one does:

I have been watching a Heltec thread and that seems to be a viable candidate as well?

I think active balancing is a 'nice to have' feature, not a necessity. Unless your cells are mismatched to the point a low current passive balancer can't keep the pack balanced during normal use in which case it becomes more than just a 'nice to have'

That said, if I was looking at a FET based BMS, I would strongly consider the heltec active balancing BMSes.

There are many others more informed on this topic than I am, this is just my 2c from the small amount of research I have done.

 

[Txglader]

50mm equals 1/0 AWG (i think)

In my ignorance, I have the same takeaway


I don't know how inverter fuses are sized, I do know a common rule of thumb for overccurent protection is: minimum fuse size of current x 1.25 (but that is not universally applicable).
Victron is a reputable and conservative company, I would trust there fuse size to be properly sized.

The fuse size might be a good proxy for BMS size.

I think a 300A BMS for a 3000W / 24v BMS is a healthy safety margin. Its large enough that even at peak surge power its technically within the BMS limits, and at a more realistic continuous or moderate surge its got a healthy safety margin.


Not necessarily. Almost no BMS popular here has active balancing.

The names are 'active' and 'passive' can be misleading. The (practical) difference is that a passive balancer uses resistors to 'burn off' energy from high cells, while an active balancer transfers energy from one cell to another. The other practical difference is active balancers tend to have higher max current.

But this one does:


I think active balancing is a 'nice to have' feature, not a necessity. Unless your cells are mismatched to the point a low current passive balancer can't keep the pack balanced during normal use in which case it becomes more than just a 'nice to have'

That said, if I was looking at a FET based BMS, I would strongly consider the heltec active balancing BMSes.

There are many others more informed on this topic than I am, this is just my 2c from the small amount of research I have done.

A common misconception on the victron 3000, is that it is 3000 watts. It it 3000va. Not sure on the 24v but the 12v is 2400 watts. Not dissing Victron as I love mine, just passing along information.

 

[Stepandwolf]

A common misconception on the victron 3000, is that it is 3000 watts. It it 3000va. Not sure on the 24v but the 12v is 2400 watts. Not dissing Victron as I love mine, just passing along information.

You are correct. I found an ad from a Victron dealer, and the title of the ad says "

Victron Energy MultiPlus 3000 Watt 24 Volt Inverter & 70 Amp Battery Charger"​

and on the Battleborn site:

Victron Multiplus Inverter Charger 3000 W 24 V​

Knowing it was VA, I still called it W.

 

[Dzl]

A common misconception on the victron 3000, is that it is 3000 watts. It it 3000va. Not sure on the 24v but the 12v is 2400 watts. Not dissing Victron as I love mine, just passing along information.

Victron does a really poor job communicating to the end user what the actual ratings of the inverter are. Actual I would just say Victron does a poor job communicating to the end user in general. Full stop.

Compare a Victron datasheet or even manual to a Samlex or Magnum datasheet and you will see what I mean. I really like Victron, but I think their documentation is not on par with other top tier brands.

My understanding is (and you will find this nowhere in the doc's but it is explained in a Victron Webinar) 3 ratings:
Using the Victron Multiplus 3000 as an example:
Continuous: 2400
30 minute: 3000
Surge: 6000

I think the 2K is:
1600
2000
4000

But I may be mistaken.

I believe this is where I got that info

 

[Txglader]

Victron does a really poor job communicating to the end user what the actual ratings of the inverter are. Actual I would just say Victron does a poor job communicating to the end user in general. Full stop.

Compare a Victron datasheet or even manual to a Samlex or Magnum datasheet and you will see what I mean. I really like Victron, but I think their documentation is not on par with other top tier brands.

My understanding is (and you will find this nowhere in the doc's but it is explained in a Victron Webinar) 3 ratings:
Using the Victron Multiplus 3000 as an example:
Continuous: 2400
30 minute: 3000
Surge: 6000

I think the 2K is:
1600
2000
4000

But I may be mistaken.

I believe this is where I got that info

I looked at several others and went with Victron because of how all components communicate with each other and one display to see everything.

You are correct on being light compared to the competition.

 

[Txglader]

UPS says in transit. Ordered November 17, UPS label created December 4 (cells obviously not in CA then), now showing shipped January 26. Should be in NH in a week or so... BMS discussion now more relevant!

You have a thread that you are following for BMS discussion?

 

[HamptonHall]

50mm equals 1/0 AWG (i think)

In my ignorance, I have the same takeaway


I don't know how inverter fuses are sized, I do know a common rule of thumb for overccurent protection is: minimum fuse size of current x 1.25 (but that is not universally applicable).
Victron is a reputable and conservative company, I would trust there fuse size to be properly sized.

The fuse size might be a good proxy for BMS size.

I think a 300A BMS for a 3000W / 24v BMS is a healthy safety margin. Its large enough that even at peak surge power its technically within the BMS limits, and at a more realistic continuous or moderate surge its got a healthy safety margin.


Not necessarily. Almost no BMS popular here has active balancing.

The names are 'active' and 'passive' can be misleading. The (practical) difference is that a passive balancer uses resistors to 'burn off' energy from high cells, while an active balancer transfers energy from one cell to another. The other practical difference is active balancers tend to have higher max current.

But this one does:


I think active balancing is a 'nice to have' feature, not a necessity. Unless your cells are mismatched to the point a low current passive balancer can't keep the pack balanced during normal use in which case it becomes more than just a 'nice to have'

That said, if I was looking at a FET based BMS, I would strongly consider the heltec active balancing BMSes.

There are many others more informed on this topic than I am, this is just my 2c from the small amount of research I have done.

You have a thread that you are following for BMS discussion?

Seems like BMS is discussed in several places, including this thread.

I find the BMS issue daunting. I'm building a 24v battery with the 8 cells I bought from Michael, and have the Growatt 3KW/24V charger/inverter Will recommended. Specs show pure sine output 3KW and input MPPT 80A. If I use the formula 3000w/24v/.88 I get 142A. Seems too close to use the Overkill so I am looking at either the Daly 150A or the Daly 200A? My first, and probably only, build so any advice/suggestions greatly appreciated! Thanks in advance!

 

[Txglader]

Seems like BMS is discussed in several places, including this thread.

I find the BMS issue daunting. I'm building a 24v battery with the 8 cells I bought from Michael, and have the Growatt 3KW/24V charger/inverter Will recommended. Specs show pure sine output 3KW and input MPPT 80A. If I use the formula 3000w/24v/.88 I get 142A. Seems to close to use the Overkill so I am looking at either the Daly 150A or the Daly 200A? My first, and probably only, build so any advice/suggestions greatly appreciated! Thanks in advance!

I am on a diy battery group on Facebook. Ther is a guy that is building a 12v 280Ah battery. He has a Daly 250amp on order. He is to report back test results when it comes in. I will let you know the results.

 

[Off Grid Thad]

Seems to close to use the Overkill

Overkill with the app is very solid, the cell controls are great. If you need more output, set it up with a relay. Will has a good Utube vid on that. I’m going to set up 2 of my 10 that way so I can have extra draw when needed. And with a replace it even if you screw it up warranty.

 

[Michael B Caro]

Seems like BMS is discussed in several places, including this thread.

I find the BMS issue daunting. I'm building a 24v battery with the 8 cells I bought from Michael, and have the Growatt 3KW/24V charger/inverter Will recommended. Specs show pure sine output 3KW and input MPPT 80A. If I use the formula 3000w/24v/.88 I get 142A. Seems to close to use the Overkill so I am looking at either the Daly 150A or the Daly 200A? My first, and probably only, build so any advice/suggestions greatly appreciated! Thanks in advance!

If you go Daly, the price difference between the 250a and the 200a is only about $25.00. I am a cheap B@st@rd, but I would still do the 200a.

 

[michael d]

Seems like BMS is discussed in several places, including this thread.

I find the BMS issue daunting. I'm building a 24v battery with the 8 cells I bought from Michael, and have the Growatt 3KW/24V charger/inverter Will recommended. Specs show pure sine output 3KW and input MPPT 80A. If I use the formula 3000w/24v/.88 I get 142A. Seems to close to use the Overkill so I am looking at either the Daly 150A or the Daly 200A? My first, and probably only, build so any advice/suggestions greatly appreciated! Thanks in advance!

what type of solar panels? if 60 cell or 72cell solar panels, then the Electrodacus SBMS0 and Electrodacus DSSR20's are great for a 24-volt battery build. the SBMS0 is the BMS. The DSSR20's are the solid-state charge controllers no mppt needed if match the panels. the SBMS0 will also work with your Mppt if that is what you choose. the SBMS0 is designed for up to 8S 24-volt battery banks.

 

[Stepandwolf]

I am trying to narrow down to a specific BMS to order. With help from here, it was determined that a 200A model would be a proper fit for my 24V 8s configuration.

I found this Daly model which seems to be appropriate but it's title and description don't agree:

Daly Electric Bicycle Scooter waterproof UART BMS 3.2V LiFePO4 Lithium Battery 24V Smart bms 8s 200a with fan

As you can see, it clearly says 3.2V batteries, but the description for the product says:
"DL 8S (3.7V rated Li-ION Battery not for 3.2V Rated LiFePO4)"

I don't want to order the wrong model. I wrote to Daly about why their title and description don't match. Perhaps someone has bought the Daly 24V 8S Smart BMS and can give me the correct URL to the correct product.
Thanks

 

[Dzl]

I find the BMS issue daunting. I'm building a 24v battery with the 8 cells I bought from Michael, and have the Growatt 3KW/24V charger/inverter Will recommended. Specs show pure sine output 3KW and input MPPT 80A. If I use the formula 3000w/24v/.88 I get 142A. Seems too close to use the Overkill so I am looking at either the Daly 150A or the Daly 200A? My first, and probably only, build so any advice/suggestions greatly appreciated! Thanks in advance!

To be clear the formula is only a way to ballapark the maximum continuous current. This is useful when sizing fuses, or sizing wire or sizing a BMS but isn't the number you should be shooting for. You'll still probably want to size up beyond this number. For 142A max load, I personally would feel more comfortable around the ~175-250A range, this preference isn't based on any rule or extensive experience or anything, just my personal preference to design in substantial safety margin where possible, particularly with off-brand/chinese electrical components which can't always be counted on for their full rating.

 

[Koontzy]

just a quick question about the same topic guys, If I'm building a 280Ah 12v battery, I should aim for 300-350A BMS ?
Also, would theses addons would be useful in my build :
50w heating pad with thermostat
low temp cutoff switch
30A inline fuse
breaker
cigarette lighter output x 2
USB output x 4
12v generic output (small gauge)
MC2 input x 2
cheap (and especially small) solar charge controller MPPT (or even PWM I plan on using 2 x 100W or maybe even 4x100W)
battery % waterproof display
Fews waterproof switch for the outside of the box for the different components

I dont want to go over the top, but I want to build a portable small power station. I even tough about combining 8 cells instead of 4 but the weight alone would be too much ?

Btw I've just noticed I'm in the wrong place for asking theses questions but many probably have done it already

 

[Dzl]

just a quick question about the same topic guys, If I'm building a 280Ah 12v battery, I should aim for 300-350A BMS ?

Not necessarily. Amp-Hours (as in battery capacity) and Amps (as in current / flow of electrons) are separate concepts/units of measurement. Your battery bank capacity doesn't directly relate to your BMS size. What is of concern is current flowing through the BMS, this is what the BMS rating (in Amps) refers to. You would guestimate this by either adding up all the loads that might be used at one time, or if your main or only consumer will be the inverter, you can get a ballpark by doing the math above to figure out the inverters max input current.

As to the other questions/rest of the comment, I think starting a thread or doing some research in the beginner section, the portable power station/solar generator section, or the general section would be best.

The discussion is wandering away from the thread topic.

 

[Stepandwolf]

Not necessarily. Amp-Hours (as in battery capacity) and Amps (as in current / flow of electrons) are separate concepts/units of measurement. Your battery bank capacity doesn't directly relate to your BMS size. What is of concern is current flowing through the BMS, this is what the BMS rating (in Amps) refers to. You would guestimate this by either adding up all the loads that might be used at one time, or if your main or only consumer will be the inverter, you can get a ballpark by doing the math above to figure out the inverters max input current.

As to the other questions/rest of the comment, I think starting a thread or doing some research in the beginner section, the portable power station/solar generator section, or the general section would be best.

The discussion is wandering away from the thread topic.

I am also guilty of discussing BMSs in this string, is there a way, for you as moderator, to move the posts that are only BMS related to a new thread? I would gladly make one in response to my own quest for a BMS but think a generic BMS thread might be better as so much of the conversation has generic value.

 

[Koontzy]

Thanks for that Dzl!!

 

[Stepandwolf]

Thanks for that Dzl!!

Thanks from me too....one battery post ended up here that should have been left behind HamptonHall 5:49a although in a later message he did go down the BMS route.

 

[Dzl]

Thanks from me too....one battery post ended up here that should have been left behind HamptonHall 5:49a although in a later message he did go down the BMS route.

Thanks, its been unmoved now