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How do I Size a BMS for a LiFePO4 Battery Pack

One thing that totally confuses me, is that if you buy a ready made consumer lifepo4 battery, there is never any mention of inverter size or draw that you can use that battery with. They all have a BMS internally, but no data relating to the expected max amp draw that applies.

Anyone able to explain that in simple terms?
 
So let's say I have 4 x 4s 12v lifepo4 320ah batteries in parallel, wired to a Lynxx Distributor common bus bar, supplying my two 3000w inverters. Let's suggest they are drawing (as an example) 5000w. How many amps would each battery be supplying?
and would this affect my choice of BMS size?
 
bds70 said:
Here's one
https://outbax.com.au/products/12v-...utm_content=&utm_medium=cpc&utm_source=google
But from what I can see, none of these companies makes reference to the current draw in relation to the BMS sizing. Not even Victron.
Confusing!
Click to expand...
In my experience, it is the exception not the rule that the seller does not give the current capability.... but sometimes it is in the form of the BMS rating. Example"....with a built-in 100A BMS...". That tells me the battery can deliver a continuous 100A.
bds70 said:
So let's say I have 4 x 4s 12v lifepo4 320ah batteries in parallel, wired to a Lynxx Distributor common bus bar, supplying my two 3000w inverters. Let's suggest they are drawing (as an example) 5000w. How many amps would each battery be supplying?
and would this affect my choice of BMS size?
Click to expand...

The BMS needs are defined by the load.... not the battery. If the load is 5000W, and the battery voltage is 48V, the total current going to the load is 5000w/48V=104.2A.

With a 12V battery arrangement of 4x4, there are 4 strings of batties in parallel and each string would supply ~ 1/4 of the total current. So for a 104.2A, the draw for each string would be 26A.

Related question, It sounds like you need a 48V system. Why not start with a 48V battery?
 
FilterGuy said:
In my experience, it is the exception not the rule that the seller does not give the current capability.... but sometimes it is in the form of the BMS rating. Example"....with a built-in 100A BMS...". That tells me the battery can deliver a continuous 100A.


The BMS needs are defined by the load.... not the battery. If the load is 5000W, and the battery voltage is 48V, the total current going to the load is 5000w/48V=104.2A.

With a 12V battery arrangement of 4x4, there are 4 strings of batties in parallel and each string would supply ~ 1/4 of the total current. So for a 104.2A, the draw for each string would be 26A.

Related question, It sounds like you need a 48V system. Why not start with a 48V battery?
Click to expand...
After having to look at all the equipment I would have to sell, all victron, 2 x MPPT, 2 x 3000w Inverters, lynxx distributor, smart shunt, and then - buy an AIO inverter/charger hybrid, new wiring, fuses breakers etc (some I might already have)
48V to 12V converter plus 1 12v battery to run existing 12v equipment, even though I would like to change, I don't think it's financially achievable for me.

Plus I know the quality of Victron, despite my recent MPPT fire, is still probably the best and safest on the market.

I have to get rid of the 2 340ah commercially supplied batteries that are useless, after ongoing issues with failing BMS's, charging, balancing, etc. Hopefully the Co is going to see sense in a full refund rather than engaging in a fight, and if so, that will allow me to buy the necessary cells and BMS's to build my own system, that will be under my control for better or worse.

If I go by the guidance here, then I think I will have an efficient long lasting off grid solution moving forward.

Back to the BMS question, so if I am getting it correct, then each of the 12v 4s batteries would require only a 100amp BMS?
 
bds70 said:
12v 4s batteries would require only a 100amp BMS
Click to expand...

Does 12V 4S in this context mean 4 cells in series with a BMS to create a 12V battery? (Given the previous post, I assume so)

Yes, a 100A BMS would *probably* do the trick. The reason I emphasize probably is surge current. 4 parallel batteries with 100A capability will easily handle a 5000W Continuous load. However, what is the surge load? An AC motor can easily create a start surge 4 or 5 times the normal run-time current. 400A for the set-up you are describing will probably handle any surge other than starting large motors. If the BMS shuts off as soon as it hits it's rated current, there may be issues if the load has large surge currents. Others can be set to allow a surge current for several seconds safely.

I don't know what inverter you have, but most of the Victron inverters are Low-Frequency inverters with impressive surge capability for both current and duration.
 
FilterGuy said:
Does 12V 4S in this context mean 4 cells in series with a BMS to create a 12V battery? (Given the previous post, I assume so)

Yes, a 100A BMS would *probably* do the trick. The reason I emphasize probably is surge current. 4 parallel batteries with 100A capability will easily handle a 5000W Continuous load. However, what is the surge load? An AC motor can easily create a start surge 4 or 5 times the normal run-time current. 400A for the set-up you are describing will probably handle any surge other than starting large motors. If the BMS shuts off as soon as it hits it's rated current, there may be issues if the load has large surge currents. Others can be set to allow a surge current for several seconds safely.

I don't know what inverter you have, but most of the Victron inverters are Low-Frequency inverters with impressive surge capability for both current and duration.
Click to expand...
Yes your correct re the batteries.
I have 2 Multiplus 12/3000/120-50 unfortunately because the existing one I had when I decided to install an induction cooktop and 3 new panels was an older model, I couldn't put them in parallel. So each one stands alone.

1 has a house style 240V fridge, smart tv, 2.5kva reverse cycle AC, 7.5kg front load washing machine plus 8 240v outlets.
The other has the induction cooktop, a second fridge, a 2kva AC, a 1300w airfryer and 6 240v outlets for other items like hair dryers etc
Perhaps 150amp BMS x 4 might be a better choice.

Thanks for all the help.
 
bds70 said:
Yes your correct re the batteries.
I have 2 Multiplus 12/3000/120-50 unfortunately because the existing one I had when I decided to install an induction cooktop and 3 new panels was an older model, I couldn't put them in parallel. So each one stands alone.

1 has a house style 240V fridge, smart tv, 2.5kva reverse cycle AC, 7.5kg front load washing machine plus 8 240v outlets.
The other has the induction cooktop, a second fridge, a 2kva AC, a 1300w airfryer and 6 240v outlets for other items like hair dryers etc
Perhaps 150amp BMS x 4 might be a better choice.

Thanks for all the help.
Click to expand...
The price difference between a 100A and 150A BMS is not that great, so going with a 150 is not a bad plan, but given the loads described, 100A BMSs are probably fine. What brand BMS are you looking at?
 
FilterGuy said:
The price difference between a 100A and 150A BMS is not that great, so going with a 150 is not a bad plan, but given the loads described, 100A BMSs are probably fine. What brand BMS are you looking at?
Click to expand...
Not sure yet, I don't hear anything good about Daly, Jbd or JK seem to have decent reviews although further up this thread on Gette's post (not sure how to do a quote)
"Well scratch JK off the list.
Seems users have been reporting issues and failures. But the worst of it is the rep on here, the manufacturer and the Ali reseller are ignoring them and not honoring warranty."

So not sure at the moment. Any suggestions?
 
FilterGuy said:
In my experience, it is the exception not the rule that the seller does not give the current capability.... but sometimes it is in the form of the BMS rating. Example"....with a built-in 100A BMS...". That tells me the battery can deliver a continuous 100A.


The BMS needs are defined by the load.... not the battery. If the load is 5000W, and the battery voltage is 48V, the total current going to the load is 5000w/48V=104.2A.

With a 12V battery arrangement of 4x4, there are 4 strings of batties in parallel and each string would supply ~ 1/4 of the total current. So for a 104.2A, the draw for each string would be 26A.

Related question, It sounds like you need a 48V system. Why not start with a 48V battery?
Click to expand...
I was just re reading your answer and I was wondering about the calculation. If I have a 4 cell 320ah 12v battery, repeated 4 times and each battery wired in parallel, wouldn't the total V still be 12, and the amps would be 1280.

So wouldn't the current calc be 5000/12 = 416A and then 416/4 = 104A from each battery?
 
bds70 said:
I was just re reading your answer and I was wondering about the calculation. If I have a 4 cell 320ah 12v battery, repeated 4 times and each battery wired in parallel, wouldn't the total V still be 12, and the amps would be 1280.

So wouldn't the current calc be 5000/12 = 416A and then 416/4 = 104A from each battery?
Click to expand...
That looks correct to me, I think there was some confusion from your earlier post, where you wrote "So let's say I have 4 x 4s 12v lifepo4 320ah batteries in parallel." The 4s makes it sound like you have 4 48v volt batteries wired in parallel which are each comprised of 4 12v batteries wired in series.
 
bds70 said:
I was just re reading your answer and I was wondering about the calculation. If I have a 4 cell 320ah 12v battery, repeated 4 times and each battery wired in parallel, wouldn't the total V still be 12, and the amps would be 1280.

So wouldn't the current calc be 5000/12 = 416A and then 416/4 = 104A from each battery?
Click to expand...
Reed Cole said:
That looks correct to me, I think there was some confusion from your earlier post, where you wrote "So let's say I have 4 x 4s 12v lifepo4 320ah batteries in parallel." The 4s makes it sound like you have 4 48v volt batteries wired in parallel which are each comprised of 4 12v batteries wired in series.
Click to expand...
Yup.... I had originally thought we were looking at a 48V system. The OP corrected me but I never went back and adjusted the numbers. Sorry for the confusion.

Now let me confuse things a bit more: What about inverter efficiency? Victron is usually pretty good, so let's assume a 95% efficiency.

For 5000W we need (5000W/.95)/12V = 439A total or ~ 110A/battery.

NOTE: 12V gives the worst case: A full LiFePO4 12V battery would be closer to 14V. (5000W/.95)/14V = 376A or 94A/ battery. However, I would recommend designing for the worst case of 110A rather than the best case of 94A. Consequently, I would recommend at least a 150A BMS.
 
FilterGuy said:
Yup.... I had originally thought we were looking at a 48V system. The OP corrected me but I never went back and adjusted the numbers. Sorry for the confusion.

Now let me confuse things a bit more: What about inverter efficiency? Victron is usually pretty good, so let's assume a 95% efficiency.

For 5000W we need (5000W/.95)/12V = 439A total or ~ 110A/battery.

NOTE: 12V gives the worst case: A full LiFePO4 12V battery would be closer to 14V. (5000W/.95)/14V = 376A or 94A/ battery. However, I would recommend designing for the worst case of 110A rather than the best case of 94A. Consequently, I would recommend at least a 150A BMS.
Click to expand...
Thanks for all your help, I am ecstatic, I finally got some math correct ????.
I think my ignorance on all things solar 8 yrs ago was a 'car salesmans' dream, if U get my drift. So 48V system which would have been cheaper and better never got a mention.
Nevertheless I have had good service from the system up to now. So I can't really complain.
Once the new year arrives I shall be starting the new build. Checking here for clarification as needed.

It's a great community, may you all have a happy safe and productive 2024.
 
bds70 said:
I think my ignorance on all things solar 8 yrs ago was a 'car salesmans' dream, if U get my drift. So 48V system which would have been cheaper and better never got a mention.
Click to expand...
8 years ago there were very few 48V systems (If any). 48V has only really taken off in the past 4 years or so.
 
In the higher Amperage output above 100A, there are few BMS options, or at least few economical ones.
I have certainly had my share of issues with Daly BMS's in low current models, however, I have never had any issues at all with the high current Daly (150A, 200A) and I have to point this out. It may be unfair to paint all their products with the same brush.
Feel free to correct me, or point out my high amperage Daly's could be just about to fail, but honestly these seem to be fine from my own experience - anyone seen differently? Curious to know.
 
OffGridForGood said:
In the higher Amperage output above 100A, there are few BMS options, or at least few economical ones.
I have certainly had my share of issues with Daly BMS's in low current models, however, I have never had any issues at all with the high current Daly (150A, 200A) and I have to point this out. It may be unfair to paint all their products with the same brush.
Feel free to correct me, or point out my high amperage Daly's could be just about to fail, but honestly these seem to be fine from my own experience - anyone seen differently? Curious to know.
Click to expand...
I am not knowledgeable enough to correct anyone lol. I just see a few less than enthusiastic reviews.
 
Oh no, reading another thread regarding BMS & inverter communication I am now confused as to whether I need some more extra equipment.

To explain my very limited knowledge. I thought that my Victron. MPPT controlled and monitored the charging of my batteries and the BMS inside the battery, prevents over/under charge, temp monitoring and active balancing of cells. Then the inverter draws the required power from the batteries to power all loads.

Then I read this

JK BMS to communicate with any inverters that support Goodwe and Pylontech Low voltage batteries via CAN
Click to expand...
The thread is very long and way over my head. But it seems to suggest that in order to get full control over charging discharging the BMS must talk to the inverter, in my case a Victron Multiplus.

Am I going down a rabbit hole?
 
Rabbit hole, the BMS is a system to monitor the battery pack and cells to protect them in the event of:
- a cell hits too high a voltage (while the pack as a whole has not reached high voltage)
- a cell hits too low a voltage (while the pack as a whole has not hit low voltage cut off)
- over current protection for the pack,
- under voltage protection for the pack.

All of the above are protections, they are not controlled by the inverter, they are in addition to it.
 
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