Charleswgibbs
Solar Enthusiast
- Joined
- Aug 5, 2022
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- 261
An inverter will always consume more power than it puts out, so if it is capable of putting out 3000w that means it's drawing more than that from the battery.
How do I Size a BMS for a LiFePO4 Battery Pack
- Thread starter Calothrix
- Start date
An inverter will always consume more power than it puts out, so if it is capable of putting out 3000w that means it's drawing more than that from the battery.
I have the exact same set up as yours except I have a 250a Daly BMS on each battery. The only reason that I have 250a is that I built one battery then built another.
A 200a BMS on each battery will provide you with ample headroom and if you look at @FilterGuy post then you can see you could even go down to 150a per battery, even more so if you are not using the inverter to its maximum.
You are correct but not quite correct in your thinking. Inverters do put out less because of their inefficiency but your maximum on the output side is 3000w. To produce that power it takes approximately 3333.33w from the batteries.
Tim
Louisvdw
Solar Enthusiast
- Joined
- Dec 18, 2020
- Messages
- 166
All the BMS that my driver support will give your the same features (some soft calculated and others from the BMS). So the choose your BMS you have to look at other things.
For a beginner the easiest BMS would be either a JKBMS or JBD.
The JKBMS has less settings in the app, but the app is easy, instructions are good, and they have large active balancers.
The JBD you can set every parameter and is even easier to set up, although the app does not look as good as the JKBMS.
Daly users have the most problems. Their protocol is the least well designed, so that limits the speed at which we can retrieve data from the BMS. Most users struggle with balancing and add an external balancer. If you have it, keep it. If you need to buy a BMS, choose something else.
Ant is a solid BMS. They just work. The set up require that you have to solder or wire up a connection for the USB TTL converter to the GX as you can't buy a pre made wire from the supplier, so it is a small bit more involved.
There are other options as well that are very good, but would be a bit more expensive or the set up is more complex. For instance the ECS GreenMeter and LiPro are Victron grade devices and also worth a look.
Good luck with your choices
Thank you all for your help!
@FilterGuy @Charleswgibbs @Tim Tim @Louisvdw etc...
I'm leaning towards 2x of the new JKBMS 4s 200amp with Heater ports for my 2x DIY 4s EVE 280ah 3.2:
https://diysolarforum.com/threads/jk-4s-200a-bms.38623/
Now on to how to top balance and compress and fit/set everything up in my 2x empty 4s battery cases.
@FilterGuy @Charleswgibbs @Tim Tim @Louisvdw etc...
I'm leaning towards 2x of the new JKBMS 4s 200amp with Heater ports for my 2x DIY 4s EVE 280ah 3.2:
https://diysolarforum.com/threads/jk-4s-200a-bms.38623/
Now on to how to top balance and compress and fit/set everything up in my 2x empty 4s battery cases.
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 now back to finding a solid (with support/warranty) 4s 12v 150-200amp BMSs to use on 2x 280ah EVE batteries with communication to Victron.
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 now back to finding a solid (with support/warranty) 4s 12v 150-200amp BMSs to use on 2x 280ah EVE batteries with communication to Victron.
bds70
New Member
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?
Anyone able to explain that in simple terms?
MisterSandals
Participation Medalist
Can you provide a link to one/some that you are referring to?
bds70
New Member
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!
bds70
New Member
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?
and would this affect my choice of BMS size?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
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?
bds70
New Member
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?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
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.
bds70
New Member
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.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
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?
bds70
New Member
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?
bds70
New Member
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?
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.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
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.
bds70
New Member
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.
