do I need four BMS's


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Anannaki

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Mar 8, 2018
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i am finishing my 4x14s200p 18650 li-ionpower wall.
do I need four 70A BMS's as I use 3 4048 PIP's. each inverter can draw 80A.
please advise which BMS to useexcept Batrium which I can't afford.
 

daromer

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You need 1 bms per battery string or a bms that can take 4 strings such as Batrium or DIY BMS.

You need a bms that can handle the full load on one string. If you never plan to remove any string thats that the 3 inverters in total can draw from the battery bank

A 4048 can easily pull up to 8kW from the battery in short time. The max charging from solar alone is 80a but combined with ac its 160A....
8Kw at lets say lowest voltage of 40V is 200A.. Thats a single unit. 3 units is then 600A.

You need to be able to surge 600A from the Battery bank.
Having 4 strings of each 14s200p should handle this. You need atleast 200A bms per battery string at minimum if you ask me!

Will it be cheap? NO. But this is something you should design for before you start building and buying stuff. You really should get a proper BMS that can handle it. You already have tons of cells, 3 rather larger inverters.. And i guess you will power a more expensive house?

Look into DIY bms and then atleast one larger 500A ABB shunt example S5 series.
 

Anannaki

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Mar 8, 2018
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daromer said:
You need 1 bms per battery string or a bms that can take 4 strings such as Batrium or DIY BMS.

You need a bms that can handle the full load on one string. If you never plan to remove any string thats that the 3 inverters in total can draw from the battery bank

A 4048 can easily pull up to 8kW from the battery in short time. The max charging from solar alone is 80a but combined with ac its 160A....
8Kw at lets say lowest voltage of 40V is 200A.. Thats a single unit. 3 units is then 600A.

You need to be able to surge 600A from the Battery bank.
Having 4 strings of each 14s200p should handle this. You need atleast 200A bms per battery string at minimum if you ask me!

Will it be cheap? NO. But this is something you should design for before you start building and buying stuff. You really should get a proper BMS that can handle it. You already have tons of cells, 3 rather larger inverters.. And i guess you will power a more expensive house?

Look into DIY bms and then atleast one larger 500A ABB shunt example S5 series.
can I use two of these( one for two 14s200p strings) BMSs:https://www.banggood.com/6S-to-28S-...-p-1658226.html?rmmds=search&cur_warehouse=CN
 

ajw22

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Nov 16, 2018
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That BMS is not a good choice for powerwalls, because it has fixed voltage settings, and balancing function will only kick in once cells reach 4.18V.
Lithium cell lifespan increases dramatically when kept below 4.1V (or lower), so you'll want a BMS that can be configured to work properly at those lower voltages.
A commonly cited figure is 2x lifespan increase for every 0.1V lower top voltage.

Also, often those cheap BMS specs are inflated. Should consider 1/2 the stated values as the long term safe level.
 

Eddy

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Jun 16, 2018
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Ibiza said:
i am finishing my 4x14s200p 18650 li-ionpower wall.
do I need four 70A BMS's as I use 3 4048 PIP's. each inverter can draw 80A.
please advise which BMS to useexcept Batrium which I can't afford.
Not in a bad light but batruim is not expensive comparing to the work it does. And can do any kind of series and parallel you want.Also you can balance from as low as 3.70v all the way to 4.20v. Can handle 350Amp charge current (in my setup) amd drain 400Amps! Chinese bms is no good. not relaible. But its my thinking.
 

daromer

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And if you cant afford batrium the question is how much you want to tinker? DIY BMS if you know your way around programming and soldering.
Daly, Ant and some others offers higher end Ebike BMS systems that can and many use for powerwalls with set able values.
 

Anannaki

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Mar 8, 2018
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Hi gents
I appreciate your efforts and willingness to answer my questions.
following your advise I'll go for four ANT BMS 300A units.
cheers and thanks again
 

Charly144

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Nov 22, 2017
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I went the route with 5 china bms and now Im switching to 5 strings each with diybms and 5 large relays. Be aware that in case 4 of my 5 bms switched off, all load is running over one bms. This happened several times in my setup and burned all my bmses...
 

100kwh-hunter

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Charly144 said:
I went the route with 5 china bms and now Im switching to 5 strings each with diybms and 5 large relays. Be aware that in case 4 of my 5 bms switched off, all load is running over one bms. This happened several times in my setup and burned all my bmses...

I guess when one relay is switched off, the whole bank will be offline then?

Would it not be simpler(with my limited knowledge) to prevent a burning bms to add some wires extra between the stringsthem self.
Basically make it one whole string (in parallel)in the "back" withsome thinner wire, so your packs can take and give each other some juice?
I assume your stringsare all small isolated islands?

https://www.aliexpress.com/item/4000001793955.html?spm=a2g0o.cart.0.0.5e063c00cKmEnp&mp=1
Or this one:
https://shop.gwl.eu/Battery-Managem...ete-Set-4-cells-with-Bluetooth-4-0.html?cur=1

Or adjust the discharge and over charge voltage in the inverter? so the bms's wont even have to go to work?

With best regards Igor


daromer said:
And if you cant afford batrium the question is how much you want to tinker? DIY BMS if you know your way around programming and soldering.
Daly, Ant and some others offers higher end Ebike BMS systems that can and many use for powerwalls with set able values.

I think in general after a certain capacity or the amount of strings added a batrium is in the long run cheaper and much safer?
If you would like to build more then 4 strings for sure

With best regards Igor
 

Charly144

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Nov 22, 2017
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I have 5 strings 14s80p, each controlled by one single 40a bms... my inverters do 150amp total in discharge mode and 105 amp in charge mode. All strings on is no issue in discharge mode, but when 2 bms switch off, it is too much for residual 3 bms... in my setup so far were no relays.

It is even worse in charge mode. 40amp bms can only handle 20 amp in charge mode, which is way too low for my setup. I did limit charge current but even in this case i run into problems when one or two bms switch off and current is too high for residual 3 or even worse one bms.

Ant bms (300A) is ok in discharge mode but it also can handle 50amp only in charge Mode.

My tip: in a reasonable powerwall, use a bms with a relay instead of mosfets...

I didnt want to do 14s300p due to monitoring and servicing reasons...

Regards
Karl
 

daromer

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Thats why you dont add multiple small bms in parallel. Its just wrong way of thinking and in the end what happens is what you found out. A bms must be able to operate alone IF something happens.

Dont use relay. Use a breaker with shunt trip function.

Ant aning others are ebike bms systems a d therefore thats why they are designed like that :)
 

Charly144

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daromer said:
Thats why you dont add multiple small bms in parallel. Its just wrong way of thinking and in the end what happens is what you found out. A bms must be able to operate alone IF something happens.

Dont use relay. Use a breaker with shunt trip function.

Ant aning others are ebike bms systems a d therefore thats why they are designed like that :)

that is what we call a learning curve :D
 

daromer

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Costly one when people dont listen :)

Ibiza then i hope its designed so that 1 bms can take full load easily? Else you end up with same issues and a waste of money
 

Anannaki

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daromer said:
Costly one when people dont listen :)

Ibiza then i hope its designed so that 1 bms can take full load easily? Else you end up with same issues and a waste of money
supplier says it's 300A unit. my 3inverter can draw 12000w/48v witch is 250A if my math is correct.
 

daromer

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if you got 3 4kw inverters your mx is alot more.
12kw is the nominal draw on the inverter side. What is the max of the inverters? If its pip 4kw inverter its 8kw..

Neither less 12kw nominal is easy 15kw at the battery side. At the battery side lets say you run 14s?
14*2.8V as minimum during sag thats 39V. 15000/39 = 390A. So just with the nominal max wattage you are way above what that bms can take!!

Then guess what happens if you have a spike as an example. If its the pip the pip is capable of 2x the factor and thats up towards 30kW at the battery side during couple of seconds. You can do the math ;)

Note that above is worst scenarios BUT thats what the BMS is there to protect for.
 

Anannaki

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daromer said:
if you got 3 4kw inverters your mx is alot more.
12kw is the nominal draw on the inverter side. What is the max of the inverters? If its pip 4kw inverter its 8kw..

Neither less 12kw nominal is easy 15kw at the battery side. At the battery side lets say you run 14s?
14*2.8V as minimum during sag thats 39V. 15000/39 = 390A. So just with the nominal max wattage you are way above what that bms can take!!

Then guess what happens if you have a spike as an example. If its the pip the pip is capable of 2x the factor and thats up towards 30kW at the battery side during couple of seconds. You can do the math ;)

Note that above is worst scenarios BUT that's what the BMS is there to protect for.
I apologize for delayed response @DAROMER . your post is very precious for us who are not so familyar with electronics and we can learn a lot from you and other guys who are willing to share their knowledge and experience .
My 3 PIPs are more for comfort then for high load. in nearly 6 years of use I found12kw spikes only twice when I welded .
Now I am at the beginning of searching for the BMS devices as I canceled previous purchase of ANT BMSs. The reason for cancellation was discovery that ANT BMS can't connect neither in parallel nor in serial connection.
 

daromer

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No you dont connect BMS systems in parallel nor in series. NONE out there are intended to it when it comes to that type of BMS

Also important is that you NEVER design for what you think you will use normally. you ALWAYS design for what the equipment can draw max when it comes to such electronics.

You got 3 Pips and those 3 together if something happens can pull heckalot of power! You need to make sure the infrastructure can handle that. You can potentially save your arse with some breakers/fuses that go off earlier but still you need to design for those.

Because suddenly you got your mother in law to visit or something and they turn on the oven and the microwave at the same time... And it may work out once or twice but suddenly that wire is toast and no insulation is left or that battery bank have been pushed to the limits 1 time to many :)

So take the advice correctly now :) Lithium batteries are dangerous if not handled correctly and 2nd hand cells even more. But as long as we take proper counter measures using them they will take care of us for a long time.

Cheaper Ebike BMS systems have a max current for a reason and thats because that what they can tollerate max. The Fuse or gear should limit well before that or you migh end up with a shorted BMS that doesnt do its job ;)
 
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