How is this Possible?

Wjm1964

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I connected a 4 cell 12 volt 100Ah Lifepo4 that has a JBD 100A bms to a 3500w inverter (fused 350A) plugged in a 3000w kettle which pulled 249 DC amps (checked with clamp meter) and fully expected the bms to shut off as it was almost 150A more than the BMS rating. But it didn't, so i tried another 12v 100aH LIFEPO4 battery that uses a different brand BMS and exactly the same outcome. e.g the kettle boiled no issue. What am i missing here about my understanding of what a BMS is supposed to do. I know they can or are meant to allow for a surge briefly but are they not meant to kill the power from the battery if you are pulling more than the rating? 100 Amp BMS =100Amps? 150A and so on. I'm confused.
 

Wjm1964

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I'm connecting direct from battery terminals using 50mm cable. Looking more at the BMS data it would seem they don't actually kill power when drawing high amps. They rely on over temperature sensors and only cut off power when over charge / discharge voltages are met. on a cell I'm even more confused now as to why they are rated in amps when they don't control / limit amps being drawn. Guess that's what fuses are for .
 

Supervstech

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I'm connecting direct from battery terminals using 50mm cable. Looking more at the BMS data it would seem they don't actually kill power when drawing high amps. They rely on over temperature sensors and only cut off power when over charge / discharge voltages are met. on a cell I'm even more confused now as to why they are rated in amps when they don't control / limit amps being drawn. Guess that's what fuses are for .
This.

They will eventually limit the amperage, once overheated, but depending on how it is mounted, and ambient temps, they could happily output surge current for quite a while… until something fails, or the temp limits are reached.
This is why critical cable fuses are required, and recommended amperage breakers are a good idea,
 

Supervstech

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Also, since your loads show the need for higher amperage, you need either a better BMS, or more of them to handle the loads you are using.
 
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3000 ac watts / .85 inverter efficiency / 12.6 volts nominal = 275.735294118 service amps

I am amazed that your battery is putting up with this treatment.
Not just the BMS but the cells themselves.

Please provide product links to the battery and BMS in question.
Also to the kettle.
In North America our kettles top out at ~1500 watts.
Guess you are in another geo.

I wonder if the FETs=field effect transistors in your BMS have already failed closed and no longer provide any protection.
 

Wjm1964

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Yes I'm in Europe and the point I'm making is that a bms has no say over how many amps are going through it when it's being discharged. It will cut off in an over charge current scenario and will also cut off when a low voltage is detected on a cell. It will balance the cells but it won't stop you pulling high amps. I used a SOK 12v 100Ah and an eco worthy 12v 100ah and 4 new 12v 100ah cells with a daly 100a bms. These are all new fully charged units. So why are BMS systems rated in Amps if they don't control amps being drawn. Yes over temp would probably kick in but probably too late to prevent damage. Hence my confusion why they are rated in amps.
 
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These are all new fully charged units. So why are BMS systems rated in Amps if they don't control amps being drawn.

They are rated in amps because FETS are in the high current path and are prone to failure due to heat stress.

A FET based BMS usually contains protection circuits designed to disconnect on over-current among other things.
The higher the current the quicker they typically disconnect.
Unfortunately FETs can fail closed.
Meaning the protection circuits can no longer disconnect loads or charge sources.
Often the user has no idea there is a problem until the BMS fails to disconnect when it should and the battery is ruined.
 

Steve_S

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Just for Clarification.
Did you attempt this with Multiple Batteries in Parallel ? That would share/divide load between all battery packs attached in parallel.
3x100AH Packs can deliver 300A for 1 Hour. Almost all BMS allow for a "short surge burst" of up to 5C for a few seconds but not all.
A Single Battery with a 100A BMS even with default settings should have kicked off.

! A WARNING !
When you look at a 100A BMS (FET Based) as advertised by virtually all makes, are to the performance edge (capacity) of those chips, It is generally suggested to DERATE any FET Based BMS by a minimum of 20% but preferably by 25%. BMS that use Relays/Contactors are more robust (hence their use in EV's and other Heavy Load systems).
 
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@Wjm1964 do you have a safe way to test that your BMS can still disconnect loads and charge sources?
If you have a smart BMS they usually allow you to administratively disable charging and discharging.
 

Wjm1964

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Just for Clarification.
Did you attempt this with Multiple Batteries in Parallel ? That would share/divide load between all battery packs attached in parallel.
3x100AH Packs can deliver 300A for 1 Hour. Almost all BMS allow for a "short surge burst" of up to 5C for a few seconds but not all.
A Single Battery with a 100A BMS even with default settings should have kicked off.

! A WARNING !
When you look at a 100A BMS (FET Based) as advertised by virtually all makes, are to the performance edge (capacity) of those chips, It is generally suggested to DERATE any FET Based BMS by a minimum of 20% but preferably by 25%. BMS that use Relays/Contactors are more robust (hence their use in EV's and other Heavy Load systems).
I used one battery for each test, they all allowed the inverter to draw 249 dc amps. This is why I'm confused as logic says they shouldn't let this happen. I'm going to repeat with a 150A bms out of sheer curiosity
 

Wjm1964

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3000 ac watts / .85 inverter efficiency / 12.6 volts nominal = 275.735294118 service amps

I am amazed that your battery is putting up with this treatment.
Not just the BMS but the cells themselves.

Please provide product links to the battery and BMS in question.
Also to the kettle.
In North America our kettles top out at ~1500 watts.
Guess you are in another geo.

I wonder if the FETs=field effect transistors in your BMS have already failed closed and no longer provide any protection.
It's not my normal use, I was curious as I bought my bms Based on its amps rating like everyone does I guess, just didn't think it would allow me to draw so many amps.
 

Supervstech

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I used one battery for each test, they all allowed the inverter to draw 249 dc amps. This is why I'm confused as logic says they shouldn't let this happen. I'm going to repeat with a 150A bms out of sheer curiosity
Ok... stop testing this limit.

The bms has ratings YOU are supposed to abide by. Exceeding them WILL result in damage to the bms, the battery cells, or your entire home...
 

Supervstech

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A bms has a single purpose. To keep the bank from getting overcharged, or out of balance.
The ratings are there to show how robust they are built... they in no way are there to show you how they will limit current.

Think of it like an extension cord.

An 8A rated extension cord will handle 30 amps for some time... but it will fail quickly and could burn down the house.

Use the ratings to organize how many units you need to safely handle the intended current.
 

Wjm1964

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Ok... stop testing this limit.

The bms has ratings YOU are supposed to abide by. Exceeding them WILL result in damage to the bms, the battery cells, or your entire home...
I fully understand this but the point is we all think the bms will protect the battery and our systems when in reality it comes down to Fuses and common sense. In normal circumstances we would never ask more from a system than it can safely deliver but im thinking what if's here.
 

Supervstech

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I fully understand this but the point is we all think the bms will protect the battery and our systems when in reality it comes down to Fuses and common sense. In normal circumstances we would never ask more from a system than it can safely deliver but im thinking what if's here.
You forgot one word in your first sentence after the "we all"...

Incorrectly
 

Wjm1964

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A bms has a single purpose. To keep the bank from getting overcharged, or out of balance.
The ratings are there to show how robust they are built... they in no way are there to show you how they will limit current.

Think of it like an extension cord.

An 8A rated extension cord will handle 30 amps for some time... but it will fail quickly and could burn down the house.

Use the ratings to organize how many units you need to safely handle the intended current.
Thank you for your clarification. As a relatively newbie I bought my bms and batteries based on my expectations of loads etc which would never exceed 30 amps. I now know more today than I did yesterday. Thanks
 
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