BMS opinions

R

Reecew89

100 mW
Joined
Aug 25, 2021
Messages
35
Hello All,

One for the Guru's, Had a discussion with a chap today who said he build packs 300/400+amps max current and he only install 30/50amp BMS's and only for charging says he never had a controller go pop.

So it made me think is there actually any fact behind this and/or potential safety aspect?.
 
Needs clarification.

Most BMSs just latch off when current goes over, they do not actually Limit the current

and may need manual resetting.

Some just smoke like a fuse and need replacing.

Usual practice is size the BMS at least 20% of the greatest current you will ever allow them to see. Cheap Chinese maybe double, buy 120A to handle say 50/80 the time factor "x amps for 10sec, y continuous" applies

Also BMS can be rigged as voltage monitor only, controls external relays rather than the power flowing through the BMS, so the relays are purchased as above to handle the current.

Charging may go through a different BMS port from discharge, and is usually a lower rate, not needing OCP so much.

The whole point of a BMS is last-ditch protection **of the battery pack** from failures in the other devices

Not afaik anything to do with the controller.
 
Controllers only use as much juice as they want to use. It's no good trying to moderate them from the battery side.

If you want to use a BMS for balancing only, and not have high or low voltage cutoff protection, then it's better to use an active balancer board and not a full feature BMS. This is because active balancers work wherever there's an imbalance, and not only at the top of charge like most BMSes. When the pack isn't fully charged, some BMSes actually push the pack out of balance by drawing on just one or two cells to keep their circuitry running.
 
john61ct said:
Needs clarification.

Most BMSs just latch off when current goes over, they do not actually Limit the current

and may need manual resetting.

Some just smoke like a fuse and need replacing.

Usual practice is size the BMS at least 20% of the greatest current you will ever allow them to see. Cheap Chinese maybe double, buy 120A to handle say 50/80 the time factor "x amps for 10sec, y continuous" applies

Also BMS can be rigged as voltage monitor only, controls external relays rather than the power flowing through the BMS, so the relays are purchased as above to handle the current.

Charging may go through a different BMS port from discharge, and is usually a lower rate, not needing OCP so much.

The whole point of a BMS is last-ditch protection **of the battery pack** from failures in the other devices

Not afaik anything to do with the controller.
Click to expand...

So the discussion was focused around controller's and power they could pull, I assumed that a controller that could demand say 250Amps would need a bms that was 300amps (To be on the safe side) He stated that a BMS would not be required as the battery pack would only send the current the controller asked for (so to speak).

So this had me thinking of a project i am underway with, 20s12p Molicell p42a Max current 540amps controller can handle 460, Now i thought if i commected battery to controller it would potentially brick the controller as there is not control over the current being fed from the battery.

Hopefully some clarity for you.
 
Chalo said:
Controllers only use as much juice as they want to use. It's no good trying to moderate them from the battery side.

If you want to use a BMS for balancing only, and not have high or low voltage cutoff protection, then it's better to use an active balancer board and not a full feature BMS. This is because active balancers work wherever there's an imbalance, and not only at the top of charge like most BMSes. When the pack isn't fully charged, some BMSes actually push the pack out of balance by drawing on just one or two cells to keep their circuitry running.
Click to expand...

Well as per my above reply to the other person, Idea was a BMS for charging only, No BMS for discharge so the controller could pull maximum power it could BUT worry it would brick the controller.
 
Reecew89 said:
No BMS for discharge so the controller could pull maximum power it could BUT worry it would brick the controller.
Click to expand...

That's not a thing. You could hook up a 35A controller to a nuclear power plant, and if all the power was only 48VDC, it would make no difference.

In stinking gas motor terms, the battery is only the fuel system. If it can't keep up with the motor, that's a problem. If it can feed a hundred times as much fuel as the motor needs, that's not a problem.

Your controller will only draw as much power from the battery and deliver as much power to the motor as it's designed to.
 
Reecew89 said:
I assumed that a controller that could demand say 250Amps would need a bms that was 300amps
Click to expand...
Yes if you wanted to use a BMS as usually implemented, that is the way to go.

> a BMS would not be required as the battery pack would only send the current the controller asked for (so to speak)

Whether you want a BMS at all is a personal judgment call, but that issue about current has nothing to do with that question.

If you know when to stop using the motor as voltage goes down, then the LVC function of the BMS - the only one used while riding - is not required.

If you trust your charger not to overcharge, then the BMS' HVC function becomes redundant.

If you know your balancing routines are sufficient, then that function is not required from the BMS.

And thus in that case a BMS may not be desired at all.


> So this had me thinking of a project i am underway with, 20s12p Molicell p42a Max current 540amps controller can handle 460, Now i thought if i commected battery to controller it would potentially brick the controller as there is not control over the current being fed from the battery.

It is not the job of a BMS to protect the controller.

Nothing "pushes" current, it is the load that **pulls** the amps it requires.

Therefore it is the battery that needs protection from the motor, and that is exactly the controller's job. The controller determines what current level it will **make available** (not push) to the motor, within its limitations.

And of course within the battery's limits.

The user can program a good controller so it does not draw more than X battery amps, or ramps the power level down when the motor starts to overheat, etc. That is where the intelligence of the system resides, other than the rider's brain watching the readouts.

>> The whole point of a BMS is last-ditch protection **of the battery pack** from failures in the other devices

 
A BMS for charging only is fine, but note that it does not actually **do** any charging.

It should act as a redundant HVC failsafe, terminating charging if the charger's controls allow the voltage to rise above its pack-level CV setpoint.

It may do balancing, starting when the charger has brought the battery voltage up to the start-balance setpoint.

Since the charger's controls are usually based on pack-voltage only, when the pack is out of balance at the top, the BMS may be needed to prevent the weaker cells from rising above its max per-cell voltage setpoint.
 
Chalo said:
Reecew89 said:
No BMS for discharge so the controller could pull maximum power it could BUT worry it would brick the controller.
Click to expand...

That's not a thing. You could hook up a 35A controller to a nuclear power plant, and if all the power was only 48VDC, it would make no difference.

In stinking gas motor terms, the battery is only the fuel system. If it can't keep up with the motor, that's a problem. If it can feed a hundred times as much fuel as the motor needs, that's not a problem.

Your controller will only draw as much power from the battery and deliver as much power to the motor as it's designed to.
Click to expand...

Appreciate the reply 👍
 
john61ct said:
Reecew89 said:
I assumed that a controller that could demand say 250Amps would need a bms that was 300amps
Click to expand...
Yes if you wanted to use a BMS as usually implemented, that is the way to go.

> a BMS would not be required as the battery pack would only send the current the controller asked for (so to speak)

Whether you want a BMS at all is a personal judgment call, but that issue about current has nothing to do with that question.

If you know when to stop using the motor as voltage goes down, then the LVC function of the BMS - the only one used while riding - is not required.

If you trust your charger not to overcharge, then the BMS' HVC function becomes redundant.

If you know your balancing routines are sufficient, then that function is not required from the BMS.

And thus in that case a BMS may not be desired at all.


> So this had me thinking of a project i am underway with, 20s12p Molicell p42a Max current 540amps controller can handle 460, Now i thought if i commected battery to controller it would potentially brick the controller as there is not control over the current being fed from the battery.

It is not the job of a BMS to protect the controller.

Nothing "pushes" current, it is the load that **pulls** the amps it requires.

Therefore it is the battery that needs protection from the motor, and that is exactly the controller's job. The controller determines what current level it will **make available** (not push) to the motor, within its limitations.

And of course within the battery's limits.

The user can program a good controller so it does not draw more than X battery amps, or ramps the power level down when the motor starts to overheat, etc. That is where the intelligence of the system resides, other than the rider's brain watching the readouts.

>> The whole point of a BMS is last-ditch protection **of the battery pack** from failures in the other devices
Click to expand...

Well i have to admit you have opened my eyes a fair shout more, Every day is a school day as they say.

So if i then went along the route of a smaller BMS rating and a good active balancer charger that is looking like the best option?.
 
All up to you, depends on your design choices.

Separate active balancers do the best job but with a new healthy pack with good cells might only be needed a half dozen times in the first 100 cycles.

Hobby chargers with balancing functionality are only cheap at 8S and lower.

BMS has to be sized for your current unless you go to external relays
 
john61ct said:
All up to you, depends on your design choices.

Separate active balancers do the best job but with a new healthy pack with good cells might only be needed a half dozen times in the first 100 cycles.

Hobby chargers with balancing functionality are only cheap at 8S and lower.

BMS has to be sized for your current unless you go to external relays
Click to expand...

Any reccomendations on balancers?

Also with the BMS talked to further people and the general opinion is they use 30/50amp bms BUT not for disacharge only cell monitoring, Bypassed so controller can pull what it requires current wise from the pack, Would you say thats the way to go with a BMS (Bluetooth/Screen)
 
Deligreen QNBBM is well regarded

1S units to handle any S count or 4S and bigger.

https://m.aliexpress.com/item/1950876697.html

https://m.alibaba.com/product/60765120782/4s-Lithium-Active-Equalizer-Lifepo4-lto.html

https://endless-sphere.com/forums/viewtopic.php?p=1674092#p1674092

https://www.expeditionportal.com/forum/index.php?posts/2674319

https://www.expeditionportal.com/forum/index.php?posts/2617706

https://www.rcgroups.com/forums/showthread.php?3283097-Advanced-parallel-charging


_______
But if you go with a BMS that includes balancing

lets you see the per-cell voltages, accurately measured

ideally lets you adjust the start-voltage and target delta

then start by using that, might be good enough for your cells.

Not having any LVC would likely shorten lifespan

but if the controller handles that function well...
 
john61ct said:
Deligreen QNBBM is well regarded

1S units to handle any S count or 4S and bigger.

https://m.aliexpress.com/item/1950876697.html

https://m.alibaba.com/product/60765120782/4s-Lithium-Active-Equalizer-Lifepo4-lto.html

https://endless-sphere.com/forums/viewtopic.php?p=1674092#p1674092

https://www.expeditionportal.com/forum/index.php?posts/2674319

https://www.expeditionportal.com/forum/index.php?posts/2617706

https://www.rcgroups.com/forums/showthread.php?3283097-Advanced-parallel-charging


_______
But if you go with a BMS that includes balancing

lets you see the per-cell voltages, accurately measured

ideally lets you adjust the start-voltage and target delta

then start by using that, might be good enough for your cells.

Not having any LVC would likely shorten lifespan

but if the controller handles that function well...
Click to expand...

Thanks for the links, Pack is being paired with an ASI8000 so i would of thought it would be ok, Assumption is the mother of all cock ups tho.

Will check the links out thanks for your help.
 
To each his own in the bms department, i have these 2 cents to add:
BMS:es of the cheap kind can be almost a larger risk than going without and i have not used bms on every battery during the last years. Unfortunately had some times when i damaged the batteries from some mistakes that could have been stopped by a bms. Leaving a light on, creep currents on an already empty battery, leaving the controller on and no undervoltage setting, stuff like that.

Right now i am using a LLT programmable / smart bms controlling a LEV200 contactor for breaking the load in case of errors or undervoltage.

I think the minimum i would recommend is hooking up a smart bms, both for balancing and since it also lets you monitor the battery easily on your phone. Going without bms is not good practice even if you can get away with it.
 
larsb said:
To each his own in the bms department, i have these 2 cents to add:
BMS:es of the cheap kind can be almost a larger risk than going without and i have not used bms on every battery during the last years. Unfortunately had some times when i damaged the batteries from some mistakes that could have been stopped by a bms. Leaving a light on, creep currents on an already empty battery, leaving the controller on and no undervoltage setting, stuff like that.

Right now i am using a LLT programmable / smart bms controlling a LEV200 contactor for breaking the load in case of errors or undervoltage.

I think the minimum i would recommend is hooking up a smart bms, both for balancing and since it also lets you monitor the battery easily on your phone. Going without bms is not good practice even if you can get away with it.
Click to expand...

Funny you mention LLT i use them a lot on smaller packs, Really good underated units.
 
Apparently same as Overkill branded.

Having a vendor shipping from the same continent

native speaker of your language

and willing to provide some after-sales support

would be just as important as build quality

and well worth paying even double or triple the price compared to the usual Ali scenario
 
I like having the bms.


.. but I keep on selling them. Doh.


Oh well. Two more on the way.



Bypass BMS hack is just that.. : A hack. A waste of design, and engineering, to circumvent a safety interlock.

Just buy the BMS with the (real) shunt and the contactor. Real contactor, not a bank of mosfets.. and a real shunt.. that can handle the current. Got 200A dreams and aspirations? Buy in the 200A-400A rated hardware. I mean, you can, if your cheap and diligent. Mindful. Observant... Ect. Hell, at this point you might as well wire the bms to the charger permanently.., switched, with double sided tape. for many reasons.. Given what any one setup might look like. ... Isolate this, run that, check those, unplug it all and go to sleep. Sure, if you can do that every day.... and have a ( pair of? ) reliable multimeter(s) for constant checking....

Who balances anyway. Lol. Balancing is overrated. Famous last words. For the poor Mr.Smuck who had a problem.. right?


Dont unlock the tagout and open the cabinet and walk away. So to speak.


They exist.

....what you want to use.... is depending on... ( yeah blah blah bah cell quality, lvc, diligence, reliability)... how stupid you are.. In your battery design.....Er, uhm...

Or how smart you are. Wait.. I forget which. Truthfully. Its one of those.

Trust me..... its the truth.
 
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