Wiki Entry Review: BMS

svetz

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Wrote up a Wiki entry for BMS: https://diysolarforum.com/ewr-carta/bms/

As I'm not an expert by any means would appreciate your review (just reply in this thread and one of the folks with wiki-authoring priviledge will update it). This was spurred by @Steve_S recent informative thread (and a few others) as an attempt to consolidate information into one place. Hopefully some of the BMS experts can chime in and help tighten it up/remove errors. Thanks!
 
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Steve_S

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I appreciate this effort and hope that ultimately it will answer many questions and misunderstandings.
It appears to be off to a good start and once references are provided and accurate supporting information is linked into it, it should resolve many points. If you wish to reuse any of my posts & links provided in other threads, feel free to do so, lots of reference info in those as well.
 

HRTKD

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Since you addressed high temp cutoff, low temp cutoff could also be added. Any LiFePO4 battery deployed in an environment that could see temperatures close or below 32° F needs low temp cutoff.
 

HRTKD

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I don't see the correlation between how common active balancing is and how many people report it not working.

In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells. Despite the obvious advantages, these systems are not as common as many members report them not working correctly. This may change as these systems mature.
 

HRTKD

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With regard to 2 wire and 3 wire, I'm not familiar with that type of BMS. My Overkill Solar BMS can cut off charge, but still allow discharge. Some models have two wires, some have three, but I think that's just for amp throughput, not functionality. Maybe I'm not understanding the concept.
 

svetz

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Since you addressed high temp cutoff, low temp cutoff could also be added. Any LiFePO4 battery deployed in an environment that could see temperatures close or below 32° F needs low temp cutoff.
It's in as LTC under common features... is there a better way to say it?

Update: Oh, I see! You mean in the "Is a BMS needed section" ... I covered the thermal runaway due to RUD. LTC keeps your investment from being destroyed but doesn't kill you.... hmmm... I'll look at it again. Okay... beefed it up a bit.
 
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svetz

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I don't see the correlation between how common active balancing is and how many people report it not working.
I agree that's not particularly clear.... hmm... how about:

Despite the obvious advantages, these systems are not as common most likely due to poor user experiences (e.g., malfunctions) with inexpensive systems.

At least that's my understanding, please correct me if I'm off base.
 

svetz

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With regard to 2 wire and 3 wire, I'm not familiar with that type of BMS. My Overkill Solar BMS can cut off charge, but still allow discharge. Some models have two wires, some have three, but I think that's just for amp throughput, not functionality. Maybe I'm not understanding the concept.
Pulled mostly from BMS common port vs seperate port, but if I'm off base please correct me.

Thank you for looking over the wiki! Really appreciate the feedback!
 
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HRTKD

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I agree that's not particularly clear.... hmm... how about:

Despite the obvious advantages, these systems are not as common most likely due to poor user experiences (e.g., malfunctions) with inexpensive systems.

At least that's my understanding, please correct me if I'm off base.

Maybe this?

Despite the obvious advantages, active balancing is not commonly found in inexpensive systems. If the feature set does not explicitly say "active balancing" then the buy should assume balancing is passive.

The app for the Overkill Solar BMS led me to believe that active balancing was taking place. It shows two cells on the screen, the high cell and the low cell, which makes it look like current it flowing from one to the other. However, the manual that Steve put together (which I just re-read with a more cynical eye) is fairly clear that it's doing passive balancing. Hence my addition above with the second sentence.
 

HRTKD

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Pulled mostly from BMS common port vs seperate port, but if I'm off base please correct me.

Thank you for looking over the wiki! Really appreciate the feedback!

I haven't kept up with the common/separate port thread. Like I said above, I may not understand the concept as well as I should to be discussing it here. Referencing 2/3 wire could lead to some assumptions.

My BMS has three wires. Ergo, it's a separate port BMS? A single wire BMS would be a common port BMS? I'm not trying to rehash the entire common/separate port thread. I'm just wondering is using the 2/3 wire reference is appropriate or just leads to confusion.
 

svetz

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... I'm just wondering is using the 2/3 wire reference is appropriate or just leads to confusion.
AFAIK they're the industry terms, but I think we need an expert to weigh in.

Despite the obvious advantages, active balancing is not commonly found in inexpensive systems. If the feature set does not explicitly say "active balancing" then the buy should assume balancing is passive.
Perhaps @Steve_S et.al. could weigh in here too. I know there's misleading marketing making things appear they have active balancing when they don't as in your case, but I've also seen the Julian Ilett video where he demonstrates a $20 active balancer working. I know they're not common, but possibly that is expense related at higher amperages rather than a lack of quality?
 

svetz

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...possibly that is expense related at higher amperages rather than a lack of quality?
In reading that it seems sorta silly. Considering power transfer is cell-to-cell rather than amps through the pack, the design amps could be quite small. Hopefully someone that knows how active balancers really work will chime in.
 

svetz

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Surfed some more BMS threads (mainly thoughts from @Steve_S, @BiduleOhm, @Craig, @Maast , @Supervstech, @Dzl, and... well quite a few of you ... you guys rock) and made some updates based on bits here and there. There's a list of substantive changes since it was public yesterday morning below to make life easier if you've already read through it.

But, just because I managed to make time to write something up doesn't mean it's correct, hey...everything I know I learned on the internet. 😲
So please help out by pointing out anything you know to be wrong, could be made clearer, or is useless and should be cut for brevity.

old verbiageNew Verbiage
Added a caveat at the top that it was a WIP.
A Battery Management System (BMS) is a device that protects a battery comprised of multiple cells (aka battery pack) from operating outside its safe operating state and keeps the individual cells voltage balanced so no capacity is lost within the battery pack. There are many varieties with different features.A Battery Management System (BMS) is a device that protects a battery comprised of multiple cells (aka battery pack) from operating outside its safe operating state and keeps the individual cells voltage balanced so no capacity is lost within the battery pack. There are many varieties with different features. A BMS is typically the only device in a system that is capable of seeing and taking actions based upon cell level conditions.
A BMS is not strictly mandatory on any battery pack. However, considering the cost of the battery investment it is usually a good idea to have one on any system that is designed to work in an autonomous fashion.A BMS is not strictly mandatory on any battery pack. However, considering the cost of the battery investment it is usually a good idea to have one on any system that is designed to work in an autonomous fashion. For example, charging even once at temperatures below 0° C can permanently harm lithium cells, destroying the investment.
In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells. Despite the obvious advantages, these systems are not as common as many members report them not working correctly. This may change as these systems mature.In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells. Despite the obvious advantages, these systems are not as common because passive balancers are less expensive and sufficient for new cells with matched capacity as typically there is very little energy dissipated. Active balancers are most useful with used or mismatched cell capacity. (ref)

Common Features​

BMS Features​

  • Monitoring - Allows various cell parameters to be monitored
  • Monitoring - Allows various cell parameters to be monitored, typically bluetooth or WiFi
  • LCD display - Shows what the BMS is doing and BMS health
  • Relay Switch - for turning secondary devices on/off (e.g., heaters/coolers, inverters)
  • Pins - General Purpose I/O pins, for e-switch/ignition key, external systems such as SOC LED, alarm buzzer
<Additions to/near Common Mistakes>Incorrectly setting the chemistry or voltage cutoffs or chemistry selection. Even if you think the settings are correct, it's prudent to test with a volt-meter to validate.

BMSes typically don't have over-current-protection. That is they won't protect the battery from exceeding the C-Rate during a short. An external fuse or breaker is required for this. Some battery packs do have cells individually fused to protect the pack from a cell gone bad, but that's within the pack and not a part of the BMS.

What's the best BMS?​

There's no one answer, but there is a thread on that. Inexpensive BMSes exist, but they might not be a bargain as they are protecting a valuable investment.
 
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Steve_S

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I've looked through this some more... There is a definite need for correction & clarity and some of the terms used ? Heck if I know where you guys pulled that from, translation issues maybe ? LVD/HVD (Lo/Hi Volt Disconnect) are just that and always have been.

Many companies produce Balancers, Active or Passive but several are K.R.A.P. and this is NOT NEW. There are many variations and some with assorted features & functions not directly related to balancing... Most issues are related to not understanding the tech, making "assumptions" and jumping to conclusions , so in other words 90% is user error, which sometimes results in doing the wrong thing and causing damage. Think of a Balancer designed for NMC being plopped on LFP Pack, voltage ranges are wrong and cause damage, folks have done it and blamed the device they chose which was the wrong one for the chemistry.

* Same applied to BMS' when someone buys the wrong BMS or misconfigures a "Smart One" for the wrong chemistry. IE a person using a Chargery on LFP but using LITO settings = not terribly brilliant ! BUT they Blame the Chargery BMS because THEY were not too bright and refuse to admit it. SEEN IT MORE THAN ONCE !

Up until recently <12 months) most BMS' on the market had no balancing capabilities at all. Some had Passive Balancing and the rare one had some form of Active Balancing. Passive is only really valuable for either small cells <25AH or for properly Matched, Batched & Binned cells.
Active Balancers (Proper Known Good units) work well on larger cells and older tired cells which are losing some capacity levels.
Many people, like myself, use dedicated Active Balancing on new High Capacity Cells (in my case 175AH used cells & EVE 280's which are new) and they do keep things nicely leveled up. I have repeatedly explained what Matched/Batched/Binned is and how it's done and far more than 3 Times. I've provided links to Docs, White Papers & Research on the subject yet still have to repeat'n repeat...

Some people get the idea that a BMS or Balancer system can FIX weak / damaged or poor cells.. They simply cannot, nothing can fix a cell, period, end of sentence.

I tried to address some of the misconceptions about BMS' and that thread had to be locked because it was headed towards Circular yapping on a topic that is not my thing. A BMS has ZERO, to do with specific cell charging in a battery pack, they do not control or direct power in Volts/Amps to any specific cell within a battery pack. But heaven's to mergatroids some folks think it does... Rule of Three Exceeded, that thread was locked down.

BMS as a topic is HUGE !
There are MANY Types of BMS' and only a "few" types are exposed in this forum. Many can't even resolve Common-Port to Separate-Port BMS'.
Decentralized, Centralised, Master/Slave, Distributed and more... Some have advanced controls for real EV's with Automotive CanBus Interfaces as well is vehicular integration for regenerative braking, heating/cooling battery systems, to gauge integration and more...

While a WIKI Entry may seem like a good idea, the volume of info required to do this is huge and complex. It would likely be more prudent to use established knowledge bases and provide a brief overview & link to the subject matter. Why attempt to rewrite something that is already well documented elsewhere.

Personally, I am hesitant to add to this Wiki Entry. It is a minefield and that's never good. I really suggest placing Known Good Verified proper technical resources LINKS and let the people learn if they want to. We can lead any horse to water but it's up to the horse to drink it or not.

Sorry, not the help / answer you wanted, it is what it is.

FYI: Ferengi Rules of Acquisition. Very True in the 21st Century.
285No good deed ever goes unpunished.
 

svetz

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Thanks Steve! I'll go through that information and see what I can to do to update the entry with it.

... that thread was locked down...

I did want to address this ASAP as there seems like there might be a slight misunderstanding... the thread was locked down as you requested that it be deleted. The mods have been discussing it since then (apologies it's taking us so long) as we don't want to lose valuable information but also wanted to honor your request. One of the thoughts was to try and capture it all in a wiki entry before following up on your request to delete the thread. Apologies if the reasons weren't adequately explained when the actions were taken. That's what I meant in the OP that the wiki entry was spurred by your thoughts.
 

Steve_S

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it was locked without any explanation or comment to me. At least it ceased things from going too far.
You can use the content from that or any other post I have put up here in the forum. Just remember attribution whenever possible.
 

svetz

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it was locked without any explanation or comment to me. At least it ceased things from going too far.
That's our bad, sorry about that. The action was taken for the reasons you cited, probably the mod that did it thought it was self-apparent.

You can use the content from that or any other post I have put up here in the forum. Just remember attribution whenever possible.
Much appreciated!
 

svetz

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Added a new common mistakes based on Steve's post above and updated the active balancing entry in the wiki... and in post #14 above to make it easier.
Have to run for now but hope to tackle Steve's paper tonight.
 

HRTKD

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svetz, edits look good. Minor issue here:

Incorrectly setting the chemistry or voltage cutoffs or chemistry selection.

Should be: Incorrectly setting the chemistry or voltage cutoffs.

I've read that setting balance on discharge will undo a good Top Balance. Maybe that should be added to the common mistakes?
 
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