How critical is a ‘good’ BMS, really

[fafrd]

Things can go wrong and for protecting the cells when that happens, I’m not questioning the value/need of a BMS.

The first 8S 24V LiFePO4 battery I built has a pedestrian 200A 6S BMS I picked up for under $40.

Any time a cell strays up to 3.75V it’ll shut off the battery.

Any time a cell frond below 2.5V, it’ll shut off the battery.

Above 3.55V it’ll ‘balance’ with a very modest current of 78mA.

But it has no temperature compensation, is not programmable, and is really just there as a failsafe in case something goes wrong.

Charge voltages will be programmed into the MPPT charge controller.

Discharge will be limited by the inverter which cuts off before 20V (2.5V/cell) and also has a SOC sensor and can be programmed to shut off at 10~ (for example).

I’m now planning a 2nd 280Ah 24V LiFePO4 battery and considering whether there are any fancier features I really need from a higher-end BMS or I can stick to my strategy of just spending as little $s as possible on the basics...

 

[snoobler]

It's a personal thing. I would regard your BMS as inadequate particularly due to the puny balancing current and the absurdly ineffective voltage at which it activates; however, it covers the basics.

I would still regard it as 'good' in the spirit of your thread subject. It manages the battery and protects it.

I would insist on being allowed to set my own cut-offs that don't just represent the BMS manufacturer's idea of "safe," but my desired limits of charge/discharge. I would want to specify the balance threshold and would hope for something more than 78mA.

ANY BMS is better than none.

 

[fafrd]

It's a personal thing. I would regard your BMS as inadequate particularly due to the puny balancing current and the absurdly ineffective voltage at which it activates; however, it covers the basics.

I would still regard it as 'good' in the spirit of your thread subject. It manages the battery and protects it.

I would insist on being allowed to set my own cut-offs that don't just represent the BMS manufacturer's idea of "safe," but my desired limits of charge/discharge. I would want to specify the balance threshold and would hope for something more than 78mA.

ANY BMS is better than none.

I appreciate the feedback. Programmability would be nice but I suspect it adds significant cost. Of the limitations you’ve listed, the measley balance current is the most annoying limitation, but that doesn’t really translate to much if the cells start balanced and age/degrade equally (stay balanced).

And if the cells don’t age equally, there is only so much a BMS can do in any case.

Bluetooth remote monitoring would be nice but I wouldn’t want to waste the power for the 99.999% of the time I don’t need to monitor, so plugging in a BattGO monitor whenever the 0.001% strikes me seems preferable.

I’d spend more on a ‘better’ BMS is I could see a clear advantage, but the EVE 280Ah LiFePO4 cells I am getting have a lifetime of of 3500 cycles when contained under pressure and ‘only’ 2500 cycles when contained without pressure (squeezed). If I have any additional resources to invest in this battery, investing to squeeze my cells together under pressure seems like a higher priority than a fancier BMS.

All of that being said, I haven’t purchased the new BMS yet and am open to recommendations for a the best performance-for-price BMSs to consider (200A or 300A, 8S LiFePO4).

 

[Dzl]

How critical is a ‘good’ BMS, really

How critical is a ‘good’ BMS, really

I suppose it very much depends on what you consider by 'good' as well as your level of experience, how actively you want to manage the system, and your use case.

Below is my opinion:

I think there are 2-3 core capabilities that are pretty essential in most cases:
1. Low Voltage Disconnect (Cell Level)
2. High Voltage Disconnect (Cell Level)
3. Cell Balancing

Beyond this there are many features that add value, or are important for a certain use case, or are just convenient or nice to have, but I wouldn't call them essential (outside of particular use cases).

Things can go wrong and for protecting the cells when that happens, I’m not questioning the value/need of a BMS.

The first 8S 24V LiFePO4 battery I built has a pedestrian 200A 6S BMS I picked up for under $40.

Any time a cell strays up to 3.75V it’ll shut off the battery.

Any time a cell frond below 2.5V, it’ll shut off the battery.

Above 3.55V it’ll ‘balance’ with a very modest current of 78mA.

But it has no temperature compensation, is not programmable, and is really just there as a failsafe in case something goes wrong.

Charge voltages will be programmed into the MPPT charge controller.

Discharge will be limited by the inverter which cuts off before 20V (2.5V/cell) and also has a SOC sensor and can be programmed to shut off at 10~ (for example).

I’m now planning a 2nd 280Ah 24V LiFePO4 battery and considering whether there are any fancier features I really need from a higher-end BMS or I can stick to my strategy of just spending as little $s as possible on the basics...

The BMS you have sounds a lot like a Daly BMS. Its one of the more popular BMS here. And is really all many people need in a basic BMS.

I think this is a good baseline BMS (if the non-configurable wide bandwidth voltage limits work for you), its simple and offers catastrophic protection if your first layer of control fails.. If you need extra features (like low temp disconnect for instance), desire extra features or configurability, if your use-case requires or would benefit from some special feature, or if you just like tweaking and tinkering and monitoring (it is a DIY forum after all), then consider more advanced options.

As for this:

I appreciate the feedback. Programmability would be nice but I suspect it adds significant cost.

And this:

Bluetooth remote monitoring would be nice but I wouldn’t want to waste the power

It may be worth re-examining these assumptions.

Regarding consumption, the three FET based BMS' I'm familiar with (JBD Smart BMS, Ant Smart BMS, and Daly BMS) all list pretty negligible (less than a few mA) self consumption at idle. I'm just going off the specs though, not speaking from experience.

As to cost here are some reference points:
Daly 100A 8S (non-bluetooth) = $51 from aliexpress
JBD (bluetooth) 100A 8s (bluetooth) = $69 from aliexpress
Ant 100A 7-16S (bluetooth) = $54 from aliexpress

Now none of these are what I would necessarily call a 'good' BMS (not that they are bad), but it does show that there is not a huge price difference between BT and non-BT BMS. I will say that from the look of it, while the other two options have BT and more functionality, the Daly looks to be the most robust and well built physically.

Where the price starts to climb steeply with the FET based BMS is with higher current models.

You will pay more if you want a mid range BMS like a Chargery or SBMS0, or a high end BMS

I’d spend more on a ‘better’ BMS is I could see a clear advantage

I would say if you don't see a need for additional functionality. There isn't necessarily a reason to 'upgrade', in fact the simplicity of a basic BMS can be a feature in and of itself if it meets your needs. I would say some basic BMS' commodity BMS' like Daly are every bit as good as other commidty BMS' with 'smart' features and connectivity.

 

[Steve_S]

Pretty well covered above. BMS features also depend on the application as well and what an individual wants.

Things like BT is nice and convenient too and if you have a system that is online 7/24/365 the overhead cost (power wise) to maintain that BT Acces up is moot, as it is part of the normally expected overhead, just like having the Inverter on. But if you have a system that is only used 1 weekend a month and is parked for the rest of the time, the daily power consumption of the BMS with BT could be an issue as the BMS continually draws power to have BT available. someone posted about turning off their BMS because of this constant drain.

As for "GOOD" BMS', there are many "Known Good and Well Used" BMS' that the membership here use. We have come across dubious ones too and folks have posted on that. I personally don't put much cred into Amazon/EBay etc reviews (too many are shady) but it is different here with people who are using, abusing & breaking things in the Real World. Magic Smoke is part of the hobby, hopefully very minimal.

 

[chevymike]

Pretty well covered above. BMS features also depend on the application as well and what an individual wants.

Things like BT is nice and convenient too and if you have a system that is online 7/24/365 the overhead cost (power wise) to maintain that BT Acces up is moot, as it is part of the normally expected overhead, just like having the Inverter on. But if you have a system that is only used 1 weekend a month and is parked for the rest of the time, the daily power consumption of the BMS with BT could be an issue as the BMS continually draws power to have BT available. someone posted about turning off their BMS because of this constant drain.

I have had my pack sitting for over 2 months with the BT hooked up (sleeping 99.9% of the time. I have only connected to it twice during the two months). My individual cell voltage has dropped by .004 volts. Figure about .002 volts per month so for a year, that's only .024 volts, per cell. This is nothing to be concerned about. If the system cannot handle a .024 volt loss per cell (4 cell system i.e. 12v is .096v total loss), per year, you have bigger issues.

 

[fafrd]

Pretty well covered above. BMS features also depend on the application as well and what an individual wants.

Things like BT is nice and convenient too and if you have a system that is online 7/24/365 the overhead cost (power wise) to maintain that BT Acces up is moot, as it is part of the normally expected overhead, just like having the Inverter on. But if you have a system that is only used 1 weekend a month and is parked for the rest of the time, the daily power consumption of the BMS with BT could be an issue as the BMS continually draws power to have BT available. someone posted about turning off their BMS because of this constant drain.

That is a very good point and I should have made my application(s) clear up-front.

The 90Ah 24V battery I’ve already built does dual duty as backup supply during occasional extended power outages here in California as well as an occasional portable power station that will be used 3-4 days a year.

As such it will be sitting unused months at a time and minimizing charge loss is a higher priority than ease of ongoing monitoring. The way I’ve got it rigged now, I can plug in a BattGO whenever I want to check cell voltages and I’m hoping it is something I will not need to worry about until Fire season approaches...

The bigger 280Ah battery I am planning will be for time-shifting and covering increased TOU charges with a 2kW DC solar system. That larger battery will be charged and discharged daily (at least for the 120 days we are hit with high TOU charges) and a few mA of current drain for the convenience of Bluetooth monitoring might be attractive.

The counter argument is the ‘one more thing to break’ argument (also stated in the post before yours).

But I appreciate the discussion - it is at least helping me understand my options.

As for "GOOD" BMS', there are many "Known Good and Well Used" BMS' that the membership here use. We have come across dubious ones too and folks have posted on that. I personally don't put much cred into Amazon/EBay etc reviews (too many are shady) but it is different here with people who are using, abusing & breaking things in the Real World. Magic Smoke is part of the hobby, hopefully very minimal.

I have not really exercised my BMS yet and I have to confess it is the ability to withstand rates current on a continuous basis that has me most concerned.

I’d probably prefer a ‘vanilla’ / non-programmable BMS that I know can handle it’s rated 200A forever to a more capable / fancier BMS that has any real risk of giving me Magic Smoke under those conditions...

 

[fafrd]

I have had my pack sitting for over 2 months with the BT hooked up (sleeping 99.9% of the time. I have only connected to it twice during the two months). My individual cell voltage has dropped by .004 volts. Figure about .002 volts per month so for a year, that's only .024 volts, per cell. This is nothing to be concerned about. If the system cannot handle a .024 volt loss per cell (4 cell system i.e. 12v is .096v total loss), per year, you have bigger issues.

In general, I agree with you, charge leakage from Bluetooth is probably inconsequential for a battery of any real size.

On the other hand 24mV can represent close to a quarter of a battery’s capacity, so it really depends where the battery is charged to.

If you use the LiFePO4 charge curves sticky thread in the LiFePO4 forum, in the ‘flat’ part of the curve between 3.27V (40%) and 3.31V (80%), 10mV translates to 10% SOC, so losing 24mV there translates to almost a quarter of your stored energy...

 

[fafrd]

I suppose it very much depends on what you consider by 'good' as well as your level of experience, how actively you want to manage the system, and your use case.

Below is my opinion:

I think there are 2-3 core capabilities that are pretty essential in most cases:
1. Low Voltage Disconnect (Cell Level)
2. High Voltage Disconnect (Cell Level)
3. Cell Balancing

Totally agree. LVD and HVD are pretty straightforward so aside from whether the BMS is programmable or not, whether it includes temperature adjustment of disconnect voltage levels or not, and it’s accuracy in measuring cell voltages, there is not much more to discuss.

‘Balancing’ is more complicated. The basic BMS I have now has a ‘Balance Turn On’ of 3.55V, corresponding to about 97.4% SOC, so there is no balancing function at until late in the charge cycle and none at all during the discharge cycle.

If I decide to stop charging my battery at 99% SOC (3.61V), there will only be a couple of hours while the solar charger is still active behore loads discharge the battery below 3.55V.

So at most, I’ll have 2 hours with a measly balance current of 78mA to balance any high cells.

That’s a maximum of ~165mAh of charge that can be moved between cells, or less than 0.2% of my existing 90Ah battery (or less than 0.06% of my new 280Ah battery).

Pretty much useless.

A Balance function that is active across the entire voltage range would be more useful, especially if the balance currents were more significant.

As I stated in an early post, the balance function is immaterial if the cells are well-balanced and age/degrade similarly (remain well-matched), and there is only so much any balance function can do to compensate for cells that are intrinsically unmatched.

That being said, I’m interested to understand whether there are any well-regarded BMSes that keep the balance function active through the SOC / voltage range (especially if they have more significant balance currents).

Beyond this there are many features that add value, or are important for a certain use case, or are just convenient or nice to have, but I wouldn't call them essential (outside of particular use cases).



The BMS you have sounds a lot like a Daly BMS. Its one of the more popular BMS here. And is really all many people need in a basic BMS.

I think this is a good baseline BMS (if the non-configurable wide bandwidth voltage limits work for you), its simple and offers catastrophic protection if your first layer of control fails.. If you need extra features (like low temp disconnect for instance), desire extra features or configurability, if your use-case requires or would benefit from some special feature, or if you just like tweaking and tinkering and monitoring (it is a DIY forum after all), then consider more advanced options.

As for this:



And this:



It may be worth re-examining these assumptions.

Regarding consumption, the three FET based BMS' I'm familiar with (JBD Smart BMS, Ant Smart BMS, and Daly BMS) all list pretty negligible (less than a few mA) self consumption at idle. I'm just going off the specs though, not speaking from experience.

As to cost here are some reference points:
Daly 100A 8S (non-bluetooth) = $51 from aliexpress
JBD (bluetooth) 100A 8s (bluetooth) = $69 from aliexpress
Ant 100A 7-16S (bluetooth) = $54 from aliexpress

Now none of these are what I would necessarily call a 'good' BMS (not that they are bad), but it does show that there is not a huge price difference between BT and non-BT BMS. I will say that from the look of it, while the other two options have BT and more functionality, the Daly looks to be the most robust and well built physically.

Where the price starts to climb steeply with the FET based BMS is with higher current models.

You will pay more if you want a mid range BMS like a Chargery or SBMS0, or a high end BMS



I would say if you don't see a need for additional functionality. There isn't necessarily a reason to 'upgrade', in fact the simplicity of a basic BMS can be a feature in and of itself if it meets your needs. I would say some basic BMS' commodity BMS' like Daly are every bit as good as other commidty BMS' with 'smart' features and connectivity.

Thanks for all those inputs. I’d be happy if my BMS is a Daly, but I doubt it. It’s a 200A BMS (900A over-current protection) that cost me less than $40.

For my ‘learner battery’ and just for DIY screwing around, I’m happy I took the chance on a budget EBay no-name, but I’m probably going to be more cautious about the BMS for the larger ‘permanent’ battery.

This discussion has been helpful because it makes me realize that reliability is a higher priority to me than added features, especially in terms of truly being able to handle rated currents on a continuous basis.

As I get my 90Ah battery up and running (just balanced for the first time and have never gone through a charge/discharge cycle yet), I’m probably going to rig up a setup to stress this little 200A BMS close to it’s rated limit.

The same vendor sells a 300A 8S LiFePO4 BMS for only a bit more than their 200A model, and if the 200A model can actually maintain that current level for a full discharge cycle of this 90Ah battery, spending a bit more for the added insurance of the 300A model may be more important to me than adding features or trying higher-priced brands.

Of course, if my stress test of the 200A BMS results in Magic Smoke, I’ll be spending a lot more time on the Forum getting recommendations of reliable BMS vendors...

 

[Dzl]

What BMS are you currently using? and what vendor?

If I were you, I would consider stress testing that BMS since it is an unknown brand, untested, and suspiciously low price compared with other high current models. If you do so, report here with your findings, the more options and the more info we have the better!

That being said, I’m interested to understand whether there are any well-regarded BMSes that keep the balance function active through the SOC / voltage range (especially if they have more significant balance currents).

I have not come across a BMS that balances through the full charge cycle / voltage range. All that I am aware of do top balancing. But there are some that start at lower SOC's.

 

[fafrd]

What BMS are you currently using? and what vendor?

I have no idea the brand. My ‘vendor’ is a Chinese reseller. He is able to get questions answered from the actual manufacturer which was good enough for me to risk $35.

Obviously no warranty and no customer support at this price.

I’ll attempt to insert my first image here:



The specs were translated, printed and attached by me from the Chinese.

After I’ve stress-tested this BMS, if I believe it delivers good value for the money, I’ll share the EBay vendor. For now, if anyone ‘recognizes’ this style of BMS (free of any plastic housing), I’m all ears about with ‘brand’ it may be.

If I were you, I would consider stress testing that BMS since it is an unknown brand, untested, and suspiciously low price compared with other high current models. If you do so, report here with your findings, the more options and the more info we have the better!

My plan exactly!

I have not come across a BMS that balances through the full charge cycle / voltage range. All that I am aware of do top balancing. But there are some that start at lower SOC's.

I just posted my experience in the LiFePO4 forum about my disappointing experience with the BattGO balance function. That device reads cells with an accuracy of +/-5mV and balances to an accuracy of <5mV and all of that combines to a worst-case balance accuracy of <15mV.

My actual battery went from actual worst-case of 11mV out of balance to 14mV out of Balance after letting the BattGO balance for 13 hours.

The lesson to me is that any balance function is only as good as it’s voltage reading accuracy, so I’m keen to get understand what other BMSs specify as far as cell channel read accuracy.

5mV read accuracy is pretty much useless in the flat part of the charge curve. In the ‘hockey stick’ above 3.6V, 5mV read accuracy may translate to something like 0.4% SOC, but if you want to be able to balance cells to within 1% SOC, you need channel read accuracy of closer to 1mV than 10mV.

Are there any BMSs that characterize cell read accuracy? (Mine does not, only the BattGO).

 

[Dzl]

5mV read accuracy is pretty much useless in the flat part of the charge curve. In the ‘hockey stick’ above 3.6V, 5mV read accuracy may translate to something like 0.4% SOC, but if you want to be able to balance cells to within 1% SOC, you need channel read accuracy of closer to 1mV than 10mV.

Are there any BMSs that characterize cell read accuracy? (Mine does not, only the BattGO).

It is my understanding that this is one of the main reasons that BMS' tend to balance at higher SOC, at or near the knees where voltage differences start to become more pronounced.

As to:

That being said, I’m interested to understand whether there are any well-regarded BMSes that keep the balance function active through the SOC / voltage range (especially if they have more significant balance currents).

I believe the SBMS0 may be worth looking into. Based on the screenshot from the manual below, It looks like it will balance through most of the voltage range (3.2V to 3.55V) by default, balance current is max of 200mA. By default on charge only, but it looks like it can be configured to balance on discharge also. Basically everything is configurable with the SBMS if you know what you are doing.



However based on everything else you have said (about simplicity, price, etc) the SBMS0 is probably not well suited to your wants.

 

[fafrd]

It is my understanding that this is one of the main reasons that BMS' tend to balance at higher SOC, at or near the knees where voltage differences start to become more pronounced.

Makes sense...

As to:


I believe the SBMS0 may be worth looking into. Based on the screenshot from the manual below, It looks like it will balance through most of the voltage range (3.2V to 3.55V) by default, balance current is max of 200mA. By default on charge only, but it looks like it can be configured to balance on discharge also. Basically everything is configurable with the SBMS if you know what you are doing.

View attachment 20449

However based on everything else you have said (about simplicity, price, etc) the SBMS0 is probably not well suited to your wants.
[/QUOTE]

Well, if I decide to dive into this ‘hobby’ with both feet (as I’ve done with others), the SBMS0 looks like a fun toy!

But for now I’m going to stick to putting together a battery that works well enough to escape the excessive inflated TOU charges my utility is in the process of imposing on me (as well as having backup power when they shut down the grid on me).

 

[Steve_S]

ACTIVE vs PASSIVE Balancing

Balancing can be active or passive.[4] The term battery regulator typically refers only to devices that perform passive balancing.

In passive balancing, energy is drawn from the most charged cell and dissipated as heat, usually through resistors.

In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells, usually through capacitor-based, inductor-based or DC-DC converters.[5]

Battery balancing can be performed by DC-DC converters, in one of 3 topologies:

• Cell-to-battery
• Battery-to-cell
• Bidirectional

Typically, the power handled by each DC-DC converter is a few orders of magnitude lower than the power handled by the battery pack as a whole.

SOURCE: https://en.wikipedia.org/wiki/Battery_balancing#Technology

 

[Mex Rider]

What BMS are you currently using? and what vendor?

If I were you, I would consider stress testing that BMS since it is an unknown brand, untested, and suspiciously low price compared with other high current models. If you do so, report here with your findings, the more options and the more info we have the better!



I have not come across a BMS that balances through the full charge cycle / voltage range. All that I am aware of do top balancing. But there are some that start at lower SOC's.

I believe that the Overkill BMS can be set to balance when charging or balance all the time but I do not know the charging current

 

[Dzl]

I believe that the Overkill BMS can be set to balance when charging or balance all the time

Thanks, good to know if true. The spec's say balancing turns on above 3.4V, but I've never installed or played with the app, so I've no idea whether its configurable or not.

but I do not know the charging current

50 mA (I believe)

 

[Mex Rider]

Thanks, good to know if true. The spec's say balancing turns on above 3.4V, but I've never installed or played with the app, so I've no idea whether its configurable or not.



50 mA (I believe)

Thanks