How seriously should you take BMS Max Charge Current spec?

[fafrd]

I’ve got a 300A Heltec BMS that I’m happy with but it has a ‘Maximum Charge Current of 100A’

It’s a common-port BMS and so my first question is to understand how the max charge current and max discharge current can be different in a common-port BMS?

And assuming there is a good reason for the difference, my second question is to understand what happens when charge current exceeds the maximum (and how seriously to design under that limit)?

I’m only charging at 45A maximum today but planning to increase the size of the PV array.

Can a 300A BMS with a 100A max charge current typically exceed that charge level for 1-2 hours by a small % without causing damage?

On the discharge side, the BMS has a Current Limit Protection of 1200A and will shut down the BMS is currents ever reach that level.

And it also has a Maximum Continuous Overcurrent of 300A (which I assume means it will shut down if discharging above 300A for long enough).

So is a Maximum Charge Current the same thing? Will it shut down the BMS if charging above 100A for long enough (but prevent damage)?

I’m trying to understand whether I can plan for Peak Charge currents (1-2 hours) that may occassional my exceed 100A by 5-10% without major concern or I need to find a different BMS before increasing PV power that much…

 

[RCinFLA]

Its maximum chargng spec is likely based on having a low cell undervoltage shutdown situation. It shuts down discharge path which adds a diode drop to charging path that increases its heating with current. It will just then shut down charging in addition based on BMS overtemp if you charge too high amperage when in cell undervoltage shutdown.

 

[fafrd]

Its maximum chargng spec is likely based on having a low cell undervolage shutdown situation. It shuts down discharge path which adds a diode drop to charging path that increases its heating with current. It will just then shut down charging in addition based on BMS overtemp if you charge too high amperage when in cell undervoltage shutdown.

Cool, thanks for the explanation. So charging over rated current will cause an over temperature shutdown but is not likely to cause any damage to transistors or otherwise.

Sounds like it would be relatively safe to try charging at whatever max current my new array puts out to see whether shutdown frequently gets triggered.

If not, I’m probably good to go and if I end up triggering a shutdown every bright sunny day, either I lower my max charge current or find a more capable BMS…

(My fall-back is to take my 8S2P 24V battery and reconfigure as 2 8S1P batteries, each with their own 8S BMS…).

Just would rather save the hassle if it’s not necessary.

But you’ve helped me to understand the difference between the max charge and the max discharge specs, so thanks again.

 

[Ampster]

or find a more capable BMS…

As a general assumption I have seen several threads suggest that most BMSs should be derated by 50 percent. Therefore even approaching the maximum might create issues. It may depend on the BMS. As you have suggested, a more capable BMS might be an alternate.

 

[chrisski]

I have exceeded the max charge rate on my 15 amp Daly BMS and it shut down. Got to 15.1 amps and shut down.

I don't see how you could exceed the max charge rate without the BMS shutting down. There's some sort of settings with programming that go like:
=======
10 System Max Charging = 15 amps

20 Set user max charging rate = [Input]

30 If System Max Charging rate > [input max user charging rate] then goto line 20

40 Check input amps

50 If input amps > user max charging rate then SHUT OFF CHARGING
========
I put that rather bad piece of programming because I think I'm missing something here.

I also think @Ampster Suggestion of having the BMS overrated by 50% is a novel suggestion especially after the nuisance tripping I dealt with.

 

[Ampster]

I also think @Ampster Suggestion of having the BMS overrated by 50% is a novel suggestion

I don't take credit for the concept. Many posters here have suggested that the ratings of imported products like BMSs are typically overstated. My main system is contactor based so I don't have to worry about the BMS capacity so I am able to focus on the optimun charging rate for the batteries versus the other parts of the system.

 

[fafrd]

I have exceeded the max charge rate on my 15 amp Daly BMS and it shut down. Got to 15.1 amps and shut down.

I don't see how you could exceed the max charge rate without the BMS shutting down. There's some sort of settings with programming that go like:
=======
10 System Max Charging = 15 amps

20 Set user max charging rate = [Input]

30 If System Max Charging rate > [input max user charging rate] then goto line 20

40 Check input amps

50 If input amps > user max charging rate then SHUT OFF CHARGING
========
I put that rather bad piece of programming because I think I'm missing something here.

I also think @Ampster Suggestion of having the BMS overrated by 50% is a novel suggestion especially after the nuisance tripping I dealt with.

Well, if the charge current can’t exceed 50% of max rating without causing a problem, these BMSs are next to worthless..

If I build 2 8S1P batteries each with their own BMS, that would be one option for me though it just occurred to me, why couldn’t I hook up two BMSes in parallel to my existing 8S2P battery (each with its own harness)??

And if getting to a safe 120A charge current for several hours proves too complicated / costly using off-the-shelf BMSs, I’m starting to think about building my own.

I don’t care about temperature or balancing so all I really need is LVD and HVD. So a 16-channel voltage monitor controlling a 300A contactor like this seems like it would solve my problem: https://www.amazon.com/Voltage-Contactor-Efficient-Normally-SPST-NO/dp/B08C57B2S1

Not ready to go there yet, but I’m motivated enough to start monitoring the DIY BMS threads for the first time…

 

[fafrd]

I don't take credit for the concept. Many posters here have suggested that the ratings of imported products like BMSs are typically overstated. My main system is contactor based so I don't have to worry about the BMS capacity so I am able to focus on the optimun charging rate for the batteries versus the other parts of the system.

Is there a thread somewhere where you discuss your system? I’ interested in what contactor you used…

 

[chrisski]

Well, if the charge current can’t exceed 50% of max rating without causing a problem, these BMSs are next to worthless..

Maybe not 50%, but at 100% it trips. There's formulas for circuit breakers which in part will avoid nuiscance tripping like expected Amps * 1.56 or ^ 1.2.

Part of the reason I'm putting this out there is to see if others have seen this where the SCC max amps = BMS max amps, but the SCC pushes .1 amp more so the BMS trips.

Will said in a video that is he needed a 50 amp draw for the small type Daly I purchased, he'd get a 60 amp BMS.

 

[fafrd]

Maybe not 50%, but at 100% it trips. There's formulas for circuit breakers which in part will avoid nuiscance tripping like expected Amps * 1.56 or ^ 1.2.

Part of the reason I'm putting this out there is to see if others have seen this where the SCC max amps = BMS max amps, but the SCC pushes .1 amp more so the BMS trips.

Will said in a video that is he needed a 50 amp draw for the small type Daly I purchased, he'd get a 60 amp BMS.

What about the idea of using a standard BMS to control a contactor rather than disconnecting the battery?

A BMS is essentially a large switch to ground, so it can be used to pull a normaly-high (24V) signal to ground when it does not want the battery disconnected (with some nominal power consumption from a pull-up resistor depending on what response time is needed).

That grounded signal can can be used to leave a normally-closed relay closed unless the BMS detaches, at which point the normally-high (24V) signal will be grounded and the normally-closed relay will open.

That control relay can control a 300A contactor so that any time the BMS wants the battery disconnected, it can do so through the control relay.

Anyone heard of any members trying anything similar to this?

I understand the Daly BMSes are designed to disconnect in exactly this way and once I’ve looked into building my own or jerry-rigging a standard BMS through control relays, I may end up deciding it’s easier and more economical to go that route…

 

[ghostwriter66]

Maybe not 50%, but at 100% it trips. There's formulas for circuit breakers which in part will avoid nuiscance tripping like expected Amps * 1.56 or ^ 1.2.

Part of the reason I'm putting this out there is to see if others have seen this where the SCC max amps = BMS max amps, but the SCC pushes .1 amp more so the BMS trips.

Will said in a video that is he needed a 50 amp draw for the small type Daly I purchased, he'd get a 60 amp BMS.

allot of changes have occured in the last 2 years with BMS ... I'm relatively new to this and I remember when virtually none had BMS or remote or cold-temp cutoff and the list goes on...

I do believe that with all these ppl doing these comparison videos on YouTube that many many companies figured that they would clean up their act and try to meet their specs otherwise they would get a bad rep ...

DALY was pretty rough now they are relatively good ...

I've been playing wiht a new BMS https://www.aliexpress.com/item/100...&terminal_id=5c38ccba0ad34f97bc6ba87cad366bbe that @Off-Grid-Garage has been testing and I will tell you - its a pretty solid BMS .. there are some things that it does that the DALY doesn't - but basically it all depends on the level of "set-it-and-forget-it" you want to do ...

so I say all of this to say this ... if its not DALY or JiKong I normally de-rate them BUT if it is a Daly or JiKong then I go with the figures that they advertise ... I am sure that there are many others that you don't need to de-rate - I just haven't played with them ...

 

[fafrd]

allot of changes have occured in the last 2 years with BMS ... I'm relatively new to this and I remember when virtually none had BMS or remote or cold-temp cutoff and the list goes on...

I do believe that with all these ppl doing these comparison videos on YouTube that many many companies figured that they would clean up their act and try to meet their specs otherwise they would get a bad rep ...

DALY was pretty rough now they are relatively good ...

I've been playing wiht a new BMS https://www.aliexpress.com/item/100...&terminal_id=5c38ccba0ad34f97bc6ba87cad366bbe that @Off-Grid-Garage has been testing and I will tell you - its a pretty solid BMS .. there are some things that it does that the DALY doesn't - but basically it all depends on the level of "set-it-and-forget-it" you want to do ...

so I say all of this to say this ... if its not DALY or JiKong I normally de-rate them BUT if it is a Daly or JiKong then I go with the figures that they advertise ... I am sure that there are many others that you don't need to de-rate - I just haven't played with them ...

Any experience with an 8S BMS that can safely charge at up to 120A?

 

[ghostwriter66]

Any experience with an 8S BMS that can safely charge at up to 120A?

I honestly can't remember what I used for 8S ... BUT with that said I would definitely look at what Daly and JiKong had ... both also have GREAT customer service ...

 

[MrM1]

@fafrd ..
Where did you get the Heltec? Link?

 

[fafrd]

@fafrd ..
Where did you get the Heltec? Link?

Here is the link to Heltec’s website: https://heltec-bms.com/shop/

I have been communicating directly with ‘Annie’ through the Alibaba messenger.

There are various ‘stores’ for Heltec BMS on AliExpress but I generally find it most effective to contact the vendor directly and then let them direct you to the appropriate ‘store’ on AlExpress.

I’m not sure how I can direct you to Annie, but if she is the sales associate you end up dealing with, I can highly recommend her.

 

[MrM1]

@fafrd OK thanks. So if I reach out to Heltec they'll put me on to a consumer sales point?

 

[fafrd]

@fafrd OK thanks. So if I reach out to Heltec they'll put me on to a consumer sales point?

I can’t recall weather I reached out first through their website or Alibaba. In any case, a sales associate should get back to you and once you get close to ordering, they will give you either the option of ordering direct or through one of the storefronts they established on AliExpress. Because shipping was slightly less through the AluExpress storefront, that’s the route I ended up going…

Seeing it, I believe I first reached out through their online messenger: https://heltec.org/about_us/contact/

Annie has excellent communication skills in English so if you do reach out, putting Attn: Annie would likely increase the chances she’s the sales associate that gets back to you…

 

[curiouscarbon]

Maybe not 50%, but at 100% it trips. There's formulas for circuit breakers which in part will avoid nuiscance tripping like expected Amps * 1.56 or ^ 1.2.

Part of the reason I'm putting this out there is to see if others have seen this where the SCC max amps = BMS max amps, but the SCC pushes .1 amp more so the BMS trips.

Will said in a video that is he needed a 50 amp draw for the small type Daly I purchased, he'd get a 60 amp BMS.

I would imagine that some devices will instantly shut down after observing an Ampere value above the software max amps.

On the JBD / Overkillsolar BMS that I use, the charge overcurrent condition is configurable with Trigger Value, Release Value, Delay system. for overcharge protection, the three numbers are: the Amps, How long to stay off, and Delay before shutoff.



130000 milliamps for 10 seconds with a 32 second delay for reset, or 130A for 10 seconds on a 120A rated BMS. one reference point. if i had active cooling on the BMS MOSFETs heat spreader, like a fan, i could test a 150A charge and observe the BMS reported temperature (one for cells, one for BMS) if it did not heat up much, i would feel comfortable bumping up the limit a bit.