BMS with limited load current

[RHTizzy]

Hi all,

I was wondering if BMS's in general limit their continuous output load current or if they just cut off the load on a current threshold? If the latter, are there BMS's available where you can adjust the maximum continuous ouput load current?

I'm putting up a 3,5kW solar array which will be used to charge a 14kWH LiFePo4 battery bank at 51,2V nominal, so 280AH per cell. I'm also wondering about the cell balancing capabilities of BMS's.

I hear good and bad things about JBD, are they worth getting and any good at these two functions? Are there better recommendations?

Thanks to anyone willing to spend the time and effort on trying to help!

 

[LakeHouse]

I was wondering if BMS's in general limit their continuous output load current or if they just cut off the load on a current threshold?

The latter, they just cut it off. Limiting current draw needs to be done on the load side.

If the latter, are there BMS's available where you can adjust the maximum continuous ouput load current?

Yes, the JK BMS's can adjust this up to the rated value. Not sure about others.

 

[Ampster]

......are there BMS's available where you can adjust the maximum continuous ouput load current?

The Orion BMSs can adjust the maximum load current, by communicating to the inverter to change the current draw.. The Orion heritage is from EVs and can adjust the motor loads under certain circumstances. One example is limp mode when SOC is low. Most production EVs do this to allow the vehicle to get to the side of the road instead of stopping while on the highway at speed.
EDIT: Added clarification that BMS only controls load by communicating to the inverter.

 

[RHTizzy]

Thanks guys, much appreciated.

After some research it appears as if the Seplos BMS is a good fit for our system and also has current limiting. I've spoken to their sales guys (the engineers happen to be on holiday because of the chinese new year) but I'm having trouble confirming whether it has adjustable [continuous current limiting].

In other words, set the charge rate to 1C ie 100A, but the continuous discharge at max 20A. This would result in for example 24H continous discharge (ie as a guaranteed backup) but still full power charging when the sun comes out.

Their software shows a Discharge limit FET which (to my lack of knowledge) sounds right, but is there anyone here who has one and can verify the discharge is configurable and not just an on/off limit switch?

 

[houseofancients]

Thanks guys, much appreciated.

After some research it appears as if the Seplos BMS is a good fit for our system and also has current limiting. I've spoken to their sales guys (the engineers happen to be on holiday because of the chinese new year) but I'm having trouble confirming whether it has adjustable [continuous current limiting].

In other words, set the charge rate to 1C ie 100A, but the continuous discharge at max 20A. This would result in for example 24H continous discharge (ie as a guaranteed backup) but still full power charging when the sun comes out.

Their software shows a Discharge limit FET which (to my lack of knowledge) sounds right, but is there anyone here who has one and can verify the discharge is configurable and not just an on/off limit switch?

You can setup both max charge and Max discharge, fail to see why, but you can..
At least in the v2 (10e)

 

[RHTizzy]

Awesome, thanks for the validation!

It's mainly a kind of backup system and will be grid tied but I dont want excess being pumped back into the grid. So by managing the discharge we keep it lower than our average usage. The idea is also that the lower discharge will benefit the longevity of the cells.

 

[houseofancients]

Awesome, thanks for the validation!

It's mainly a kind of backup system and will be grid tied but I dont want excess being pumped back into the grid. So by managing the discharge we keep it lower than our average usage. The idea is also that the lower discharge will benefit the longevity of the cells.

This is not something you should handle in your BMS, rather your inverter.
The BMS is a safety device to protect your batteries, nothing else.

Highly recommend to NEVER do it this way

 

[RHTizzy]

Would you happen to have a screenshot of the config option btw?

 

[houseofancients]

Would you happen to have a screenshot of the config option btw?

Not near my batteries this weekend, so can't make a screenshot.
But I did upload the manual for the v2 , which includes all options.
You can find it in the resources section here

 

[Ampster]

This is not something you should handle in your BMS, rather your inverter.
The BMS is a safety device to protect your batteries, nothing else.

I think what you are saying is when discharge reaches the max discharge setting, the BMS will shut down the pack. Any control on discharge should be done at the inverter since the BMS should be a last resort fail safe device not a device that manages loads. That is what @LakeHouse said in an earlier post

 

[RHTizzy]

I think what you are saying is when discharge reaches the max discharge setting, the BMS will shut down the pack. Any control on discharge should be done at the inverter since the BMS should be a last resort fail safe device not a device that manages loads. That is what @LakeHouse said in an earlier post

Uhm, that then sounds like it's NOT a setting for a continuous maximum but rather a shuttoff switch.

I'm looking for the former, not the latter. Is it indeed a simple cuttoff at an adjustable threshold? Or is it a maximum for a continuous discharge rate which I thought we had just confirmed?

 

[houseofancients]

I think what you are saying is when discharge reaches the max discharge setting, the BMS will shut down the pack. Any control on discharge should be done at the inverter since the BMS should be a last resort fail safe device not a device that manages loads. That is what @LakeHouse said in an earlier post

Better worded, but exactly what I mean

 

[houseofancients]

Uhm, that then sounds like it's NOT a setting for a continuous maximum but rather a shuttoff switch.

I'm looking for the former, not the latter. Is it indeed a simple cuttoff at an adjustable threshold? Or is it a maximum for a continuous discharge rate which I thought we had just confirmed?

Again, you are missing the point entirely..
There is, but it shouldn't be used in the way you want to..
Limiting should never be done on the BMS but on the inverter.
Setting a rate limit means a hard limit, some peak demand comes in unexpected and your inverter will simply shut down.
You are trying to cut corners where you shouldnt

 

[Ampster]

Or is it a maximum for a continuous discharge rate which I thought we had just confirmed?

Current is typically determined by loads. Therefore controlling loads is the optimal method to control current. BMSs are safety devices. The only way they are used to control loads are to turn off current when maximum current setting is reached. They can also do this when typically in closed communication with an inverter by telling the inverter that SOC is getting low as I mentioned in earlier. In that case the inverter will pull less current. Most EV motors are controlled by 3 phase multi frequency inverters. Sorry for the confusion. Your application does not sound like it is an EV use case so I should probably not have mentioned that.

 

[RHTizzy]

Sorry for the confusion guys and thanks for all the opinions. I have been working with solar panels since 2017 and understand your concerns. Just dont know the Seplos BMS and the sales rep is demonstrably not a reliable source.

As long as the Seplos can be regulated to NOT deliver more current than my ADJUSTABLE requirement then I'm happy. I DONT want the BMS SHUTTING the battery down when it reaches MY CONFIGURED load current. I have other options for overcurrent protection.

Have a great weekend guys!

 

[Ampster]

As long as the Seplos can be regulated to NOT deliver more current....

Just so there is no confusion, a BMS can NOT be configured to deliver a regulated current. It is a safety device which shuts off current when the configured maximum setting is reached.

 

[RHTizzy]

The Orion BMSs can adjust the maximum load current. The Orion heritage is from EVs and can adjust the motor loads under certain circumstances. One example is limp mode when SOC is low. Most production EVs do this to allow the vehicle to get to the side of the road instead of stopping while on the highway at speed

Now you're confusing me, because previously you also said this. Are you saying the ancient misunderstood every time what i was asking? You now seem to understand the question though, even if your answers are contradictory to themselves....

 

[Ampster]

Now you're confusing me, because previously you also said this.

I should not have over explained by using the EV example because it is not relevant to what you are trying to to do. Reread the thread. I am saying the same thing that @houseofancients and @LakeHouse said.
Do I understand what that you are trying to do is control the maximum current your inverter pulls from your batteries? If that is correct then the way to do that is with your inverter. When your inverter reaches the setting it will begin supplementing the battery current with grid current. It does this on the output or load side of the inverter as the load increases. This is a very common way a hybrid inverter controls the amount of battery power is used. I use a SolArk (Deye/Sunsynk) and it has a max battery discharge setting. I assume your inverter can do that as well. What inverter do you have?

 

[RHTizzy]

No you misunderstand. I am trying to limit the current that is output by the BMS. You dont know what I'm trying to do other than that. Which is why they say that people who "assume" make an ass out of u and me. Even with the best intentions, a simple question is best served by a simple answer. Leave peoples motivations to themselves and dont treat them like children by assuming you know whats best for everyone all the time in every situation and they dont have a clue. You are trying to help which is appreciated, but it appears are getting mislead by what others are saying or have said. I have a few people saying multiple times including here, that continuous current draw can be limited by the BMS without switching off the discharge. You have said yes and no. Do I believe everyone else? Are they misunderstanding? Do I believe you? Or are you misunderstanding? Do they have information that you dont? Or the reverse? The question is simple: "can it be done". Leave answering the "why's" when people ask it..... I have ancient saying multiple times that it can, he just doesnt agree with it. Fine, I respect his opinion. But stop with the "No, because you dont want to" answers. Unless you know for a fact that the seplos cannot do it. Ancient has one, he says 'Yes', or is he getting ancient beyond his years and losing his mind?

 

[Ampster]

The question is simple: "can it be done".

The answer is simple. A BMS cannot control a load other than switch it off or on. Forget everything I said earlier that implied anything different. @LakeHouse and @houseofancients are saying the same thing.