Your Charge Controller "Charges" the line between your Battery.

Supervstech said:

The BMS has several features, but the MAIN AND ONLY IMPORTANT use for a BMS is to disconnect the charging or discharging when parameters are exceeded from a fault.

Click to expand...

Yes 100% accurate, and this can be confirmed just by looking at your typical default threshold settings on your average LFP BMS for example.

An example pulled off of spec sheet for a typical JBD BMS...


The high-volt threshold (Overcharge Detection Voltage) at 3.75v /cell, and low-volt threshold (Undercharge Detection Voltage) at 2.7v /cell, which are both considered to be within extreme limits to protect a cell from damage due to some infrequent charge/discharge event, but definitely not within standards (best practices) for repeated cycling in that range to maintain long battery cell life...


As you can see, both of those threshold voltages are well into the high and low red zones in the diagram below (if any kind of decent cell life is the objective):





Both the charger profile cycle voltages, and the inverter low-voltage cutoff should always be calibrated to limit SoC movement within the desired high limits and low limits of the battery bank which are within acceptable range for normal operation.

BMS was intended to be 'the last line of defense'...

More info here in this PDF, good read:

Michael77 said:

Okay. The SCC terminals and App show the same @ 14.7
The Battery BMS App shows 13.6 and I used the multimeter to read 13.6. I took the readings from a pair of cables coming off the Distribution Bus Bars that connect the main battery, charge controller, and inverter, 12v DC Panel, the wires are for adding a second battery in parallel. So, they are before the BMS.

Yes, you will have voltage drop, and the wires get hot and add & compound the problem, Agreed.

Click to expand...

your wires should never ever get “ hot” , if they do they are seriously unsized

Michael77 said:

My system shows 14.7 at the SCC and 13.6 at the battery, and I am 100% positive that regardless of the wire thickness (bigger is better, or course) this is common for all modern high amp MPPT/LFP Battery Systems.

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No, it’s simply you have way too much resistance between the mppt ( your solar unit is only PWM by the way , not mppt) and the battery. If that 1.1v drop is occurring at 30 A , your have a 33 watt wire heater consuming solar power needlessly

I have a victron mppt controller and decent wires , I have approx .15v drop at full current 20 A

again the fact you are positive , does not invalidate ohms law. If you have a voltage difference , you must have the resistance in between.

Michael77 said:

The best thing you could do is this: just for testing, disable the shunt and any communication devices (just to eliminate the variable) and under peak sun conditions, measure your SCC App Voltage and Terminal Voltage, and then measure your Battery BMS Voltage and Terminal Voltage

If I am correct, you will see similar results as my set up. Are you 12, or 48 volts?

Click to expand...

If you want to carry 30 A , you need about 8 or 6 AWG , these are big cables. When you install those voltafe drops around 0.2v will be typical

the wiring you have now is unsafe

Michael77 said:

as I upgraded components, batteries, chargers, panels, etc all showed the same phenomenon, the Charge Controller would "Charge The Line" from the Controller to the Battery. aka charger shows ~14.7 and battery shows ~13.6 in each iteration of the system/s = people be totally freaking out. (I predict, you might see similar results)

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Everything except your cables which are causing the issues.

Where the heck did you even get that "charge the line" phrase from? LOL That craziness is what make me look at the thread in the first place.

Your prediction would be absolutely wrong.

Michael77 said:

The best thing you could do is this: just for testing, disable the shunt and any communication devices (just to eliminate the variable) and under peak sun conditions, measure your SCC App Voltage and Terminal Voltage, and then measure your Battery BMS Voltage and Terminal Voltage

Click to expand...

I told you I'd get numbers for you. It wasn't quite peak sun and I had some loads running, but here's my numbers.
Charging at roughly 55.2 amps.

Charger measurement/display: 53.5 volts
Actual measurement at charger terminals: 53.03 volts
BMS measurement/display: 52.93 volts
Actual measurement at batt terminals (after BMS/shunt) 52.86 volts

You can see the inaccuracy in my charger's voltage measurement, hence the insistence to ignore this voltage reading until you know how it compares at different current.
In total, my system has 0.17 volts of loss in: 2 breakers, 1 shunt, 2 bus bars, and 25 feet of 0000 cable.

You measured over a volt at about half the current. Your wiring/connections are the first thing to address.

Supervstech said:

The BMS has several features, but the MAIN AND ONLY IMPORTANT use for a BMS is to disconnect the charging or discharging when parameters are exceeded from a fault.

Click to expand...

Yep, last line of defense sort of thing.
The charge controller should be programmed to function properly and the BMS should only kick in (and open) when you have a failure elsewhere in the system.

400bird said:

I told you I'd get numbers for you. It wasn't quite peak sun and I had some loads running, but here's my numbers.
Charging at roughly 55.2 amps.

Charger measurement/display: 53.5 volts
Actual measurement at charger terminals: 53.03 volts
BMS measurement/display: 52.93 volts
Actual measurement at batt terminals (after BMS/shunt) 52.86 volts

You can see the inaccuracy in my charger's voltage measurement, hence the insistence to ignore this voltage reading until you know how it compares at different current.
In total, my system has 0.17 volts of loss in: 2 breakers, 1 shunt, 2 bus bars, and 25 feet of 0000 cable.

You measured over a volt at about half the current. Your wiring/connections are the first thing to address.


Yep, last line of defense sort of thing.
The charge controller should be programmed to function properly and the BMS should only kick in (and open) when you have a failure elsewhere in the system.

Click to expand...

And the BMS last line of defense is on the cell level, not the entire battery level. So, the BMS main and most important job is to detect cell imbalance to prevent pack failure… not to control the pack voltages.

400bird said:

You measured over a volt at about half the current. Your wiring/connections are the first thing to address.

Click to expand...

Yes, agreed. I put a heavy (4 my system) ~900 watt continuous load on my system and I am still seeing exactly 1 volt difference between the Charge Controller and the Battery BMS Voltage.

Cables are hot, but not unsafely so, and the cable is technically well within spec, so yea, as I said I will be replacing/upgrading it, and I will double check each and every connection in my system to make sure there aren't any loose wires, or connections, etc... again...

With that said, every iteration of my build, first with PWM and lead acid, and then PWM with LFP, and now MPPT with LFP, every one of my iterations of my solar system always had a high delta from the Controller to the Battery. (I have also seem many people mentioning this same issue, and they were told that "something catastrophic is occurring."

If it was really just resistance in the wire, I would be surprised, the wire would probably be getting hotter, but I am willing to, I am changing it out anyway, if for nothing else, the certainty and efficiency.

I do not know if you read my post on how I cranked up my "allowable voltage" on my charge controller, and I turned off unnecessary features, timers, etc and just let my MPPT send all available amps to my system... Other then the warm wire, and the higher (then what people think is bad) Voltage Delta between Charger & Battery, I am getting more "juice" then every before!

Good luck with your system, and I offer you to read this thread, it may be worth you manually setting & tinkering around with your system, if you can get an extra 15/20% out of an existing installation, while still having peace of mind, and a safe system... Good luck.

I still would suggest as other people had already suggested, check Vdrops between two ends the wires, on the circuit breaker/fuse, bussbar, etc, then if you change the wires then you will have a ref. to compared to, then you can also try tighten the connection and see if there are changes in the Vdrops, it is so easy to do to measure the Vdrops between 2 points.,
BTW, do you have breaker/fuse between SCC and the batteries?

Michael77 said:

Cables are hot, but not unsafely so,.

...

If it was really just resistance in the wire, I would be surprised, the wire would probably be getting hotter, but I am willing to, I am changing it out anyway, if for nothing else, the certainty and efficiency.

Click to expand...

You just said they are hot. Any heat is due to resistance. Resistance=voltage drop. Not sure why you are still so reluctant to accept that the cables have issues. Someone calculated about 33 watts being dissipated. It's not like that is going to make the cables glow or anything

Michael77 said:

I am getting more "juice" then every before!

Click to expand...

And you would get more if you weren't loosing it to heat in your cables.

After you get the cable issue fixed, I highly recommend you crank the controller down to the recommended charge voltage and stop relying on the BMS to cut things off.
.

Goboatingnow said:

If you want to carry 30 A , you need about 8 or 6 AWG , these are big cables. When you install those voltafe drops around 0.2v will be typical

Click to expand...

Hi, I read your reply, and agree with you. I do, by the way, have thin strand high temp silicone 8 gauge AWG wire at 3 feet. It is well within spec, but as I have mentioned before to others, I will be swapping that out for 6 AWG wire and double checking, triple checking all connections, wires, etc.

The Delta from the Charge Controller to the Battery IS something I am aware of as being "an unknown" or questionable, but I have had 3 completely different iterations progressively getting larger and more powerful, from 2 12v2P Lead Acid Batteries with a PWM, then I upgraded to a Large LFP Battery, and they have ALL had this "high" Delta

And, now; I just upgraded from 4 Solar Panels to 6, and upgraded to an MPPT Charge Controller. I initially wasn't getting any more energy from the additional 2 Solar Panels and MPPT, and so I did some research and turned off the "Flood Timer" and turned up the allowable voltage limits, and now I am getting a lot more power out of my system, and I able to use my small system to cook for about 4 hours every day (~750/900 Watts @ ~4 Hours) which it really really good for me, especially if SHTF which I think everyone these days is a little worried about.

But, though I flat out disagree with several people in the forum, I do like it when I see people who take the time and comprehensively point out their observations, based on their experience. So, thanks for the info, and after I swap out the cables I will be sure to let everyone know the results. Thanks.

Texas-Mark said:

You just said they are hot. Any heat is due to resistance. Resistance=voltage drop. Not sure why you are still so reluctant to accept that the cables have issues. Someone calculated about 33 watts being dissipated. It's not like that is going to make the cables glow or anything


And you would get more if you weren't loosing it to heat in your cables.

After you get the cable issue fixed, I highly recommend you crank the controller down to the recommended charge voltage and stop relying on the BMS to cut things off.

Click to expand...

And he could change the charge controller back to normal/recommended settings and go back to using his BMS "properly"
But, oh well, the OP seems happy to have not have the BMS acting as the last line of defense and instead they have no back up protection.

Michael77 said:

Yes, agreed. I put a heavy (4 my system) ~900 watt continuous load on my system and I am still seeing exactly 1 volt difference between the Charge Controller and the Battery BMS Voltage.

Click to expand...

Do you run this 75 amp load through the same wiring? Or is there a bus bar in common? Voltage drop is bad and should be located.
As I've said before, could be in amy connection, fuse, breaker, bus bar, etc.

Michael77 said:

I do not know if you read my post on how I cranked up my "allowable voltage" on my charge controller

Click to expand...

I did.
I flagged it because of the poor advice. You're giving advice that recommends others run with no safety (it's not got practice to use the BMS as routine charge cut off), ignore battery charge settings, etc.

Bud Martin said:

I still would suggest as other people had already suggested, check Vdrops between two ends the wires, on the circuit breaker/fuse, etc, then if you change the wires then you will have a ref. to compared to, then you can also try tighten the connection and see if there are changes in the Vdrops, it is so easy to do to measure the Vdrops between 2 points.,
BTW, do you have breaker/fuse between SCC and the batteries?

Click to expand...

Hi, I will be replacing the 3 Foot 8 Gauge AWG Thin Strand Silicon wire that connects to my Charge Controller to the Busbars (250A Rated) on the system. (wires from busbar to battery are 2AWG.) I am just waiting on delivery of the new cables. I say this first because my system is in a cubby under my kitchenet, and in in order to do any real work or maintenance on it, I have to take out the countertop... etc. So I will just do it all at once and I will make sure to report back. I will also double/triple check all connections etc.

There is no fuse other then the built in fuse in the MPPT. There is a T90 Connector. I will eliminate that when I swap the wire. May opt for an "Anderson T50 Connector" - they seem to be very popular. Thank you.

400bird said:

And he could change the charge controller back to normal/recommended settings and go back to using his BMS "properly"
But, oh well, the OP seems happy to have not have the BMS acting as the last line of defense and instead they have no back up protection.


Do you run this 75 amp load through the same wiring? Or is there a bus bar in common? Voltage drop is bad and should be located.
As I've said before, could be in amy connection, fuse, breaker, bus bar, etc.

I did.
I flagged it because of the poor advice. You're giving advice that recommends others run with no safety (it's not got practice to use the BMS as routine charge cut off), ignore battery charge settings, etc.

Click to expand...

Hi, Thank you, you have been really helpful. I really appreciate your advice. I am running the 40Amp MPPT Charge Controller through a 3 Foot, 8 Gauge AWG Thin Strand Silicon Cable with a T90 Connector to the 250 Amp 4 3/8" Stud Busbar, and that connects to the Battery with a 2AWG 2 foot Cable, and a 2AWG Cable to the 2200W Pure Sine Inverter. (Will be upgrading the 8AWG cable to a 6AWG Cable, as I already said & double check all wires, connections, etc.)

Do you think its bad for the BMS to cut of the charge to the battery?

As for the charge controller settings, I appreciate your concern, and your caution. But I have been playing with this for about a week and I am very happy with the function and operation of my system, it is working great, and it is mid summer, very hot, and everything is working great. Again, will upgrade that wire for efficiency & peace of mind.

As we move into the cooler Fall months, I will continue to monitor and use the system daily, cooking, and even seeing if I can cook and run a small 12v 300 watt heater, and have enough juice. I will monitor the system and I will report back on the progress. I have confidence in my Batteries, and though I may be "shaving years off of them" by using aggressive charging parameters, I am more than sure they will hold up just fine for a very long time, and if not, its not a big deal, they are the cheapest Chinese Packs I could get from China. They could fail for any reason at any moment, and if they die, they die. Based on the safety data on these batteries, there is more of a chance that any other component will fail before the battery cells themselves fail, and if that happens I will tear them apart and harvest them.

Thank you for all your great help and advice. Thanks.

Texas-Mark said:

You just said they are hot. Any heat is due to resistance. Resistance=voltage drop. Not sure why you are still so reluctant to accept that the cables have issues. Someone calculated about 33 watts being dissipated. It's not like that is going to make the cables glow or anything


And you would get more if you weren't loosing it to heat in your cables.

After you get the cable issue fixed, I highly recommend you crank the controller down to the recommended charge voltage and stop relying on the BMS to cut things off.
.

Click to expand...

Hi, I don't know if I am supposed to reply to everyone who replies to me, due to me repeating my plans and answers, so if you already read my most recent replies, sorry.

Thank you for your very insightful analysis on this. 33 watts is a lot of energy and I think you guys might be right. I don't know why 8 Gauge AWG Thin Strand Silicon Cables wouldn't be able to handle HALF their rated Amps, but as I have already said several times, I will be swapping that out with a 6 Gauge AWG Cable and double checking all the connections, etc. I am all about keeping things as cool as possible and as efficient as possible.

There seem to be a lot of people in this forum who think it is a bad idea to use the BMS to stop the charging cycle. The way I see it, and the way I have seen literally every person on YouTube who does battery capacity tests AND MORE - including the Man himself ^ - they all let the BMS "do its job" and cut off the charging cycle when the battery is full.

For the first 2 years all I used was a simple PWM Charge Controller, and there was no way to program it, other then select the "Blue Light" for Li. (Renogy Wanderer 30 Amp LFP Model) and the Battery would charge until it was full and cut off.

Even with my new MPPT Controller, there really is no way to stop it from sending energy into my battery, I literally have no way at all to stop it from charging, and I have to rely on my BMS to halt the charge cycle.

Am I missing something?

Aggressive settings; Yea, I LOVE THEM and I am getting more juice into my system then ever before and to quote myself:

"
As for the charge controller settings, I appreciate your concern, and your caution. But I have been playing with this for about a week and I am very happy with the function and operation of my system, it is working great, and it is mid summer, very hot, and everything is working great. Again, will upgrade that wire for efficiency & peace of mind.

As we move into the cooler Fall months, I will continue to monitor and use the system daily, cooking, and even seeing if I can cook and run a small 12v 300 watt heater, and have enough juice. I will monitor the system and I will report back on the progress. I have confidence in my Batteries, and though I may be "shaving years off of them" by using aggressive charging parameters, I am more than sure they will hold up just fine for a very long time, and if not, its not a big deal, they are the cheapest Chinese Packs I could get from China. They could fail for any reason at any moment, and if they die, they die. Based on the safety data on these batteries, there is more of a chance that any other component will fail before the battery cells themselves fail, and if that happens I will tear them apart and harvest them. "

Thank you for all your great help and advice. Thanks.

Michael77 said:

Do you think its bad for the BMS to cut of the charge to the battery?

Click to expand...

Yes
I like a little safety.

Normal BMS use is to program your charger to do exactly what you want. The BMS is there to act as a safety (protect cells) if/when the charge controller fails. There evidence around here that the controllers can fail shorted (short PV to battery) and not regulate charge current when they should enter float or stop charging.
In this situation, the charge controller fails, the BMS steps in and protects the cells.

The way you currently have it set up is the BMS has to stop charge. The BMS most likely has MOSFETs. When the fail, they fail shorted. Meaning they can't be opened to stop the current.
So, a single failure (BMS) and you've got some ruined cells because they are severally overcharged.

400bird said:

Yes
I like a little safety.

Normal BMS use is to program your charger to do exactly what you want. The BMS is there to act as a safety (protect cells) if/when the charge controller fails. There evidence around here that the controllers can fail shorted (short PV to battery) and not regulate charge current when they should enter float or stop charging.
In this situation, the charge controller fails, the BMS steps in and protects the cells.

The way you currently have it set up is the BMS has to stop charge. The BMS most likely has MOSFETs. When the fail, they fail shorted. Meaning they can't be opened to stop the current.
So, a single failure (BMS) and you've got some ruined cells because they are severally overcharged.

Click to expand...

Ahh, okay.

There is no way for me to turn off the charging from my MPPT Charge Controller - there is no "cutoff voltage" setting.

How do I tell my charge controller to stop charging?

As I have said, for over 2 years now I have relied on my BMS to stop the charge cycle with my old PWN Charger and even my 5 Amp LIFEP04 Charger only stops charging once my Battery BMS stops the charge cycle. Am I missing something?

How do you stop the charge cycle in your system? Thanks

Supervstech said:

And the BMS last line of defense is on the cell level, not the entire battery level. So, the BMS main and most important job is to detect cell imbalance to prevent pack failure… not to control the pack voltages.

Click to expand...

Ah, okay, a lot of people seem to be saying this.

There's no way for me to turn off charging from my MPPT Charge Controller - there is no "cutoff voltage" setting.

How do I tell my charge controller to stop charging?

For over 2 years now I have relied on my BMS to stop the charge cycle with my old PWM Charger and even with my 5 Amp LIFEP04 Wall Charger, it only stops charging once my Battery's BMS stops the charge cycle. What am I missing?

How do you stop the charge cycle in your system? Thanks

Michael77 said:

Even with my new MPPT Controller, there really is no way to stop it from sending energy into my battery, I literally have no way at all to stop it from charging, and I have to rely on my BMS to halt the charge cycle.

Am I missing something?

Click to expand...

Yes. Set your charge controller to the proper settings after you get the cables fixed.

Michael77 said:

How do you stop the charge cycle in your system? Thanks

Click to expand...

I set the "bulk termination voltage"
It's name is a carry over from lead acid. But, it acts like a CV/CC charge and the bulk termination voltage is the changer over point for it to exit CC and transition to CV.

Have you shared a link to your MPPT charge controller? You've changed settings, I'm sure it has something to set the bulk/absorbtion voltage. Some manufacturers give it a different name.

Can you share screenshots of the settings menus or options on you charge controller?

Texas-Mark said:

Yes. Set your charge controller to the proper settings after you get the cables fixed.

Click to expand...

Yes, thank you. But even with the default settings, the Battery BMS kicks in and stops the charging cycle. There is literally no way for me to stop the charger from sending energy to the batteries. What am I missing

How do you stop your charging cycle? Thanks again

I would no longer use either the BMS or the charge controller for voltage values only the BMS for current. I would check the battery terminals and the charger terminals with a voltmeter. You have no idea if either of those are accurate, and even if your voltmeter is inaccurate, it should be relatively accurate such that the difference between the two values is correct.

NOT here:


But ON the battery terminals.

and here:




And given that those are puny little Phillips that already look partially boogered up, I would looking at this connection very closely.

Also, what BMS are you using?

Mattb4 said:

Ideally your BMS should not be turning off charging. Your charge controller should be handling that.

Click to expand...

Hi, Thank you. I now see that almost everyone is saying the same thing, that I shouldn't use my BMS to stop the charge cycle.

For 2 years I used a simple non programmable PWM Charger to charge My LFP Battery and the only way the charge cycle would stop is when the Battery's BMS would stop the charge cycle.

With my new, programmable MPPT Charge Controller - even at default settings, or even in the User settings, there is no way to stop the charger from sending energy to the battery and for the first week I owned it and had it in the LFP preset mode, the only way the battery stopped charging is when the Battery's BMS stopped the charge cycle

How do you stop the charge cycle on your Lithium Iron Phosphate Battery?

sunshine_eggo said:

NOT here:
View attachment 108786

Click to expand...

I just noticed that he is measuring on a lug that is not even connected to the battery. Why?????

Texas-Mark said:

I just noticed that he is measuring on a lug that is not even connected to the battery. Why?????

Click to expand...

How did I not see that either?