BMS common port vs seperate port

[LFP addict]

Last pic your'e connecting the C and P ..

 

[Dzl]

Last pic your'e connecting the C and P ..

Ahh, I see. Not my intention I'm just a really bad artist, the hall sensors I'm familiar with look like 'donuts' that the current carrying wire(s) pass through. Like this:

So the purple circle in the schematic is meant to represent the 'donut' and is isolated from the current carrying wires that pass through it, even if it doesn't look that way.

 

[veikop]

The split port has a charge port and a battery port. The BMS can cut off Charge separate of battery (Load). The common port the load and charge port are together. If the split port sees high voltage It can shut off charging. If it sees low voltage, It can disconnect load.

Hey, i am confused. If i am using common port BMS and BMS detects overcharge - what then happens - will it disconnect only charging or both - charging and discharging. From Overkill BMS manual it seems that this is not so (charge is disconnected but discharge is not - page 16)

 

[BiduleOhm]

Depends on the BMS. Some common port ones have separate disconnection, while others don't.

As you saw in the manual, the Overkill one is of the first kind (and I had a discussion about that with the manufacturer so I can confirm 100 % it's correct).

 

[veikop]

Depends on the BMS. Some common port ones have separate disconnection, while others don't.

As you saw in the manual, the Overkill one is of the first kind (and I had a discussion about that with the manufacturer so I can confirm 100 % it's correct).

Thank you! But then, why there are separate port ones? What is the advantage of separate port? As far as i understand, separate port BMS has a disadvantage of low charging current.

 

[John Frum]

Thank you! But then, why there are separate port ones? What is the advantage of separate port? As far as i understand, separate port BMS has a disadvantage of low charging current.

Beginners Summary of BMS Functions, Types and Features

This is a beginner's summary of the common types, functions and features of BMSs. The descriptions do not attempt to list all information of all BMSs, but it is intended to give you a base understanding of BMSs The document is in 6 sections: 1...

diysolarforum.com

 

[BiduleOhm]

Thank you! But then, why there are separate port ones? What is the advantage of separate port? As far as i understand, separate port BMS has a disadvantage of low charging current.

The problem with doing different disconnects with a common port BMS is that when you disconnect charging for example, the discharge current must pass through the body diodes of some of the MOSFETs (this is inherent to how MOSFETs are arranged in BMS and how they work/are built) and in doing so you have far greater losses and risk overheating the MOSFETs. Of course it works exactly the same if you disconnect discharging and still have charging (basically the circuit is mirrored internally).

That's why I wouldn't use a common port BMS to more than 1/4 to 1/3 of its rating if it's in disconnect mode and why I had the discussion with the manufacturer (which according to him: it's not a problem because the BMS shouldn't be in that mode too long and there's thermal sensors on the FETs... Well, maybe but thermal impendance is a thing and that's why you can still burn the FETs easily when in that mode), but I digress).

With a separate ports BMS you don't have this problem because charge and discharge currents take different paths. The fact they handle only a fraction of the discharge current for the charge current is just because of cost savings... you can have equal charge and dscharge currents or even a higher charge current than the discharge current, it all depends on the MOSFETs you chose and how many of them you put on each side.

 

[veikop]

The problem with doing different disconnects with a common port BMS is that when you disconnect charging for example, the discharge current must pass through the body diodes of some of the MOSFETs (this is inherent to how MOSFETs are arranged in BMS and how they work/are built) and in doing so you have far greater losses and risk overheating the MOSFETs. Of course it works exactly the same if you disconnect discharging and still have charging (basically the circuit is mirrored internally).

That's why I wouldn't use a common port BMS to more than 1/4 to 1/3 of its rating if it's in disconnect mode and why I had the discussion with the manufacturer (which according to him: it's not a problem because the BMS shouldn't be in that mode too long and there's thermal sensors on the FETs... Well, maybe but thermal impendance is a thing and that's why you can still burn the FETs easily when in that mode), but I digress).

With a separate ports BMS you don't have this problem because charge and discharge currents take different paths. The fact they handle only a fraction of the discharge current for the charge current is just because of cost savings... you can have equal charge and dscharge currents or even a higher charge current than the discharge current, it all depends on the MOSFETs you chose and how many of them you put on each side.

OK, thank you, it is getting complicated and interesting

As my Overkill BMS is 120A (4S/12V) and i am planning only to pull max 15-20 amps, then i should not worry. In short term in "mosfet mode" where charging is disabled, it can probably handle it. In real life BMS cutoff should not activate at all as smart charger should stop charging before that.

 

[GXMnow]

This is correct. In a separate port BMS there are 2 separate current paths. In the on state, they are very low resistance, and when one opens, the other is not effected. With a common port design, the 2 switches are effectively in series. So even when both are on, there is likely to be more on resistance, and when one switches off, the other direction is conducted through a diode drop which will create a power loss and heat generation, either in the mosfets, or at best in a parallel diode.

But here is another argument where the common port is better. You have a large solar array, and a big inverter. With a separate port, ALL of the power is going both in and back out of the BMS. While the sun is shining, you have 100 amps coming in, heating those FETs, and let's say 80 amps are going back out driving the inverter, heating those FETs. The battery is only charging at 20 amps, but the BMS is working hard. With a common port design, the 80 amps is only going through a fuse from the charge controller to the inverter. The common BMS port only sees the 20 amps left over that is charging the battery. This results in far less heating in the FETs. You might also get away with a much lower charge current rating, assuming the inverter has to work while the sun is shining.

If your system is properly setup and the cells are good, the BMS should never have to shut off the current path. The charger(s) should limit the top voltage to below where the cells exceed the maximum safe voltage, and the loads (inverter(s) etc. should all be set to shut down before the cells get down to too low of a voltage. In those cases, the BMS just sits there and monitors for a fault. The one additional function is balancing the cell voltages. If the cells are well matched and properly top balanced, then even the balancing won't really have to do anything. The BMS is there for when something goes wrong. If a cells goes too high or too low, it will shut things down to protect the system from further damage. Either case is bad and should never happen. The BMS is not a charge controller. The only real exception is blocking LiFePo4 cells from charging below freezing. But even that should be setup in the charge controller and not depend on the BMS to save it.

 

[veikop]

This is correct. In a separate port BMS there are 2 separate current paths. In the on state, they are very low resistance, and when one opens, the other is not effected. With a common port design, the 2 switches are effectively in series. So even when both are on, there is likely to be more on resistance, and when one switches off, the other direction is conducted through a diode drop which will create a power loss and heat generation, either in the mosfets, or at best in a parallel diode.

But here is another argument where the common port is better. You have a large solar array, and a big inverter. With a separate port, ALL of the power is going both in and back out of the BMS. While the sun is shining, you have 100 amps coming in, heating those FETs, and let's say 80 amps are going back out driving the inverter, heating those FETs. The battery is only charging at 20 amps, but the BMS is working hard. With a common port design, the 80 amps is only going through a fuse from the charge controller to the inverter. The common BMS port only sees the 20 amps left over that is charging the battery. This results in far less heating in the FETs. You might also get away with a much lower charge current rating, assuming the inverter has to work while the sun is shining.

If your system is properly setup and the cells are good, the BMS should never have to shut off the current path. The charger(s) should limit the top voltage to below where the cells exceed the maximum safe voltage, and the loads (inverter(s) etc. should all be set to shut down before the cells get down to too low of a voltage. In those cases, the BMS just sits there and monitors for a fault. The one additional function is balancing the cell voltages. If the cells are well matched and properly top balanced, then even the balancing won't really have to do anything. The BMS is there for when something goes wrong. If a cells goes too high or too low, it will shut things down to protect the system from further damage. Either case is bad and should never happen. The BMS is not a charge controller. The only real exception is blocking LiFePo4 cells from charging below freezing. But even that should be setup in the charge controller and not depend on the BMS to save it.

Thank you for your detailed response. I am setting my system up in a way, where BMS is only for balancing and acting as a last safety for battery. My initial fear was the situation, where charger fails to disconnect charge and BMS shuts down both - charging and loads - this situation can be dangerous as my navigation and running lights are depending on power (unknown areas, low visibility).

 

[BiduleOhm]

As my Overkill BMS is 120A (4S/12V) and i am planning only to pull max 15-20 amps, then i should not worry. In short term in "mosfet mode" where charging is disabled, it can probably handle it. In real life BMS cutoff should not activate at all as smart charger should stop charging before that.

Yes, it's totally fine

Yeah but even if it should not activate it may, and if it does the last thing you want is for it to fail as something already gone wrong (else it wouldn't have activated).

 

[william2021]

Hi, sorry for cutting in but I'm trying to understand this issue such as Will's post at https://diysolarforum.com/threads/bms-common-port-vs-seperate-port.468/#post-4145 .

If I understand correctly, he said for separate port, Daly's BMS has charge current at 10A. While this Daly's product page says for their 150A model, the charge current is 40A: https://www.aliexpress.com/item/32921412658.html . Is that correct?

Also, does he mean that, if I buy a separate port model, then I can either connect the charger to the BMS's charger port, or directly connect the charger to the battery? Does the BMS work only if I have to connect the charger to it?

I'm only planning to buy a 280Ah Lifepo4 battery pack, with a MPPT charge controller, so GXMnow's argument for the common port above doesn't apply here, I think.

 

[william2021]

To add to my post above, here is the specification of the Daly BMS: https://ae01.alicdn.com/kf/Hed02da12b1bc4f139a07cb9e07309ed28.jpg linked from the Daly page above.

It says for separate port, the charge current is 10A for up to 60A models, and 40A for models from 80A.

 

[GXMnow]

Specs can change all the time, so make sure you have the specs for the exact model you have. The page you sent does show 40 amps of charge current for all separate port models from 80 amps and higher. What is your charge source? If it is greater than 10 amps, then you need a 40 amp charge model. If it could exceed 40 amps, then you have to move up further.

For the BMS to be able to do it's protection functions, the current has to go through the switch controlled by the BMS. In the case of these Daly units, my JK BMS, or most others, that means the current is going through the board inside of the BMS. Some larger systems like a Chargery can use an external protection contactor or solid state relay for the protection, but it is much more common to have a mosfet based switch inside of the BMS on the circuit board.

What is your load going to be? Will you be using much power while the sun is shining? How much power will you be running overnight? And what bank voltage are you running (to calculate the current and wattage values)?

 

[william2021]

Thanks. I currently have 4 100W panels in serial, may add another 4 in serial, and then parallel the two serials, so my charge current will be within 20A if I'm not wrong. The battery bank is 12V, will be a 280Ah one for now. My load can be 1500W occassionally in the evenings or while the sun is shinning, of course there are lower loads at other times. I have a MPPT charger controller and a pure sine ware inverter.

So looks like the Daly BMS I buy should be separate port, and I can:

* either connect that MPPT charger to the BMS's dedicated charger port, for BMS's protection,

* or directly connect that MPPT charger to the battery bank, since that MPPT already has quite some protection and no worry for 0 C degree in my case, so leave that BMS only to protect discharging.

is that correct?

 

[GXMnow]

The charge current should still go through the BMS for protection. If the cells do go out of balance, it is possible for one cell to go too high and be damaged, even if the total pack voltage is safe.

1,500 watts on a 12 volt system could exceed 130 amps. A 150 amp BMS would be the minimum I would suggest for that. Even running a microwave for 90 seconds would probably shut down a 120 amp unit.

 

[william2021]

Ok, thanks for the explanation of protecting charging with BMS. So I will buy a separate port and connect the BMS to both the charger and the inverter to the corresponding ports.

 

[william2021]

Just a second thought, if the BMS disconnects the battery from the charger, wouldn't the charger burn out from the panel? I remember Will's video said I should always connect the battery to the charger before connecting the panel.

 

[BiduleOhm]

Trying to destory my SCC's: I can't do it!

So I took every EPEver/Victron/PWM/Renogy solar charge controller, and tried to fry it by connecting it only to solar panels, and not to a battery... guess which one survived? EVERY SINGLE ONE! I couldn't destroy any of them, and I left them connected to a high voltage array (80 volts) for 4+...

diysolarforum.com

 

[Horsefly]

Just a second thought, if the BMS disconnects the battery from the charger, wouldn't the charger burn out from the panel? I remember Will's video said I should always connect the battery to the charger before connecting the panel.

Some charge controllers can handle being disconnected from the battery while still connected to the panels, and some cannot. My Schneider MPPT-60/150 has specific wording in the manual that says it is OK. I've heard on other boards about MPPT controllers burning out in seconds when the panels are connected without the battery.