BMS common port vs seperate port

[GXMnow]

If your charge current and load current are from the same device, you have no choice, you have to use a common port BMS. This is my case with an inverter/charger being the only thing connected to the battery bank.

If you have a separate charge controller and inverter, then you have to decide which is better. For most systems, I agree, the common port is still better, and I will try to explain why.

Imagine it is near noon. The solar panels are producing a lot of power. The charge controller is pushing that power to the battery. But you have a large inverter, and you decide to turn on the microwave, make some toast, dry your hair, and vacuum the floor. The inverter is pulling 3,000 watts while the solar panels are making 3,000 watts. Where is all the power coming from and going to?

With a common port BMS, the BMS sees almost no power at all. The current coming in from the solar charge controller is going straight to the inverter and powering the loads. In this situation, the MOSFETs or relay for the BMS does not need to carry any current.

Now we do the same thing with a separate port BMS setup. ALL of the current from the solar charge controller has to go through the BMS over charge cut off switch. And all the inverter current has to then also go through the BMS over discharge cutoff switch. In a MOSFET based BMS, you now have 2 banks of FETs, both carrying 3,000 watts of power.

With the common port setup, the BMS will never see as much power. Even if the system was charging at noon, and discharging at midnight, you still only have the power going through one half at a time. And as both run at the same time, the BMS only has to deal with the difference in power. The separate ports has to deal with the sum of the charge and discharge power.

My only separate port BMS units are on my jump start pack. I only use a small 5 amp wall pack to charge it. I was going to go separate port for my e-bike batteries, but it turned out to be just as easy to just plug the charger into the same connector that goes to the speed controller on the bike. I never want them both in at the same time anyways. If I ever do add a solar panel and charge controller on the bike, I can still "Y" cable it between the battery pack and speed controller.

 

[Tex]

Hi Will, I'm new to the forum a few months now. I'm now seeing your post in 2022 which you wrote in 2019. I know alot has changed in the last 2 to 3 years, and I'm wondering if you're still recommending same port BMS for the reason that they have/had low charging current. I'm looking to build quite a large AH lifepo4 battery and trying to finalize my BMS of choice. They are so many options to choose from but I am really warming up to a relay type BMS.

Been using a Daly separate port for about 3 years now, 50 amps in x 200 amps out (max 400 x 10 seconds) out in a caravan, no issues to date.
All other equipment is Victron.
I believe Daly BMS have built in Bluetooth now.
Just double check their wiring instructions, mine were grossly incorrect.

 

[Henderson]

If your charge current and load current are from the same device, you have no choice, you have to use a common port BMS. This is my case with an inverter/charger being the only thing connected to the battery bank.

If you have a separate charge controller and inverter, then you have to decide which is better. For most systems, I agree, the common port is still better, and I will try to explain why.

Imagine it is near noon. The solar panels are producing a lot of power. The charge controller is pushing that power to the battery. But you have a large inverter, and you decide to turn on the microwave, make some toast, dry your hair, and vacuum the floor. The inverter is pulling 3,000 watts while the solar panels are making 3,000 watts. Where is all the power coming from and going to?

With a common port BMS, the BMS sees almost no power at all. The current coming in from the solar charge controller is going straight to the inverter and powering the loads. In this situation, the MOSFETs or relay for the BMS does not need to carry any current.

Now we do the same thing with a separate port BMS setup. ALL of the current from the solar charge controller has to go through the BMS over charge cut off switch. And all the inverter current has to then also go through the BMS over discharge cutoff switch. In a MOSFET based BMS, you now have 2 banks of FETs, both carrying 3,000 watts of power.

With the common port setup, the BMS will never see as much power. Even if the system was charging at noon, and discharging at midnight, you still only have the power going through one half at a time. And as both run at the same time, the BMS only has to deal with the difference in power. The separate ports has to deal with the sum of the charge and discharge power.

My only separate port BMS units are on my jump start pack. I only use a small 5 amp wall pack to charge it. I was going to go separate port for my e-bike batteries, but it turned out to be just as easy to just plug the charger into the same connector that goes to the speed controller on the bike. I never want them both in at the same time anyways. If I ever do add a solar panel and charge controller on the bike, I can still "Y" cable it between the battery pack and speed controller.

Ahhh, this makes perfect sense...thank you.

 

[Henderson]

Been using a Daly separate port for about 3 years now, 50 amps in x 200 (max 400 x 10 seconds) out in a caravan, no issues to date.
All other equipment is Victron.
I believe Daly BMS have built in Bluetooth now.
Just double check their wiring instructions, mine were grossly incorrect.

Thanks Tex.

 

[PatrickG]

View attachment 665

Also, I posted this elsewhere sometime ago:

Quick update for advanced LiFePO4 raw cell systems using a Daly BMS:
On my website I recommended using a separate port BMS for over voltage protection for the mppt connection (if common port BMS is used, possibility of destroying mppt during low voltage disconnect).

Well yesterday, a viewer and I finally received our separate port BMS from Daly, and the amp rating was not as advertised on the listing. The separate port can only handle 10 amps!

Considering the likelihood of over voltage situation from most high quality mppt, and the chance of matched LiFePO4 cells going out of balance is rare (and BMS will correct for cell drift over time), and that LiFePO4 can be over charged to 4.2v per cell before electrolyte degradation... I would say its safe to connect mppt directly to the battery bank, and bypass the BMS entirely. We have been doing it this way for years, but people still want to use a BMS.

I would say use BMS for loads, and not for chargers. If you have mismatched cells, and some hit a higher voltage at high SOC quicker than others, drop the upper limit voltage of your controller. 14.0-14.2v is a safe charging voltage that can give full capacity with LiFePO4 12v.

I hope this helps! I bet most people building these systems will figure this out when they see this problem, but if you are a beginner trying to build an advanced level system, then this bit of information will be very useful. Let me know if you have any questions

I know this is an older thread, but I have a question. I have an Overkill 120 BMS set up for8/s 24 volts. This is a common port BMS, but I noticed that when the cell(s) get to the determined voltage (in my case set to 3.45) it will disconnect the charging but still allow discharging. The same goes for the other way around, if there is low voltage, it will disconnect the discharging and allow charging to occur. Sorry if this is a silly or stupid question, but from what I am hearing a common port should not be able to do what I am stating above? I only have a "B- and C-" on the BMS. does a seperate port do something different that what I am describing above? I have only ever used Overkill BMS, so I am not sure if this is the same behavior I would see from other common port BMS.

Thanks in advance


-Patrick

 

[Pierre]

Quite normal. Clever electronics with FET’s and switching.
Great BMS aka JBD. Use a lot of them and have sold a lot - no problems to date.

 

[Tex]

Not sure, but maybey you can find the answer here: https://diysolarforum.com/threads/bms-common-port-vs-seperate-port.468/post-117963

 

[Tex]

I just wish they would package the damn things properly!

 

[PatrickG]

Quite normal. Clever electronics with FET’s and switching.
Great BMS aka JBD. Use a lot of them and have sold a lot - no problems to date.

Thank You, I've been reading all over this forum, and wanted to confirm, I had thought about the Fets, and now I feel reassured. Yes, they are Great BMS's, No issues here for quite some time now!

 

[GXMnow]

The easy explanation is the BMS can act like a diode when it turns off only one direction. If the cells are too hig, it will only allow current to flow out. If the cells are too low, it will only allow current to flow in.

There is one small issue though. If you have a fairly high charge current flowing and a cell tops out, the battery looks like an open circuit to the charge controller. Many charge controllers say to never have solar panels connected without a battery connected. This can look just like the battery was disconnected. In most cases, the capacitance of the devices will slow the voltage rise when the BMS goes "open", and the Charge controller should be able to see the voltage climb and switch to constant voltage absorb or float mode. But it is still a good idea to have a surge suppressor on the DC bus so the voltage can't shoot up too high. Most larger inverters will have a lot of input capacitance, and that can save you as even a 20 amp charge rate won't make the voltage rise too fast.

Keep in mind that this is an error condition which should not happen. The BMS is NOT a charge controller. It is a safety device to protect the battery. Think of it like a fuse. It should be sized and setup to never trip unless something goes very wrong. If it doe have to disconnect, figure out why and fix the problem. in most cases it is poor cell balance. Do not depend on the BMS to keep saving you.

I highly recommend having at least 2 parallel battery strings with separate BMS units. I had my entire system on a single 200 amp BMS. One of the cell monitor wires had a bad connection and it caused it to disconnect. I was lucky it did not lead to other damage this time. With a second battery string, you will still have a battery connected to keep the system stable. I now have 3 battery strings. Each string can accept my maximum charge rate on it's own. If one shuts down for a fault, it won't hurt anything.