BMS common port vs seperate port

[Solarfun4jim]

Knowing that bottom balanced pairs of paralled cells 'self balance' , would a single 16s BMS protect all 32 cells in a 2P16S configuration? Any downsides?

 

[maxx123]

sure it would, brings up the ah too

 

[Hdrider337]

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

That's the answer I was looking for but on another thread.

 

[Buzzard Bait]

I bought the 100A Daly BMS with separate port, and the description on the actual BMS says 20A charge current, 100A discharge current.
I spoke to Daly Official Store on aliexpress, and apparently they checked with the engineer and it can charge up to 100A.

Not sure if there is a language barrier where he doesn't understand my question...

I bought the 150A Daly BMS with separate port, and the description on the actual BMS says 80A charge current, 150A discharge current.

 

[KC7NOA]

I'm working on a very small system using a Separate Port BMS. I use a relay to disconnect the PV when the BMS does the High Voltage Disconnect.
The relay can handle much bigger amps than the BMS.

Well ... I try to not use relays ... the coil does use some power, but I guess if the battery bank was large the usage might be negligible ...

 

[Stupid Monkey]

Well thanks to all the contributors.

Seems there was no point buying a low temp disconnect BMS if I'm going to have to charge by-passing the BMS anyway this might have pushed me over to doing a 24v system rather than 12 for the charge power via BMS.

 

[Svetlana]

Hi im new to lifep04 batteries and i have setup a pack with BMS with separate load and charge terminals, i am using 4S connections and then i used it to power some loads, then one set of batteries reached 2.5V , automatically the BMS cut off the supply to the load but it also cut off from the Solar Charge Controller. Most people that i talked to said that when you cut off the batteries from the Charge Controller while the solar panels are connected will surely damage the controller. How will i solve this problem? In another occasion, I setup another lifepo4 battery with BMS but this time, i followed what WILL PROWES said, I connected the charge controller directly to the batteries bypassing the BMS, but the loads are still connected to the BMS, what happens this time is that, one set of the batteries reached 4.2V, while the other 3 sets have 3.3V, 3.6V and 3.3V respectively.
NOW IM TOTALLY DEPRESSED about Lithium and BMSs, and thinking about my discarded Lead Acid Batteries. Pls. Help...

 

[Buzzard Bait]

Well thanks to all the contributors.

Seems there was no point buying a low temp disconnect BMS if I'm going to have to charge by-passing the BMS anyway this might have pushed me over to doing a 24v system rather than 12 for the charge power via BMS.

I like the idea of the low temp

Hi im new to lifep04 batteries and i have setup a pack with BMS with separate load and charge terminals, i am using 4S connections and then i used it to power some loads, then one set of batteries reached 2.5V , automatically the BMS cut off the supply to the load but it also cut off from the Solar Charge Controller. Most people that i talked to said that when you cut off the batteries from the Charge Controller while the solar panels are connected will surely damage the controller. How will i solve this problem? In another occasion, I setup another lifepo4 battery with BMS but this time, i followed what WILL PROWES said, I connected the charge controller directly to the batteries bypassing the BMS, but the loads are still connected to the BMS, what happens this time is that, one set of the batteries reached 4.2V, while the other 3 sets have 3.3V, 3.6V and 3.3V respectively.
NOW IM TOTALLY DEPRESSED about Lithium and BMSs, and thinking about my discarded Lead Acid Batteries. Pls. Help...

I have a few questions. You are new to LiFePo4 batteries and so am I. So please excuse dumb questions. Your BMS C- terminal is connected to the to the neg of the SCC, and the P- terminal is connected to the neg side of your load maybe an inverter, and B- terminal is connected to the batteries neg terminal is this how you arranged your charging system? Will may have advised that a person could bypass the BMS if the batteries had been bottom balanced first, but I believe he also advised that if the BMS was bypassed that the batteries would still need to be monitored. Have you changed your SCC charge perimeters from FLA to LiFePo4? Hang in there we are learning together.

 

[kesper]

Hi, would it be possible to use 2 common port bms, one for the loads and the other one for the charge? Lets says 200A for loads (2kw inverter) and a 60-100A for charging (60A max charger). I have a 2p4s lifepo4 setup.

 

[John Frum]

Hi, would it be possible to use 2 common port bms, one for the loads and the other one for the charge? Lets says 200A for loads (2kw inverter) and a 60-100A for charging (60A max charger). I have a 2p4s lifepo4 setup.

I wonder how the balance mechanisms would interact.

 

[kesper]

I wonder how the balance mechanisms would interact.

What do you mean?

 

[John Frum]

What do you mean?

Each BMS is comprised of protection circuitrty and a cell balancing mechanism and a balance lead per cell and a cell balancing mechanism that utilises them.
The bms won't function without voltage in range on each of those leads.
Two bms's means 2 leads per cell.
My guess is this would just double the balancing load on the "high" cell.

 

[kesper]

Each BMS is comprised of protection circuitrty and a cell balancing mechanism and a balance lead per cell and a cell balancing mechanism that utilises them.
The bms won't function without voltage in range on each of those leads.
Two bms's means 2 leads per cell.
My guess is this would just double the balancing load on the "high" cell.

Do you think 2 leads per cell can be an issue?

 

[John Frum]

Do you think 2 leads per cell can be an issue?

I don't know but I am curious.

 

[Svetlana]

I like the idea of the low temp

I have a few questions. You are new to LiFePo4 batteries and so am I. So please excuse dumb questions. Your BMS C- terminal is connected to the to the neg of the SCC, and the P- terminal is connected to the neg side of your load maybe an inverter, and B- terminal is connected to the batteries neg terminal is this how you arranged your charging system? Will may have advised that a person could bypass the BMS if the batteries had been bottom balanced first, but I believe he also advised that if the BMS was bypassed that the batteries would still need to be monitored. Have you changed your SCC charge perimeters from FLA to LiFePo4? Hang in there we are learning together.

Yes Sir, C- is connected to my SCC Negative Terminal, -P is connected to inverter and lastly B- is to the battery Negative.
Another setup that i made is according to Will's advise to bypass the BMS but i haven't bottom balanced the batteries first. huhuhu (i think thats where i made the mistake) thanks for reminding me sir. I have changed my SCC parameters to LifePo4 when I made the two setups, but i didnt bottom balanced them.

 

[Buzzard Bait]

I don't know but I am curious.

Me too.

 

[Fred]

Yes Sir, C- is connected to my SCC Negative Terminal, -P is connected to inverter and lastly B- is to the battery Negative.
Another setup that i made is according to Will's advise to bypass the BMS but i haven't bottom balanced the batteries first. huhuhu (i think thats where i made the mistake) thanks for reminding me sir. I have changed my SCC parameters to LifePo4 when I made the two setups, but i didnt bottom balanced them.

I am learning here myself but it seems like the C- could be used to disconnect ground from the solar array in order to block the solar from further charging. The charge controller would still be connected to the battery. The voltage and current from the solar array is variable so you would need to make sure the BMS could handle any extremes. I haven't tried this and may be missing something.

 

[SolarRat]

... I would say its safe to connect mppt directly to the battery bank, and bypass the BMS entirely.

You always talk about the importance of a low-temp charge cut-off in the BMS. Won't this just bypass that low-temp protection? (not that I care in Florida, just wondering)...

 

[grizzzman]

You always talk about the importance of a low-temp charge cut-off in the BMS. Won't this just bypass that low-temp protection? (not that I care in Florida, just wondering)...

If the controller does not have it. (Some do)You would be correct ?

 

[kesper]

Hi, would it be possible to use 2 common port bms, one for the loads and the other one for the charge? Lets says 200A for loads (2kw inverter) and a 60-100A for charging (60A max charger). I have a 2p4s lifepo4 setup.

Could this work?