BMS common port vs seperate port

An other question I can't find the answer for is, I have a Victron Multiplus. It is a inverter and charger in one. So a Load and a Charger. Then I use solar when on the road. The question is, do I use a Common or Seperate port BMS? And where do I mount the Victron Multiplus on?

tucarb said:

I know but the 8th module could give me another 400+Wh

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7s for 3.6 or 3.7 cells...8 for lithium iron phosphte

Echo, On the split port, does current back feed on the discharge side?

Svetlana said:

... 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....

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I do not at all understand that the SCC can be damaged by an open circuit condition on the battery. This can be caused by many many things (i.e., a blown fuse, a BMS cutting of the battery, etc.). This makes no sense whatsoever. If the SCC can be damaged by lack of a battery connection this is simply poor (VERY POOR) engineering. I know many switching power supplies (which a MPP SCC is) require a minimum load to operate PROPERLY, but they don't get damaged, they simply output a perhaps not very controlled voltage. So this could in theory damage other components down the line (i.e., if the SCC outputs too high of a voltage it could damage a 12 V radio). So yes, in principle there could be a problem but not with the SCC. I guess that having a separate port is safer then.

Disconnecting the SCC from the battery under moderate to heavy load i.e. like pulling a plug may cause some arcing and possible damage to the SCC but normally it is advised to connect the SCC to the battery first and then PV to SCC. This just helps the SCC to determine the correct battery voltage as most SCC are multi voltage units. If PV is connected first before the battery it may take a few seconds before the battery starts charging.

at least for my mpp solar unit, it wont start up without a battery.

I was told by Redarc manufacturer that they tell their clients to jumper a external 12v source to the battery terminal to restart their solar controller.
Their new combination dc to dc / mppt controller will repower it self from a solar or vehicle 12v power source without 12v from battery.
I have a question that i can't find a clear answer so far on the forum.
With a common load/charge connection BMS in low voltage disconnect, will the BMS allow the battery to charge.
And will the BMS allow the battery to discharge when high voltage disconnect is activated.
I'm referring to most BMS's and particularly to the
Overkill Solar 12v 4S 120A BMS
That will Prowse has on his web site.
And this one
DYKB smart BMS 4S 12V 60A 80A 100A 120A Li-ion LifePo4 Lithium Protection Board balance High Current Bluetooth APP software GPRS
you can get on Aliexpress they look the same.
For us in Australia, freight it way too expensive from USA so have to source from CHY NA.

grey nomad said:

I was told by Redarc manufacturer that they tell their clients to jumper a external 12v source to the battery terminal to restart their solar controller.
Their new combination dc to dc / mppt controller will repower it self from a solar or vehicle 12v power source without 12v from battery.
I have a question that i can't find a clear answer so far on the forum.
With a common load/charge connection BMS in low voltage disconnect, will the BMS allow the battery to charge.
And will the BMS allow the battery to discharge when high voltage disconnect is activated.
I'm referring to most BMS's and particularly to the
Overkill Solar 12v 4S 120A BMS
That will Prowse has on his web site.
And this one
DYKB smart BMS 4S 12V 60A 80A 100A 120A Li-ion LifePo4 Lithium Protection Board balance High Current Bluetooth APP software GPRS
you can get on Aliexpress they look the same.
For us in Australia, freight it way too expensive from USA so have to source from CHY NA.

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My experience with the JBD BMS ( same as Overkill but sourced from CH ) :
The unit has 2x FET control indications displayed , so answer to both questions is ‘yes’ . If any of the FET’s indicate ‘red’ , that function will not be allowed and conversely if any of the FET’s indicate ‘green’ that function will be allowed.
I am a firm believer in the JBD BMS and for the price and functionality it makes a great buy.
I reviewed this unit some time ago in the BMS Section.
https://diysolarforum.com/threads/dgjbd-bluetooth-bms-a-short-review.5290/

My BMS (Ji Kong JK BMS) is a common port, but it has separate control for charge and discharge protection. When it goes into under volt protect mode, it will not pass current out from the battery to the load, but if there is charge current coming in, it will still allow the current into the batteries to raise the voltage. And on the flip side, if the voltage goes too high and hits over volt protect, it will go open and not accept any more charge current, but if will still supply current out to the load. I have not tested the over current protection, but I would assume it works the same way. Testing the over and under volt was easy, I just adjusted the trigger voltages to make it switch off and on in each case. I was concerned it would be a problem switching off to stop an under volt, and then not being able to charge, but they thought of that and it works great.

I am setting my inverter/charger voltage limits to always stay inside of the BMS limits, so it never should go into protect, but the BMS will always be there as an extra safety factor.

Will Prowse said:

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!

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I just got a 4s seperate port was rated at 20 amps for charge and 100amp for discharge
I think you have to ask them to send a 20amp version though (At least thats why I did)

Another member mentioned that Daly can make a separate port charge input up to 50A. Nimi the sales rep from Dongguan Daly Electronics has comfirmed this on Alibaba

Ok, I am wondering why you couldn't put in a low voltage cutoff in the load line? If low voltage would be the primary reason in a warm area that the the BMS would cut off. Use a programmable board in conjunction with a 100 amp relay. As long as the voltage is high enough the current flows, but if the drops then the relay cuts off the flow. With the only draw being the MPPT controller. When voltage rises above the predetermined level then the current flows once again.

Likewise, a relay could be used to switch the MPPT over to a very small lead acid battery when the BMS switches off. This would keep the MPPT connected to a battery and preserve the MPPT.

rippysig said:

Ok, I am wondering why you couldn't put in a low voltage cutoff in the load line?

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A low voltage cutoff would need cell level access to voltages to be effective.
Imagine if the battery voltage was 11v and it did not cutoff when the cells are 3v, 3v, 3v, 2v?

Not good.

If I have a seperate port 150Amp Daly BMS, can I just not use the the center port and use it like it's smaller brother?

There is no lead hooked on to the center post.

Also, tried to hook it up with 2 120Ah cells in series and 4 sets in parallel for 240Ah 12V, but the BMS won't activate. Tried the normal "reset" jumper.

Lithium Charger is 50 amps. Ideas?

Will, it would be great if you would do a short video on the separate port...I know I'm lost.

jpc333 said:

can I just not use the the center port and use it like it's smaller brother?

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Sure, why not?
I am not sure whether the "center port" is the charge port or load port but surely there will be times (many!) that they are not being used (not charging and/or no load) so it makes no difference whether its physically connected or not.

Depending on how the MOS FET's are laid out, it is possible the load port might not like current going backwards during charging. And the charge port probably won't like load current going out. Without a schematic, it would be hard to know for sure what effect it would have. But my fear is it would go pop like charging through a Victron battery protect. Though on an E-Bike, I know they can push charge power back from the motor with dynamic braking, So.... it might work, but.... I wouldn't try much current in the wrong direction.

jpc333 said:

If I have a seperate port 150Amp Daly BMS, can I just not use the the center port and use it like it's smaller brother?

There is no lead hooked on to the center post.

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Can you clarify what you mean here.

The 'center port' is the P- (loads)?
And when you say "use it like its smaller brother" are you referring to using it as a common port BMS running everything through C- (charging)?

GXMnow said:

out, it is possible the load port might not like current going backwards during charging.

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Yes, i suspect you are right. I did not anticipate one would charge and discharge on the same port on a split port BMS.
Good catch.

I think you have to use one lead for charging and the other for discharging if I remember right

P- is discharge
C- is charge
B- connects to the battery btw

Most on here are referring to a 24 volt system. So I am assuming you are using a inverter for most of the load. It looks like you could just set the low volt cut out on it to disconnect before the bms sees the lowest cell. In my RV I have the inverter LVCO at 11.5 volt I have tested and not seen bms on any of the 12v 200ah. I don’t want to get down into the discharge curve very far away. Most Mppts recommendation on PV volts. 12 volt system is around 75 volts. 24 volt is around 100 volts and 48 system is 150 volts. This is for conversion efficiency. So on a 24 volt system or lower you are not going to see a spike of 50 plus volts from battery disconnect. If you were on 48 volt system and pv strings at the full 150v. I would be more worried about spikes in volts from the sun coming out from behind a dark cloud. I have had my 48 volt system. Alarm on pv over volt and drop pv for a short time. Schneider Mppt 60-150. I am fairly new with lithium. My home system is rolls surrette 880 AH 48 volt. My RV I have 2 200AH 12 volt parallel.

When everything is working correctly, the BMS is just along for the ride. If the cell balance goes out, it will try to pull it back in. IF the cells are well matched, it does not have much to do. A few milliamps for a few hours will keep god batteries in balance. The Charger will stop charging before the batteries get too high, and the inverter will shut down before they get too low, so the voltage protection in the BMS should never be needed.

BUT>>>

There is always a but.. If the cells do drift out of balance for any reason, and the BMS balancing is not able to keep them in range, you could run into a cell going to high during charge or too low during discharge. Depending on how far the inverter or charger is trying to use the pack, an imbalance could be enough for the rouge cell to go far enough out to hit the protection threshold in the BMS, even if the whole pack is still in the safe voltage range. Without monitoring each cell, the charger and inverter do not have any way of knowing if one cell is out of range. The better the batteries are matched, the less the BMS needs to do, but due to how lithium batteries work, a BMS is always needed to make the system safe. Even perfectly matched cells could age differently and become unbalanced as they age.

The question becomes what happens if the BMS has to disconnect. If it does, it means something is wrong. It should never need to happen. So the main thing is we don't want it to damage anything else. Disconnecting the battery from a solar charge controller is not a good thing. Ideally, it should see the voltage jump up and go into constant voltage mode. This would be a good question to ask and should be in the manuals since more and more people are turning to Lithium batteries. If a charge controller can't withstand the battery being pulled due to a BMS cutoff, then I would say that charge controller is not compatible with lithium batteries.

On the discharge side, most inverters will just shut off when the battery is disconnected. It is certainly no worse than a fuse blowing. And that can happen easily from an overload. If an inverter can't survive it, there is a bigger issue.

Unlike a fuse blowing due to an overload, a BMS shut down could have a few different reasons. If a cell keeps going out of balance, it is telling you to repair the battery pack. If it is a current shut off, it is being overloaded, and you need to adjust things to prevent that. And if it is a temperature issue, you need to correct the cause. If it is working correctly, it never should shut off.

Since my system is based on an inverter/charger unit, I had to use a common port BMS. But it is nice to know it can stop charge and still allow discharge, and stop discharge and still allow charge. If a battery is run to low, we certainly want to be able to charge it again.

If a system has separate charge controllers and inverters, then it makes sense to use the separate port design. but common port will still work. It would be interesting to look at the diagrams and see how they layout the mosfets to see if you could easily convert between a split and common port system.

interesting read.

A separate port seems to be preferable if it handle decent charge rates. Can anyone confirm a working 50A charge current ( as Will's OP is already a year old) with a daly bms?

After reading the first two pages of this thread again, i have a question: Because the charge rate of the separate (charge) port is too low, it is advised to get the common port bms, and connect the SCC directly to the battery (bypassing the bms).

But how do you protect the battery from over voltage in that case? Does the SCC need to take care of that? It cant, because it does not know the voltage of the individual cells...So how is the battery protected against over voltage?

No, you should not bypass the BMS without some form of protection. Will did do a trick where he used a lower current BMS, but had it's output lead drive a contactor that connected the inverter to the system, but this would also work for the charge controller. The whole point of a BMS is to protect all of the cells from over voltage, under voltage, over current, and temperature extremes. When using the relay bypass trick, it does eliminate the current protection, but you can (and should) still have a fuse or breaker to limit the maximum current incase something goes wrong. The rest of the protection systems still work because when the BMS goes open, it drops the contactor (relay) and shuts off the load or charger. If you were going to do the relay trick, I would recommend a separate port BMS and 2 relays. One for the charger, and one for the load. This way if it runs low, it can still charge, and if it goes too high, the loads can still pull it down.

When using a common port BMS, the inverter, charger, and any other loads, should all connect on the output lead of the BMS. Mine is a common port that allowed me to set different maximum current for charge and discharge direction. And even though it is on the same physical wire, it can block charge when the voltage is high, or block load when the voltage is low, but still allow the other direction so a low batt can be charged etc.

The SCC should be set for the whole pack voltage. Set it a couple tenths of a volt below the desired max cell voltage times the number of cells. In my case, I have MNC cells that are safe to 4.2 volts. I have 14 in series for a max voltage of 58.8 volts, but I have my absorption voltage set to 57.6 volts, which works out to 4.117 volts per cell. Add in a little wire voltage drop, the BMS drop, and some calibration error, and my BMS reports right at 4.1 volts per cell when the charger stops. The BMS will not disconnect until 4.2 volts. on a cell. On the low side, I am even more conservative. I have the inverter shutting off at 44 volts, which is still 3.2143 volts per cell. The BMS is set to 3.0 volts per cell, or just 42 volts for the pack.

boatmonkey said:

After reading the first two pages of this thread again, i have a question: Because the charge rate of the separate (charge) port is too low, it is advised to get the common port bms, and connect the SCC directly to the battery (bypassing the bms).

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I would argue that this is not 'what is advised' (I know in some of his earlier vidoes, Will suggested bypassing the BMS, but beyond that I haven't come across that advise, and I'm not sure he still stands by that advise for beginners or people buying grey market cells--in fact mostly I've come across exactly the opposite advice).

The reason for this is because your concerns below are valid:

boatmonkey said:

But how do you protect the battery from over voltage in that case? Does the SCC need to take care of that? It cant, because it does not know the voltage of the individual cells...So how is the battery protected against over voltage?

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People that suggest it is acceptable to bypass the BMS for charging to save money because they argue risk of overcharging is minimal. Setting aside whether that is true or not, it seems like the wrong solution to a problem that can be solved in safer ways. There are many BMS on the market that can handle higher charge rates, some are FET based many use relays or other measures. Bypassing the BMS pokes holes in your most essential layer of protection. That is my 2c