BMS common port vs seperate port

[tom.b]

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

So i am a newby trying to figure all this out to design a lifepo4 battery for a sailboat motor.

My understanding with a common port BMS is that despite sharing a connection, their is a seperate FET for the charge and a separate FET for the load and depending on the direction of the current depends on which one opens. Otherwise if there was only one FET, a discharged battery could never be charged as the BMS would disconnect both when below voltage, correct? I imagine that if this is the case that this switching may not be fast enough to avoid damage to the MPPT. But, for a battery to run low it has to be connect to a load, and MPPT being connected to same load, would not the load absorb the voltage spike? seems a disconnect would also occur when the load is connected and the FETs go from charging to load. but again, would not the load be absorbing the voltage from the MPPT?

Also, would a small lead acid battery connected in parallel to the lifepo4/bms be able to absorb any spike from disconnect.

I am finding myself quite reluctant to connect my MPPT directly to the battery.

You'll have to excuse me if i dont' understand something, like I said I am a newby and there is a lot to learn.

 

[gpower07]

I am currently have 2 sets of battery with different BMS. One with com port, other one is separate port. I want to know if I can use them together or not.

 

[MisterSandals]

I want to know if I can use them together or not.

What do you mean by "use them together"? Connect them in series? Parallel?
Are the batteries identical or similar other than the BMSs?

 

[gpower07]

What do you mean by "use them together"? Connect them in series? Parallel?
Are the batteries identical or similar other than the BMSs?

Parallel. All battery same size. I have 16. And want to run 2 group of 8.

 

[HRTKD]

Parallel. All battery same size. I have 16. And want to run 2 group of 8.

That should work. Which BMS are you using?

 

[GXMnow]

So i am a newby trying to figure all this out to design a lifepo4 battery for a sailboat motor.

My understanding with a common port BMS is that despite sharing a connection, their is a seperate FET for the charge and a separate FET for the load and depending on the direction of the current depends on which one opens. Otherwise if there was only one FET, a discharged battery could never be charged as the BMS would disconnect both when below voltage, correct? I imagine that if this is the case that this switching may not be fast enough to avoid damage to the MPPT. But, for a battery to run low it has to be connect to a load, and MPPT being connected to same load, would not the load absorb the voltage spike? seems a disconnect would also occur when the load is connected and the FETs go from charging to load. but again, would not the load be absorbing the voltage from the MPPT?

Also, would a small lead acid battery connected in parallel to the lifepo4/bms be able to absorb any spike from disconnect.

I am finding myself quite reluctant to connect my MPPT directly to the battery.

You'll have to excuse me if i dont' understand something, like I said I am a newby and there is a lot to learn.

You are correct. There are some BMS units that will just disconnect the battery, and you have to manually trigger it back up to resume charging or discharging, but many can act like a diode and allow one or the other. But keep in mind, this BMS disconnect will only happen when something has gone wrong. If your system is properly setup, the MPPT charge controller should stop charging the battery before the BMS sees a cell going too high, and your loads, should all disconnect before the BMS sees a cell go too low. So let's say you are cruising using electric power and have weak sun on the solar panels. The electrical loads should shut down with some battery energy left. If you have several loads, you can set the big currents to turn off sooner to "load shed" and allow you to still have lighting etc. As long as the cells are fairly well balanced, then the BMS will be fine and see your pack is down to 20% charge, but nothing is wrong.

Now you are anchored, not using much power, and the sun is shining bright. The battery bank tops up and reaches a solid full charge. The MPPT charge controller, goes into float charge, or even shuts off completely when the battery bank is completely full. The BMS protection voltage should still be a little higher than what the MPPT is set for. So the charge current drops, but again, the BMS sees all is fine at 98% charge or whatever. Once again, as long as the cells are balanced, you won't have a shut down. If your battery bank is out of balance, and one cell tops out too soon, that can trigger the BMS into a shut down. This is a problem, as any more charge could damage that overcharged cell. The BMS is protecting the battery bank as it should. At this point, you need to blead off some energy from that cell, and then reset the system. Some BMS's will reconnect once the voltage drops, some need to be triggered. But your MPPT really should be connected through it, because it is there to save the battery bank from being damaged.

It does sound like you have a pretty good understanding, and I am just ensuring you, that your thinking is correct. As for the load surges and spikes, you should be fine. Although there are some MPPT controllers that warn about a battery disconnect, in any real world setup, it should not be a problem with a well made unit. And yes, your load will help hold down any spikes. Any good charge controller should see the voltage climb up as the load is removed, and reduce it's current output to keep the voltage below the max absorb voltage setting. If the MPPT can't do that safely, then I would not use it in my system. You do need to ensure that the worst case no load VOC cold voltage from your solar array is safely within the rating of the MPPT charge controller. When the system has to greatly reduce it's current draw on the solar panels, the voltage will climb to the VOC rating. And when it is cold, it can go up quite a bit more. This would be the most likely reason for a failure in the MPPT controller.

 

[Horsefly]

This is very reassuring, and makes sense of some of the things I've been seeing on-line. The overkill BMS has a separate low-temp discharge cut-off, and low-temp charge cut-off. That only makes sense if series MOSFETs on both the charge and discharge directions, separately controlled by the BMS. So the BMS can cut off charging while still allowing discharging.

 

[tom.b]

This is very reassuring, and makes sense of some of the things I've been seeing on-line. The overkill BMS has a separate low-temp discharge cut-off, and low-temp charge cut-off. That only makes sense if series MOSFETs on both the charge and discharge directions, separately controlled by the BMS. So the BMS can cut off charging while still allowing discharging

You are correct. There are some BMS units that will just disconnect the battery, and you have to manually trigger it back up to resume charging or discharging, but many can act like a diode and allow one or the other. But keep in mind, this BMS disconnect will only happen when something has gone wrong. If your system is properly setup, the MPPT charge controller should stop charging the battery before the BMS sees a cell going too high, and your loads, should all disconnect before the BMS sees a cell go too low. So let's say you are cruising using electric power and have weak sun on the solar panels. The electrical loads should shut down with some battery energy left. If you have several loads, you can set the big currents to turn off sooner to "load shed" and allow you to still have lighting etc. As long as the cells are fairly well balanced, then the BMS will be fine and see your pack is down to 20% charge, but nothing is wrong.

Now you are anchored, not using much power, and the sun is shining bright. The battery bank tops up and reaches a solid full charge. The MPPT charge controller, goes into float charge, or even shuts off completely when the battery bank is completely full. The BMS protection voltage should still be a little higher than what the MPPT is set for. So the charge current drops, but again, the BMS sees all is fine at 98% charge or whatever. Once again, as long as the cells are balanced, you won't have a shut down. If your battery bank is out of balance, and one cell tops out too soon, that can trigger the BMS into a shut down. This is a problem, as any more charge could damage that overcharged cell. The BMS is protecting the battery bank as it should. At this point, you need to blead off some energy from that cell, and then reset the system. Some BMS's will reconnect once the voltage drops, some need to be triggered. But your MPPT really should be connected through it, because it is there to save the battery bank from being damaged.

It does sound like you have a pretty good understanding, and I am just ensuring you, that your thinking is correct. As for the load surges and spikes, you should be fine. Although there are some MPPT controllers that warn about a battery disconnect, in any real world setup, it should not be a problem with a well made unit. And yes, your load will help hold down any spikes. Any good charge controller should see the voltage climb up as the load is removed, and reduce it's current output to keep the voltage below the max absorb voltage setting. If the MPPT can't do that safely, then I would not use it in my system. You do need to ensure that the worst case no load VOC cold voltage from your solar array is safely within the rating of the MPPT charge controller. When the system has to greatly reduce it's current draw on the solar panels, the voltage will climb to the VOC rating. And when it is cold, it can go up quite a bit more. This would be the most likely reason for a failure in the MPPT controller.

Hey thanks for the response Gary. I just looked up the manual of the controller I am interested in (Genasun GVB-8-Li-56.8V-WP) and indeed they give the same warning about the VOC voltage in cold temperatures. Thanks for the heads up. I'll read you response a couple more times to soak it all in.
Also in the manual they mention: "LITHIUM BMS WARNING: Genasun recommends using a lithium battery with a Battery Management System capable of disconnecting the solar charge controller in the event that any cell in the pack is outside of its rated temperature, current, or voltage range. Failure to do so may result in property damage, injury or death."
which of course makes me think that the sudden disconnect from BMS is not an issue for this device.

 

[Baserati]

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.

Does this mean you can connect a relay to any BMS? Logic tells me you should be able to. I would like to use a relay or two with my Daly BMS

 

[HRTKD]

which of course makes me think that the sudden disconnect from BMS is not an issue for this device.

I would not assume that. Genasun may be thinking that your BMS will turn off charging, while discharge is still enabled. Make sure that YOUR BMS really does work that way.

 

[gpower07]

That should work. Which BMS are you using?

Daly. They told me not to parallel them.

 

[Campin]

Solar Charge Controller

...had to Google That one Lol

 

[MezzBatt]

This will not be an issue for a Victron controller, others I am not sure about.

This is just one of many points and threads I am currently following trying to get an overview of what and how I need to build my system. So I have an older VE Blue Solar (metal cased) SCC and I need not worry about VOC disconnect issues if using this MPPT...??

 

[who dat]

Upgrade ... "only when charger is on" version (and corrected NC to NO)

View attachment 5007

Yes, your concept is sound. But if I may offer some suggestions, take the power for terminal 86 directly from the battery and move your relay to the input side of either the charger or controller from the output side.

 

[GXMnow]

I would never recommend charging without BMS protection. Even at 13.8 volts, strictly regulated (Which alternators are not) that would be 3.45 volts per cell, which is fine, but when the balance get's off at all, an LFP cell can run away from the others when they get near full charge. Let's say the one cell hits a bad 3.8 volts, the other 3 would be at 3.333 volts which is still in the flat plateau. Your alternator has no way of knowing that the one cell went rouge. The BMS balancing leads would still detect this, and it would shut down it's charge input port, but with what you propose, it would not be able to stop the alternator from charging it further into destruction.

I have a though for you. How about using a contactor that would use the BMS charge lead and a switched ignition to connect the alternator output to the battery? That way, the BMS mosfets only have to take the contactor coil power, maybe an amp tops. Will did it on the discharge side to use a large inverter with a small BMS. But I think this could also be made to work with a separate port BMS having the charge side operate a contactor.

Does anyone have a separate port BMS to try this with?

 

[GXMnow]

What BMS are you using? I would have to think about this one a bit. Might be able to share some current through the BMS, but it could be tricky to make it work. I'll try to find a data sheet on that Cyrix contactor also.

 

[Tex]

The big problem with separate port BMS is the current is extremely limited. Not uncommon for a 100a separate port bms to have a charge line that can only handle 8-15 amps. If you are building a system for solar, use a common port for your loads, and connect the SCC directly to the battery.

Not true, they don't really make it clear but you have to order what you want, they actually do a lot of 250/200 units nowadays. I have a 200/50.
The big problem with Daly is the lack of any sort of monitoring, you really have no idea if it is working or not most of the time.
The other problem is QA , it seems to be non existent, I had one unit DOA which was replaced with some difficulty, both the original and the replacement had the balance plug wired in a completely different manner, and the one charging port was incorrectly labelled!
Add to that the fact that their wiring instructions are completely incorrect and you have a recipe for disaster.
I corrected and rewrote their instructions for them, for which they didn't even thank me), but I believe they are still sending out the incorrect version BEWARE!

 

[iquibalamenpau]

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

Hello,
Thanks for your time, your information is acurate and practical !. I have a question, because you said that would be better to bypass the BMS to use the mppt. But in the Instrucion manual clearly says you shouldn't. What should i do ?

 

[timmeh]

So i am a newby trying to figure all this out to design a lifepo4 battery for a sailboat motor.

My understanding with a common port BMS is that despite sharing a connection, their is a seperate FET for the charge and a separate FET for the load and depending on the direction of the current depends on which one opens. Otherwise if there was only one FET, a discharged battery could never be charged as the BMS would disconnect both when below voltage, correct? I imagine that if this is the case that this switching may not be fast enough to avoid damage to the MPPT. But, for a battery to run low it has to be connect to a load, and MPPT being connected to same load, would not the load absorb the voltage spike? seems a disconnect would also occur when the load is connected and the FETs go from charging to load. but again, would not the load be absorbing the voltage from the MPPT?

Also, would a small lead acid battery connected in parallel to the lifepo4/bms be able to absorb any spike from disconnect.

I am finding myself quite reluctant to connect my MPPT directly to the battery.

You'll have to excuse me if i dont' understand something, like I said I am a newby and there is a lot to learn.

In testing my new smart 400a bms common port i can confirm if i switch off the charge mosfet in the bluetooth app i can still discharge but not charge. same goes the other way around if i switch off the discharge mosfet i can still charge. crazy cool.

 

[Daddy Tanuki]

In testing my new smart 400a bms common port i can confirm if i switch off the charge mosfet in the bluetooth app i can still discharge but not charge. same goes the other way around if i switch off the discharge mosfet i can still charge. crazy cool.

Timmeh... you would not happen to own a mazda rotary would you???