diy solar

diy solar

BMS common port vs seperate port

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.
So yeah I’m concerned about this. Also it just seems better to not have the chargecontroller charging when the battery power to it is shutoff. That’s why I’m using separate port (one relay on the pv wire into charge controller) but I keep reading that I should be using common port instead.. what happens if the chargery bms disconnects the common port due to over discharge. How do I charge the batteries back up with both charge and discharge cutoff? And if the battery gets overcharged and common port shuts off then how would I turn the inverters on to discharge the battery?

If this is discussed somewhere else please direct me to it. I’ve been searching
 
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If you are using actual relay contactors, you can connect a high current diode across the relays. The charge disconnect relay would have a diode that still allows current in the discharge direction, and the discharge disconnect relay would have the diode to allow current in the charge direction. Then the two relays are in series. The one big advantage to a common port setup is when there is a lot of sun on the panels, AND the inverter is drawing a lot of power running loads. The bulk of the power is going straight from the charge controller to the inverter, and not even going through the BMS disconnect system. With separate ports, all of the charge controller current is always going through the charge port of the BMS, and all of the inverter current is coming back out of the discharge load port of the BMS. All of the mosfets (or contactors) are carrying all of the current all of the time.
 
If you are using actual relay contactors, you can connect a high current diode across the relays. The charge disconnect relay would have a diode that still allows current in the discharge direction, and the discharge disconnect relay would have the diode to allow current in the charge direction. Then the two relays are in series. The one big advantage to a common port setup is when there is a lot of sun on the panels, AND the inverter is drawing a lot of power running loads. The bulk of the power is going straight from the charge controller to the inverter, and not even going through the BMS disconnect system. With separate ports, all of the charge controller current is always going through the charge port of the BMS, and all of the inverter current is coming back out of the discharge load port of the BMS. All of the mosfets (or contactors) are carrying all of the current all of the time.
Are you saying to use common port with 2 relay/contactors? I thought with common port only one relay is used
 
Let's be completely honest and real here. The BMS is for protection when something goes wrong. If the system is reliable and working properly, there is no reason the BMS protection contactor(s) will ever open. That only happens when a cell's voltage goes too high or low, or the temperature goes out of range. If you system is properly set up with well matched and balanced cells, then a single contactor will be just fine. If some freak thing happens and causes a shut down, then you need to figure out what went wrong and manually reset the system.

On the other hand, if you are using bulk cells that are not well matched, the balance can drift quite a bit. And if you are pushing the charge limits to get the most you can out of them, there are good chances that the BMS may have to go into a protect mode from time to time. In this case, having the two separate contactors can really save you a lot of hassles. Let's say you are trying to charge your cells to 95%, but one cell is a little low on capacity, and it's state of charge creeps up compared to the other cells. The rest of the bank is only at 85%, but that weak cell starts to run away. When that rogue cell hits 3.65 volts, the protection contactor opens. With a single contactor, the system is now dead. It can only reset with manual intervention. The cell might droop down enough to turn back on, but if the charge controller is still putting out, it will just trip out again. With the two contactors and the diode, the relay that can prevent charging will open, but the diode will allow the inverter to pull power from the battery bank and this will pull the rogue cell down and get the system back to a safe state much quicker. You can set the charge recovery voltage a little lower so it will stay off a bit longer and hopefully not need to trip out again, but even if it does, all of the loads ae still staying powered through the second contactor and the diode. You have no interruption to your power, and the solar is actually still powering your loads straight to the inverter. A decent MPPT charge controller will see the voltage climb up and switch to Absorb and/or float mode to regulate the voltage and keep running the inverter and use very little to no battery power. With separate ports, the charge controller would be fully disconnected until the battery comes down.

On the other end, the opposite happens. Let's say it is the middle of the night, and someone left an extra fan running and you had weak sun today, whatever the reason, a cell in the battery runs too low. Again, with a single contactor, you are done. You have to manually reset the system again. With the separate contactors, the charge controller stays connected through a diode and can start charging as soon as the sun comes up. Put too much load on the inverter though, and it may cause a shut down if the solar is not making enough power yet. The separate port does have a little advantage here as it will not power up the inverter until the battery recovers a bit. You could wire the discharge cut contactor after the charge controller to gain this function. That is something a true common port mosfet BMS can't do.
 
GXMnow, after reading your post I got to thinking, if using separate port (like I do now from a chargery bms) it will hide a potentially serious battery problem. I agree the HVD LVD should next to never happen if cells are well balanced. So if the bms does make a relay open then I would want to know about it. With separate port if HVD was reached the bms would open the charge relay but still allow discharge which would bring the battery voltage back down therefor hiding the problem. Opposite would happen if LVD was reached and problem would still be hid. Some bms send alerts when a relay is opened. The chargery however does not. You would have to check the lcd unit to see if there was a problem. Having separate port could continually hide a problem that should have been fixed immediately.
By intervening manually when a relay opens I suppose you are saying to adjust the bms setting to allow discharge or charge. That seems like a better way to do this. To use common port and one relay. That way if a problem occurs I would know about it because the system will be shut down till I fix the battery problem.

I can only see one problem with that. Shutting down the chargecontroller by opening the relay when there’s a LVD or HVD event. Still thinking it’s not a good idea to shut off the chargecontroller by disconnecting the battery when the chargecontroller is charging. It would be better for the chargecontroller if the bms shutoff charge by disconnecting the solar. Then that requires 2 relays because that charge relay wouldn’t disconnect the discharge. Although since like you said an HVD or LVD event should not (rarely) happen with well balanced good condition cells. So maybe I shouldn’t be concerned about shutting off the chargecontroller via a common port single relay after the chargecontroller since the opening of the relay might never happen. Although if an HVD or LVD needed to happen I’d hate for my chargecontroller to get damaged from being shutoff via relay after the chargecontroller while it was charging. Thoughts on how likely that would be to damage the chargecontroller? I know it’s not likely for HVD or LVD to happen anyways with my battery.

So from all this I’ve decided that if an issue with my battery/cells was occurring I would want the entire system to shutdown till I noticed there was an issue. Separate port would continue to cover up a serious problem. So common port it is. Now I just have to figure out if I’m ok with shutting off the chargecontroller with the common port single relay that basically just kills the battery to the entire system.
 
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So the chargery port has 4 wires. 2 for discharge and 2 for charge. I assume that each set of 2 can open one relay if LVD happens and the other relay if HVD happens therefor being separate port? Is this a correct assumption? For me to use the chargery as a common port all four wires would go to one relay correct? That way the battery gets shut off to the entire system if HVD or LVD happens?
 
GXMnow, after reading your post I got to thinking, if using separate port (like I do now from a chargery bms) it will hide a potentially serious battery problem. I agree the HVD LVD should next to never happen if cells are well balanced. So if the bms does make a relay open then I would want to know about it. With separate port if HVD was reached the bms would open the charge relay but still allow discharge which would bring the battery voltage back down therefor hiding the problem. Opposite would happen if LVD was reached and problem would still be hid.

The same thing happens with my common port BMS, which sort of acts like a separate port. HVD will get reset when the voltage comes down, either due to discharge or to the natural voltage loss.
 
I do not have a Chargery, but that does sound right. I know their own DCC solid state contactors have a 4 pin input when used as a common port. There may be setup options on how to use one or two contactors.

As for the system recovering on it's own... You have to decide what is most important. Most people would want it to recover on it's own, but if you want it to lock off and not recover without help, there are a few options. A single contactor is a pretty sure want to have it shut down until you manually reset the system.
 
I do not have a Chargery, but that does sound right. I know their own DCC solid state contactors have a 4 pin input when used as a common port. There may be setup options on how to use one or two contactors.

As for the system recovering on it's own... You have to decide what is most important. Most people would want it to recover on it's own, but if you want it to lock off and not recover without help, there are a few options. A single contactor is a pretty sure want to have it shut down until you manually reset the system.
Yes I do want the system to shutdown till I see what caused the LVD or HVD to open. With my batteries if the bms triggered LVD or HVD it would have to be something serious. I would want it fixed, not continuously covered up. I’d think that anyone with good cells that are not being charged/discharged with high amps, would want the same. So one contactor/common port seems to be what I’d want.

But if I use one contactor, if one cell starts going bad or drift (might not ever happen) and HVD/charge relay is opened by the bms, it would instantly shutoff the chargecontroller while it’s charging. What’s the possibility of that damaging the chargecontroller. If there is a better chance of damaging the chargecontroller when using common port/1 relay vs 2 relays/separate port with one before charge controller on solar, then idk what to choose.

Seems like I have to choose between putting my chargecontroller at risk of damage (1 contactor/common port), or potentially continuously covering up a serious problem (2 relays/separate port). If the chargery would send alerts to my phone then Separate port/2 relays seems better because I would know about a problem when it happens and still could easily bring the battery out of HVD or LVD and no chance of damaging the chargecontroller due to having 1 of the 2 contactors on the solar.
 
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does the outback have an input for a temp sensor? what happens when you purposely pull the temp sensor? The TriStar that I use if you pull the temp sensor it stops charging, so a simple relay that opens the circuit of the temp sensor will immediately stop the SCC from charging. (this is an approved method from morningstar.) I bet if you talk with outback support they will say theirs is the same. that or disconnecting the battery sense (if outback uses one).
 
does the outback have an input for a temp sensor? what happens when you purposely pull the temp sensor? The TriStar that I use if you pull the temp sensor it stops charging, so a simple relay that opens the circuit of the temp sensor will immediately stop the SCC from charging. (this is an approved method from morningstar.) I bet if you talk with outback support they will say theirs is the same. that or disconnecting the battery sense (if outback uses one).
I currently use separate port / 2 relays. You are saying to have a 2nd relay and use separate port like I already am doing. Your saying to instead put a charge relay on the temp sensor or battery sense wires off the chargecontroller. Right now I have the charge relay on solar coming into the chargecontroller. It is fine to shutoff the chargecontroller the way I’m already doing it without damage to the chargecontroller. Only problem is that with separate port it hides the problem with the battery/cell as I explained above. Using a smaller relay on the temp/battery sense wires doesn’t help that problem.
 
IF the charge controller is able to shut down charging when the battery bank is too cold, that is better than the BMS doing it.
 
IF the charge controller is able to shut down charging when the battery bank is too cold, that is better than the BMS doing it.
Agreed that would be better. Here’s a pic of what my chargecontroller has. It’s called RTS. I don’t know how that could shutoff the chargecontroller.
 

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wouldn’t that still require separate port and 2 relays. That would still allow an HVD or LVD event to be hidden.
 
From what I've read, temperature compensation for LiFePO4 and lead acid work the opposite way for cold. The colder it is for a lead acid battery, the more volts are needed. A LiFePO4 battery should get fewer volts (really amps) the lower the temperature approaches 32° F.
 
For LFP you should disable the temperature compensation. The RTS (remote temperature sensor) is used by the charge controller to raise the voltage in colder temps, as @HRTKD said. The compensation value (volts per °C above 25°) should be set to 0mV for LFP. The Outback charge controllers do have an AUX function that can turn a relay on or off, but as far as I can tell temperature is not a trigger you can choose for the AUX function.
 
For LFP you should disable the temperature compensation. The RTS (remote temperature sensor) is used by the charge controller to raise the voltage in colder temps, as @HRTKD said. The compensation value (volts per °C above 25°) should be set to 0mV for LFP. The Outback charge controllers do have an AUX function that can turn a relay on or off, but as far as I can tell temperature is not a trigger you can choose for the AUX function.
I use the outbacks aux float function to turn on my hot water heater when batteries hit float so I can’t use it for anything else. I think ken was saying to put a relay on the temp wires somehow. I don’t see how that could work with the outback.
 
" I think ken was saying to put a relay on the temp wires somehow. I don’t see how that could work with the outback."
That's exactly what I meant, but as I said consult with outback tech first. this works for the Morningstar controllers and it kind of makes sense to work on other top tier units. why? because if your temp sensor goes out, the SCC does not know if it should attempt to adjust the voltage. with morningstar, you set the temperature compensation to zero, and then put a relay in the temp sensor wiring.

you can now use any trigger you can build a circuit for. under temp, use one of those little temp controllers. under/over voltage use one of the voltage sensing relays that opens or closes a circuit depending upon settings.

in your case you are worried about having an issue and not being able to notice it if you don't run a relay contactor system that once shut down needs manual resetting correct? well you can do the same thing but on a low voltage/low amperage scale if your solar controller operates in the same fashion as mine does. it all depends upon the architecture of your controller which only Outback can tell you.

for instance, in my case vice having the common port BMS shut down everything when its too cold to charge i run a temperature controller. if temp is below xx degrees then it opens a relay. solar controller will not charge. This is of course set at a higher temp then the BMS's temperature protection. Temps come up to a safe level and it automatically turns on the SCC. Defense in depth. Multiple triggers that come on line prior to the BMS making the ultimate decision. Not only that but I can put multiple relays on the line... over charge, relay stops the solar controller, under temp, same I can put multiple temp controllers w/sensors so that I can auto shut down charging depending upon location within the battery pack. since my inverter is set to shut off at 3.00 volts per cell i don't have to worry about over discharge. but strangely enough, my inverter has an input that I can use to shut of the inverter with... all it needs is a 12-48 volt signal. once again defense in depth.

Best of luck
 
Agreed. All the documentation I have been able to fin says to disable or set the temperature voltage compensation to minimum on Lithium batteries. I have not found any different wording for LFP or NMC cells. Schneider also says set it to zero. Sadly, even with all of the charger settings, the software version on my Schneider XW-Pro does not have a setting to disable charging when the batteries are too hot or too cold. That should be a feature in any charging device.
 
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