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BMS Array Cutoff and Bypass Switch

MisterSandals

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I could have added this to quite a few different threads but did not want to hijack anybody's thread.

There have been a few conversations about disconnecting batteries from a SCC while the PV array is still hot/connected. There are several scenarios where a battery could be disconnected from a controller such as temperature BMS cutoff, fuse/breaker cutoff or negligent manual battery disconnect. All these have the potential to irreparably damage your SCC (See Morningstar Best Practices, page 20 - my controller is Tristar MPPT 45). I recall folks mentioning Outback and other SCC's having similar warnings.

I REALLY like the idea of the BMS controlling 12v 200A switches. It allows for a small amperage BMS to switch high amperage loads. Not my idea but i expanded on Will's concept and his drawing to use switches to control the PV array load to the SCC.

The problem with this setup which was pointed out in a different thread, failed in the case of the BMS cutting off for low voltage: there would be no solar charging to overcome the low voltage. I am wondering if anybody sees a problem with installing a manual switch, operated at the judgment and peril of the user, to bypass the BMS controlled switch to the PV array in a low voltage cutoff scenario in order to resume PV charging.

See attached, hopefully the schematic explains this better than my words.

I intend to have temperature sensors connected to the BMS to protect the LiFePO4 cells from charging in a freeze condition (temp sensors and wiring not pictured).

And of course if anybody has a way to make this better or even automated with smarts, PLEASE help.
 

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There are differing ways to accomplish it. Also depends if you have a solar controller charging or a charger using charging or an Inverter/Charger

Simply put, disconnect the solar controller from the battery s a BAD idea, now controllers handle not having a battery can either fail & get damaged, lose their configuration settings or more. IF possible program a solar controller to TWO Stages… NO EQ, No Float or Low Volt float. (Most decent controllers allow for it) Just BULK at the appropriate voltage. Lo Volt Cutoff should be set properly as well as Hi Volt (in solar controller) VERIFY VOLTAGE @ BATTERY & @ Solar Controller and adjust to match / compensate for line loss. IF Inbound power can't be stopped or LOWERED TO SAFE, IT IS BEST to shutoff Solar Panel INBOUND Power, allowing solar controller o see the battery and get it's voltages. HI Volt Cutoff on BMS should be matched to Solar Controller with -0.1 V so if BMS is set to 29.2 solar should be 29.1

IF using a dedicated external charger with it's own separate power input line, then you simply shut-off that input on Hi Volt... Lo Volt is moot for charging. IF using a proper Smart Charger which has links to the BMS the it manages both (not all companies have such, Chargery BMS' Do).

IF using an Inverter/Charger Combo, then you get into a series of Relays.... Lo & Hi Volt cutoff relays (1 each) in front of the battery pack.

**
If you have more than one battery pack (same type) and use a (+) & (-) bus bars the each pack is controlled on the BATT Side of the Busbar.
IF you have a Mixed setup of LifePo & FLA for example, then other trickery & complications play into it... and it depends on how the charging of the odd balls is done which will guide the ways to do it.

NOT ALL BMS' are the same ! Some do not use external relays and they throttle voltages by controlling the NEG line. Some have an external charge port which is great IF you have a dedicated external charger. While using External Relays may be a tad clumsy in some installations they can general handle more amperage and are safer & more reliable in general terms.

Here is 2 pic's from Chargery which uses external relays (up to 600A) and a Pic of what I plan on for my system with mixed battery types. Hopefully that is some help. Maybe it adds to confusion..... REF for Chargery: http://chargery.com/

My System (planned configuration)
*Midnite Solar controller can be programmed easily to manage charge profile)
Samlex Inverter/Charger will be left programmed to top up FLA Bank when necessary using external power.
Our Solar Diagram V3 (12-2019).jpg

CHARGERY MODE 1 With Inverter, or Inverter/Charger
chargery-2.JPG

Chargery Mode-2 with dedicated external charger.

chargery-1.JPG

Hope it helps !
Steve
 
Thanks for the feedback and pictures!
Your system looks great, i cannot say i understand it all (yet!).

depends if you have a solar controller charging or a charger using charging or an Inverter/Charger

I have a Morningstar MPPT 45, which is a solar charge controller. Plan to have 4x 206AH LiFePO4 cells at 12v. Possibly changing to 8x 206AH at 12v down the road if necessary.

Simply put, disconnect the solar controller from the battery s a BAD idea,

I agree 100%! I am trying to avoid disconnecting the solar controller from the battery. That is my entire point, sorry if i was not clear. My diagram is intended to show no breaks/switches between them (but i will need/want a breaker there, right?).

I was wondering how you handle low temperature cutoff to stop charging batteries at or below freezing? I REALLY want to handle that in particular. And my understanding is that its best to let the BMS handle that condition.

Similarly, do you plan to have a breaker/fuse/cutoff between the SSC and battery(s)? Do you have a plan to disconnect the solar array if that connection is broken? I have a breaker and if that trips, that meets our definition of BAD.

And I also concur that the day to day charge voltage handling can be managed by the SSC. But what happens if/when a cell gets out of sync with the others and is greatly overcharged or undercharged? I suspect that the entire group of cells is at risk and with 45 amps going into "non-conforming" cell(s), there is likely some fire risk too. And with my RV sitting in the sun charging all day, everyday, its unattended most of the time. How long it takes for cells to show signs of failure or how likely it would be for me to inspect regularly enough is a big unknown and concern (i expect the BMS to manage this too and STOP CHARGING... somehow).

Thanks again, welcome to all feedback!
 
Thanks for the feedback and pictures!
Your system looks great, i cannot say i understand it all (yet!).

I have a Morningstar MPPT 45, which is a solar charge controller. Plan to have 4x 206AH LiFePO4 cells at 12v. Possibly changing to 8x 206AH at 12v down the road if necessary.

I know Morningstar controller's are programmable but to what extent I don't know.

I agree 100%! I am trying to avoid disconnecting the solar controller from the battery. That is my entire point, sorry if i was not clear. My diagram is intended to show no breaks/switches between them (but i will need/want a breaker there, right?).
There should always be a way to A) Disconnect panels from CC and B) to protect the CC from anything coming from the Panels & C) Lightning Protection ! no one considers It till their system goes POOF !.

I was wondering how you handle low temperature cutoff to stop charging batteries at or below freezing? I REALLY want to handle that in particular. And my understanding is that its best to let the BMS handle that condition.
Most BMS' will disconnect charging if temps are out of set parameters, some have a Hi & Lo while others only have a Hi cutoff. BMS is best left to manage that as log as it has temp sensors & capability to do so (not all do). Will has posted about battery warmers which turn on at a setpoint and uses the battery power to warm them... There are other possible solutions as well... depends on your setup and how & what will work best.

Similarly, do you plan to have a breaker/fuse/cutoff between the SSC and battery(s)? Do you have a plan to disconnect the solar array if that connection is broken? I have a breaker and if that trips, that meets our definition of BAD.
As shown in my diagram, the panels go through a breaker before hitting the SC, also there is Lightning Protection too. The breaker in the E-Panel allows me to disconnect solar-in BUT the 2 strings are still "hot" and this is where a combiner with a breaker for each string can shut them down entirely.

And I also concur that the day to day charge voltage handling can be managed by the SSC. But what happens if/when a cell gets out of sync with the others and is greatly overcharged or undercharged? I suspect that the entire group of cells is at risk and with 45 amps going into "non-conforming" cell(s), there is likely some fire risk too. And with my RV sitting in the sun charging all day, everyday, its unattended most of the time. How long it takes for cells to show signs of failure or how likely it would be for me to inspect regularly enough is a big unknown and concern (i expect the BMS to manage this too and STOP CHARGING... somehow).

With a good BMS it should cutoff charging as/when needed. Programming the Solar Controller to remain within the thresholds is important. IF everything is setup properly, the system would require no attention at all. Refer to your your Solar Controller & Inverter documentation on how to program your gear. As long as you can program the charging parameters to fall within the specs, your good to go. IF you cannot program these devices properly, put them on Craigslist and chalk it up to lessons learned...

Thanks again, welcome to all feedback!
 
...
Most BMS' will disconnect charging if temps are out of set parameters
...
With a good BMS it should cutoff charging as/when needed.

But HOW does the BMS cutoff charging? I can only see two options:
1) cutoff switch from panels to SCC (my proposal above)
2) cutoff switch from SCC to batteries (we've both agreed this is BAD)
3) ???

The BMS in my diagram is controlling the load side as many have suggested
is the best practice.
 
Seems a small cheap FLA would make things much easer.
K.I.S.S

I LOVE simple!
How would I implement it? I am guessing I cannot hook up a FLA in parallel to a bank of LiFePO4 batteries because of the charge voltage differences, so i am mystified.

Lets start with Will's diagram without my additions, the BMS detects low/high voltage or its sensor says its freezing, and cuts the load portion. What are you suggesting to "stop charging"?

Thanks for any info!
 
One type of BMS will drop the NEG line. Another type will drop the POS side. Another can use a relay which can be setup in different ways to shut off.

By dropping the load side it applies to the inverter on LOW VOLT signal, you don't want to pass that threshold and hit 0 V, right. IF in LO volt, you sure do not want to shutoff the solar panels though, only in a high volt situation would you consider that.

Programming the Solar Controller for your lifepo settings is the only way to go really and allow it to do a lower volt float (if you can't disable it) with EQ off or set to 1 minute a the lower voltage as well. With the inverter on, it will be pulling a bit of juice anyways, or even another small draw on the system should pull enough to neutralize crossing the threshold.

Temp cutoff on Hi Temp is to protect from over heating, On Low Temp is to prevent charging below freezing point 0c/32f Daly handles that internally somehow, at the cell level and by the relay external if so programmed as I understand it... I do not have a Daly. By not having a separate port for charging, it drops the common port to prevent charging but that also kills output too. With a separate port, it would just drop the input charge line and allow output to continue. LiFePO can output below zero, I think to -20C if mem serves off hand. There is LiFePo Y which can go to much lower temps but at higher cost of course.
 
A small SLA battery with the lifepo4 will survive at lifepo4 voltages, allowing the lifepo4 to disconnect without harm to the charge controller. And may give ya time to fix or at least Intel a flashlight is found. And is cheaper then the electrical mess that is proposed.
 
IF in LO volt, you sure do not want to shutoff the solar panels though,

Exactly, but I cannot discern why the BMS cut off (that i know of). And the "LO volt" is EXACTLY the scenario as to why I would put the bypass switch in... to manually allow the panels to power the SSC which would charge the batteries. THIS is what I am asking about in this thread, whether this array cutoff AND bypass switch has any adverse consequences.

The only consequence (that i know if!) of the array cutoff switch is the "LO volt" issue you pointed out (array cutoff will never charge out of LO volt).
The bypass switch (human intervention is the only way i know how to do it) solves that issue: perhaps waiting until the sun is up, hit the switch to turn on the array and charge the batteries out of the LO volt situation. Eventually the BMS will get out of LO volt and turn on the load ... thats the plan anyway.

In a freeze condition, I would want the array to be cutoff and I would NOT use the bypass until temperature is appropriate (as determined by the BMS). This works EXACTLY how i would want it to (but someone correct me if i am missing something!).

Programming the Solar Controller for your lifepo settings is the only way to...

Yea, I know i have to program the controller and handle a lot of other things, I'm only trying to get feedback on the solar array cutoff and bypass switch (as the title says...).

So far, nobody has said "Your array cutoff and bypass switch will cause your therambulator to flark".
 
A small SLA battery with the lifepo4...

Sorry, i don't know what a "small SLA battery with the lifepo4" looks like. Are they hooked in series? Parallel? Switched? I got nothing. I know they won't play nicely if connected...

I agree that keeping the SCC alive with a SLA battery would be great. How would that be done, with wires and stuff?

And is cheaper then the electrical mess that is proposed.
Is it really a mess?
Is it much different than the 12v 200A relay that Will proposed? (its his diagram except the dotted lines on the right)
The battery cutoff switch is overkill and $40
The 12v 200A relay switch is under $10
That's all I am adding to Will's schematics to protect my SSC and battery bank.
 
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Sealed Lead Acid battery (calcium based would servive the longest) In parrell.
 
Sealed Lead Acid battery (calcium based would servive the longest) In parrell.

It would probably "survive the longest" but then what happens when it stops surviving on an unattended RV? An SLA getting LiFePO4 level charge voltages 24/7/365?
No way.

Seriously, that is far from safe.

Thats not even close to something i would consider, especially when a single switch and relay are cheap and easy.
 
That 200A relay has always bugged me a bit, it's a tad low. The relay has to handle the absolute worst amperage that could hit it, so consider the inverter fully maxed out, The amp rating is for what is passing through the relay itself so I would consider a 300A to be on the safer side of that. You aid you paid $10 for it... Hmmm,,,, I just paid $48 ea for 12V/300A for industrial grade direct from China…. and that was discounted from $60. makes one wonder a bit... and they are actually coming from the BMS manufacturer (Chargery in my case)
 
I would consider a 300A to be on the safer side of that. You aid you paid $10 for it... Hmmm,,,, I just paid $48 ea for 12V/300A for industrial grade direct from China…. and that was discounted from $60

Can you provide a link to the relay you recommend?

I was thinking that with 400 watts of panels (to a 45 amp SCC) that 200A would be plenty, many times over. Am i missing something?

This is the relay:

It is recommended by Will here:

He also recommends a 500-800 amp model which seems ridiculously overkill. Amazon has an 80 amp version (same model, same ad). Is one of these better for what I am trying to do?

 
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