BMS Array Cutoff and Bypass Switch

[MisterSandals]

What I did not expect was load sharing.

I am not sure what you mean.

I have a question about your schematic: Are the two switches (bottom and upper right) to determine what battery bank (LiFePO4 or CG2) your load uses? I am assuming you are isolating the two battery banks from one another, so how/when are these switches operated?

 

[MisterSandals]

Hmm, actually when the bottom switch is open, then the LiFePO4 and CG2 batteries are connected (what you call load sharing?). Is this intended or recommended?

 

[grizzzman]

In use, they are connected. The reality is that the lifepo4 Pac has a higher working voltage then the FLA bank. As such the LFP bank does all the work. The FLA bank gets a trickle charge the whole time, with an exception under high load the FLA bank assists. This let me know that upgraded wiring for the LFP battery was needed (working on that now) When brewing coffee the FLA bank would contribute 20 amps while the LFP contributed 55 amps.

 

[Maast]

If you have direct access to the cell terminals I can't envision a situation where you would ever WANT to disconnect your solar charge controller and not also disconnect it from the panels. If you don't have access to the cells then I think your best bet would be to place a supercapacitor parallel to the battery bank. If it's a 24V bank use two supercapacitors with a 1.5 megaohm resistor across each capacitors terminal to keep them in balance.

The SC also will help the bank meet sudden large amp draws and help filter any ripple to the bank and you dont have to even consider charge voltages, it's voltage will be whatever the main banks voltage is. And it'll never wear out in your or your grandchildrens lifetime.

 

[MisterSandals]

I can't envision a situation where you would ever WANT to disconnect your solar charge controller and not also disconnect it from the panels.

Exactly, this is the nut I have been trying to crack!

Unfortunately you lost me at super capacitor. I really am trying NOT to disconnect battery from SCC! I am trying very hard to figure out how to disconnect array from SCC as you suggest.

Can you have a look at this thread?

Relays: NC vs NO or other solution?

I am hoping someone can help me find a solution. I am learning about relays and hoping to incorporate one to cut the power between my array and solar charge controller. 400w array, 45A SCC. My "thought" was to use an NO (normally open) relay but wondering how i would power a "close" in the...

www.diysolarforum.com

 

[grizzzman]

The FLA bank stays connected and in the trailer. It stays connected to the inverters. (Caps stay charged ) The switch is when I remove the LFP bank. ( when not in use it lives in the comfort of the house). (A/C when hot,heater when cold) With that said I'm pulling the trigger on a super capacitor bank, 6 caps with the balancing board. Gonna need to update my wiring diagram. I just ordered the Chargery BMS8T.

 

[Maast]

Unfortunately you lost me at super capacitor. I really am trying NOT to disconnect battery from SCC! I am trying very hard to figure out how to disconnect array from SCC.

Well you might get stuck if the battery has a internal BMS like a battleborn that you can't get to without cracking the case. If it gets below freezing it might shut itself off, depends on the battery BMS. It's okay to discharge below freezing - just dont charge.

The supercapacitor I refer to is the ones the boom box car audio guys use like this one.

I replied to the other thread. The hard part of the whole thing is finding a temperature sensitive trigger. If it were my system I'd quit trying to shut off the input power to save the batteries (I WANT them to keep working in sub-freezing temps) and instead build a heating system for the batts that activates when it gets cold.

 

[Keith C]

Mister; What am I missing about the "disconnect issue" can't you disconnect charging by opening a contactor in the lines coming from the array? The SCC just thinks the sun went out. All systems normal.

grizz; SLA's commonly choke with a shorted cell. (One of my scariest adventures!) This has any paralleled bank suddenly trying to get the now one-cell-short battery back up to the original voltage. In this case your LFP bank is going to dump into the SLA. If the event is big enough your fuses will do the trick. But, if the drain is just less than the fusing how are you going to prevent a far-undercharged LFP bank?

 

[MisterSandals]

Mister; What am I missing about the "disconnect issue" can't you disconnect charging by opening a contactor in the lines coming from the array? The SCC just thinks the sun went out. All systems normal.

You're not missing anything. My original thought was that holding a NO relay open used too much power and was not a good solution for 24/7365 use. Essentially held open all the time.

But Maast and you have shed new light on this and I am comfortable with the NO relay being a good solution. SCClockDr is on board with this too with his $3 relays that he swears by. I think I'll carry a spare or two just in case.

I am REALLY appreciative of all the input on this. I'm listening and learning!

And to wrap up the bypass switch aspect of this thread, I am comfortable "bypassing" the BMS low voltage cutoff situation with a switch to temporarily
close this relay to charge until sufficiently charged.

Thanks all! I think I got this!

 

[grizzzman]

.

grizz; SLA's commonly choke with a shorted cell. (One of my scariest adventures!) This has any paralleled bank suddenly trying to get the now one-cell-short battery back up to the original voltage. In this case your LFP bank is going to dump into the SLA. If the event is big enough your fuses will do the trick. But, if the drain is just less than the fusing how are you going to prevent a far-undercharged LFP bank?

Thank you for your opinion Keith. I appreciate your taking the time to respond. There is a 100 amp fuse on the CG2 bank. The BMS on the LFP bank will not allow more then 120 amps(ish) and would shut the LFP bank down at around 12.00 volts. Many decades ago I was leaning over a battery that had a charger going on it. while the cust. tried to crank the car I wiggled a terminal and the top of the battery blew off spraying acid in my eyes and face. I understand FLA batteries very well. What really keeps me safe is "Trends and Tendencies"

 

[Keith C]

Mister; I'm with you on running some coil forever and would probably look for a push-me-pull-me solution. A latching relay. That three legged thing mentioned up above is latching. Haven't figured out where to get it or what it costs. Depending on your array and the array voltage a latching relay might be pretty easy to find.

grizz: Yikes! That's worst than my experience. I had a diesel with a 150A alternator and two batteries. One battery shorted a cell the other, and the alternator, attempted to straighten things out. The battery went into a full roiling boil you could hear from 40 feet away with steam spraying out of all the orifices. I had to disconnect it because we were in the middle of nowhere and would be unable to restart the vehicle if we safely left it to "calm down" as the second battery would be fully discharged by coming down to the failed battery's level. It was pretty terrifying trying to disconnect that battery.

 

[Keith C]

Digikey Latching Relays 60-120A

 

[MisterSandals]

Digikey Latching Relays 60-120A

I'm open for the idea, it sounds great but I have not found a real life solution to implement it (a solution that does NOT involve relay boards or Arduino's - where i lost interest).
Closing on the latching relay was straightforward. The solution that I failed to see was how to open it after losing power (from "BMS P-" cut or simply batteries disconnected by breaker or other means).

I think we beat this horse quite a bit on this thread:

Relays: NC vs NO or other solution?

I am hoping someone can help me find a solution. I am learning about relays and hoping to incorporate one to cut the power between my array and solar charge controller. 400w array, 45A SCC. My "thought" was to use an NO (normally open) relay but wondering how i would power a "close" in the...

www.diysolarforum.com

 

[Buzzard Bait]

Question if the main battery fuse goes during solar panel's PMax will we lose our SCC?

 

[SCClockDr]

Question if the main battery fuse goes during solar panel's PMax will we lose our SCC?

Possibly & it will likely be a minor annoyance in comparison to what caused the battery fuse to blow.

 

[Keith C]

Possibly & it will likely be a minor annoyance in comparison to what caused the battery fuse to blow.

LOL. True-dat!

 

[Buzzard Bait]

Possibly & it will likely be a minor annoyance in comparison to what caused the battery fuse to blow.

Question if a blown fuse could cause the loss of the SCC wouldn't it be better to reduce your losses to open the SCC input with a NO relay with the loss of the fuse? I understand the relay will cost me a few watts around the clock, but if the battery is required to open a switching relay it could not open with out power. Thank you in advance for your further assistance.

 

[Keith C]

Question if a blown fuse could cause the loss of the SCC wouldn't it be better to reduce your losses to open the SCC input with a NO relay with the loss of the fuse?

The devil is in the details. The instant the fuse blows the SCC chokes. There is no time to subsequently open the SCC's supply. If fuses go then you're probably dealing with an emergency over-current situation and you're in fire-prevention mode, damn-the-SCC at that point. More applicable is a BMS, of whatever ilk, needing to interrupt further charging to prevent over-charge damage. That has at least a couple of seconds if not minutes to respond to. That's when you have time to remove the SCC's input source.

 

[Buzzard Bait]

The devil is in the details. The instant the fuse blows the SCC chokes. There is no time to subsequently open the SCC's supply. If fuses go then you're probably dealing with an emergency over-current situation and you're in fire-prevention mode, damn-the-SCC at that point. More applicable is a BMS, of whatever ilk, needing to interrupt further charging to prevent over-charge damage. That has at least a couple of seconds if not minutes to respond to. That's when you have time to remove the SCC's input source.

First let me thank you for your response. Now I think I have to apologize for being a little unclear. Please let me try to explain. My thinking is that the PV cable could go through a NO relay that is to say if there is power to the relay it is closed to let power from the PV panels enter the SCC if power is lost to the relay the connection of the panels to the SCC would end. If there were an interruption of power to the relay for any reason a battery fuse, BMS, Over or Under Temp. Over Charge would have to activate a different relay to some kind of dump load. This would have to be automated in some way to protect itself, because I may leave it unattended for weeks at a time.

 

[SCClockDr]

I concur with your logic. I start from the basic tenant a safety device must fail to a safe state.The only risk factor with a NO relay its the return spring.

Over Charge would have to activate a different relay to some kind of dump load.

I'd use your same logic here as well in this case, fail to NO pin to battery & NC pin to the dump load.
Better yet disconnect the PV's from the SCC as the first scenario. (Safer)