Low voltage triggers BMS cutoff, cannot recharge because MPPT cannot see battery

[Roger Rosthwaite]

I know this is a known issue and I apologize if it's been addressed here, but I cannot find any satisfactory answers.

The title says it all:
Battery is drained so BMS cuts off.
When sun comes back MPPT cannot see the battery so it doesn't charge the battery.

The only way is to address it manually, whether its bypassing the BMS or charging the battery manually until the voltage is high enough for the BMS to turn back on.

What are some automatic solutions?
I have an automatic transfer switch and I would like this to be a fully automatic system.
I have a LiFePO4 with 4s 280ah EVE cells and an overkill solar BMS.

Overkill Solar addresses this issue in their documentation but they only offer manual solutions.

 

[meetyg]

What are you using this battery for ? Only DC loads or with an AC inverter?
Basically, you should set your inverter's low voltage cutoff to a value higher than what the BMS is set to. That way the BMS won't go into LVD.

 

[Roger Rosthwaite]

What are you using this battery for ? Only DC loads or with an AC inverter?
Basically, you should set your inverter's low voltage cutoff to a value higher than what the BMS is set to. That way the BMS won't go into LVD.

I’ve got an AC, off-grid home solar setup.

Unfortunately my inverter isn’t programmable. I’m using a Renogy 1000W Inverter. The low voltage cutoff is hardcoded to 10v

 

[meetyg]

I’ve got an AC, off-grid home solar setup.

Unfortunately my inverter isn’t programmable. I’m using a Renogy 1000W Inverter. The low voltage cutoff is hardcoded to 10v

Since it has a "remote switch" port, I think it should be fairly easy to "hack" the inverter using a voltage sensing relay:

There are relays which you can set to "open" when a low voltage is reached, and "close" again when the voltage has risen above a certain value.
As I suppose the remote switch port is probably a 2 or 4 wire phone plug, you could use the relay to act as the switch.
That way it will turn your inverter off when battery reaches say 12.0v and turn on when above 13.0v (or whatever).

 

[meetyg]

Since it has a "remote switch" port, I think it should be fairly easy to "hack" the inverter using a voltage sensing relay:

There are relays which you can set to "open" when a low voltage is reached, and "close" again when the voltage has risen above a certain value.
As I suppose the remote switch port is probably a 2 or 4 wire phone plug, you could use the relay to act as the switch.
That way it will turn your inverter off when battery reaches say 12.0v and turn on when above 13.0v (or whatever).

I've been using this guy for controlling a small DC load:

https://www.aliexpress.com/item/4000602255417.html

Do you have the remote switch for the inverter?
If you do and are interested in doing this hack, I can try to walk you through it...

 

[Roger Rosthwaite]

Since it has a "remote switch" port, I think it should be fairly easy to "hack" the inverter using a voltage sensing relay:

There are relays which you can set to "open" when a low voltage is reached, and "close" again when the voltage has risen above a certain value.
As I suppose the remote switch port is probably a 2 or 4 wire phone plug, you could use the relay to act as the switch.
That way it will turn your inverter off when battery reaches say 12.0v and turn on when above 13.0v (or whatever).

That’s just what I was thinking. Seems like the most straightforward solution short of buying a programmable inverter.

 

[meetyg]

That’s just what I was thinking. Seems like the most straightforward solution short of buying a programmable inverter.

If you can take a picture of the remote switch plug, that will help. I suppose it's a 4 wire (could be 6, since there are 2 LEDs on the remote).
Anyhow, you will need to open the remote switch and see which wires go to the physical ON/OFF switch.
Those are the ones you will want to control via the relay.
You don't need to destroy the remote switch, just identify which wires go to the physical switch, and trace back thier positions on the phone plug (RJ11 ?). Then you can just take an old phone cable and splice it, identifying the wires that correspond to the positions above. Then you can connect those wires to the relay.
Of course the relay itself will have to also be connected to the battery, but its power consumption is very little, so it probably won't drain your battery that fast.

If you just want to test before buying the relay, you can do as I mentioned above. Just to make sure that you identified the correct 2 wires, I would check them with a DC volt meter (DMM) to make sure they have no voltage (otherwise you may have identified the wires for LEDs). If they don't have any apparent voltage, I would think it's safe to test just by connecting them manually to see that the inverter really powers on.
I think that the inverter's main ON/OFF switch has a special third position for remote switching, so check that it's in the proper position before testing.

 

[Roger Rosthwaite]

I've been using this guy for controlling a small DC load:

https://www.aliexpress.com/item/4000602255417.html

Do you have the remote switch for the inverter?
If you do and are interested in doing this hack, I can try to walk you through it...

I was looking at the exact same relay on amazon.

Seems like it would be pretty straightforward. Just did a test with the renogy remote and it looks like the middle two pins on the RJ-11 have continuity when the remote is on, and none when it’s off.

I’m assuming this relay can handle triggering that continuity based on programmable cutoff voltages?

 

[meetyg]

I was looking at the exact same relay on amazon.

Seems like it would be pretty straightforward. Just did a test with the renogy remote and it looks like the middle two pins on the RJ-11 have continuity when the remote is on, and none when it’s off.

I’m assuming this relay can handle triggering that continuity based on programmable cutoff voltages?

Yes, exactly.

 

[Roger Rosthwaite]

Yes, exactly.

Great! Thanks for the help. Good to know that relay will handle the job.

I’ll buy that relay and see what I can do. Hopefully the documentation is dicepherable and/or exists ?

 

[meetyg]

Great! Thanks for the help. Good to know that relay will handle the job.

I’ll buy that relay and see what I can do. Hopefully the documentation is dicepherable and/or exists ?

This should help with setting it up:

 

[Roger Rosthwaite]

This should help with setting it up:

Thanks!

I didn’t realize the automatic transfer switch I was planning on buying works via battery voltage. I assumed it simply switched when one source of power is lost, but no, it switches from inverter to grid via low voltage detection.

So that’s an option.

Although, it would be nice to have the inverter turned off when grid power is connected, because then the inverter wouldn’t be drawing several watts of standby power (plus the couple watts required to power the AC smart plug I have plugged in to monitor solar powered AC usage).