Low voltage disconnect questions

[DavidH]

Hi all,

So I've been getting the components of my system purchased and reading up as much as I can. I've a couple of questions on best practice for low voltage disconnect that I'd appreciate anyone with experience weighing in on because I'm aware that the power to the charge controller should never be cut off while the PV array is continuing to feed power into it.

Since my last post I've currently got the following: Midnite Classic 200, 24/1200 Victron Phoenix Inverter and an ATS (MOES Dual Power Controller 50A https://amzn.to/32NZ2MZ ). This complements the 24V 100Ah AGM battery bank (the mppt and the inverter are the investment to cope with future expansion and I will not use the inverter anywhere near full capacity). My system is a back-up but I want to put it to work to supplement the power I'm using in the house without tying it to the grid. I'll try to give as much info as I can.

Firstly, the inverter has a low voltage disconnect which is adjustable, the factory default settings for it are;
"Low battery shut down (adjustable) 18.6"
"Low battery restart & alarm (adjustable) 21.8"
"Battery charged detect (adjustable) 28.0V"
"Zero-load power 9.5W"
"Default zero-load power in ECO mode (default search interval: 2.5sec, adjustable) 1.7W"

I currently don't know how to determine what voltage of my AGM 24V battery = what SOC, so I can set the voltages. Any info on that would also be appreciated as I'm looking to not go below 60% if I can help it.

The ATS has the inverter power running into it, the grid power going into it and also the battery power going into it. A low voltage disconnect can be set, so when the battery reaches a certain voltage it cuts the load, transferring from the inverter to the grid power. The ATS also has a voltage reconnect that can be adjusted where it will transfer the load again to the inverter. The drawback is that if the inverter turns off first, the ATS does not switch because it's the battery voltage that determines this.

So if the ATS kicks in, transferring from the inverter to the grid, I'd have the inverter drawing 9.5W or 1.7W if in ECO mode until a full shutdown at a voltage that I can specify. However, I'd still have the draw of the Midnite Classic (if anyone knows what this is I'd appreciate it). I believe the Midnite has some sort of low voltage protection, or a nighttime low power mode but I'm still trying to find exact details. Is there a way for it to break the PV array circuit and also power down automatically as the last fail-safe to protect the battery? If there is an alarm I guess I'm pretty sure I would be around to do it manually but auto never hurts if I happened to be out of the house.

Any insight or thoughts are much appreciated.

[Mike90250]

It's going to be tough. You have to protect batteries and charge controller, switch HV DC PV array lines and get gear working with mixed brands.

First you need to understand your battery discharge voltage under loads. Battery could be 90% charged, but heavy loads can pull it's voltage down to cut-off, when it will instantly recover from the load. Will your transfer switch be "smart" enough (as in uSeconds to prevent motors and things from crashing and powering up in a different phase)

Personally, I'd rely on the inverters low voltage shutdown, and let that trip the transfer switch. With AGM, you don't need to be switching the PV off and on. Just let the charge controller regulate the battery charge. Li-Ion batteries need a PV disconnect to prevent overcharge, but not AGM.

[DavidH]

Thanks Mike, I hadn't realized that about the battery discharge voltage. That'll be interesting to see - from the price of the transfer switch, I doubt it'll be smart enough. So I'll have to be very careful about loads and watch carefully how the ATS behaves if I use it. Since the ATS runs off the battery, I'm not 100% sure how I could get it to switch when the inverter shuts down. If it was powered by a 120 to 12V converter off the inverter, it would shut down the ATS. I know it uses a mechanical relay - so it might still switch to grid power despite being off. I'll have to investigate and see.

[Glock24]

Hi, what ATS model/make do you have?

[DavidH]

Hi, I got the following: MOES Dual Power Controller 50A https://amzn.to/32NZ2MZ My thinking behind it was that I could put my smaller system to work and whenever the battery got too low, it would switch to grid power... then once battery charges up, switch back over to the battery.

[Glock24]

I have this one: https://spartanpower.com/product/spa...-spts4500plus/

I'm pretty sure it's the same one with a different label. I have not installed i yet, but I did some testing. It is powered by DC via the voltage sensing cables that go to the battery. When the voltage is between the LV and HV range the AC output is connected to the "Inverter" input. When the voltage goes below the LV it transfers to the "Public Power".

If the ATS has no power from the batteries or if you switch it off (there's a on-off switch on the top) it switches to the "Public Power" input.

It only transfers based on battery voltage, it does not sense if there's power on the "Inverter" or "Public Power" inputs. If you are on "Public Power" because the battery voltage was below the threshold it won't transfer back to the "Inverter" AC input If there's a power outage because it won't notice.

[DavidH]

Yes, that really looks like the same one just rebranded. Ok that's really helpful information! Have you tested it to see what its power draw is on the battery? That's something I'm really interested in but have not been able to find any information on.

Ok, if it switches to public power input when it turns off that means that I could use the inverter to control the ATS (to avoid the problems Mike mentioned). I'd have to use a 120V to 12V transformer that's powered by the inverter and connect the ATS to the 12V circuit. That way when the inverter switches off it kills the power to the transformer, turning off the ATS switching it to the public power. When inverter switches on again it'll turn on the ATS which will recognize HV and switch the load to the inverter.