Low Voltage cut off

Good morning all I have a question about low-voltage cut off. I am building a emergency backup system for my home. I have a grid tied solar system already installed in my home. I have two sony boy 5000 watt inverters with a handy power outlet Available. I can pull 1500 W off each inverter in case of a grid power failure. This system works great on sunny days however I wanted to have a nighttime storage solution. Not knowing much I Purchased 4 100 amp hour 12 volt AGM batteries and a 4000 W Pure sine wave inverter. I have wired it into my homes transfer switch which I already had for my generator and it powers some lights and small appliances perfectly. I am charging the system with a 55 amp 3 stage charger Which I keep plugged into grid power. The batteries stay topped off and ready to use. In the event of a power failure I just simply plug the charger into these auxiliary AC power outlet off one of my inverters which remember are powered by my Pv on the roof. I was hoping to be able to power my refrigerator overnight but it seems I could only get 3 to 4 hours before my battery voltage drops too low. My refrigerator pulls 4 amps and I have some lights as well so I’m probably pulling about 600 W or so during my test. I did not let it drop below 12 V But I know upon recharging I was below 50% of my charge capacity. So this is my question how do I install a low voltage cut off? I purchased a 220 amp Victron battery protect however from what I read it seems I should not install that between the battery and the inverter. The inverter will shut off at 9.5 V which seems very low. Also how much capacity do I actually need to run 600 W for 6 to 8 hours

If you are doing a continuous 600W that's about 4.8kWh. Your lead acid battery can only just manage that but by being taken all the way down to dead flat. That's only going to get about 250 cycles out of the batteries before they are done and need replacing and would not actually hold the load up as they get towards discharged (can't hold the voltage up with 50 amps draw) anyway. IMO you should probably have twice the amp hours in the battery bank.

When it comes to low voltage switch off the ways I see to do it are pushing the power button on the inverter; disconnecting the DC from the inverter; disconnecting the AC load from the inverter. I'd be approaching this from a more technical view so I'd be pushing the button but maybe someone else already knows of a suitable voltage based switch that could control a contactor (DC side switching) or a relay (AC side switching). The sensing would need to have a low water mark at which the unit 'turns off the inverter' and a high water mark at which it 'turns on the inverter', ie some hysteresis to avoid things rapidly turning off / on as the battery voltage rises with no load.

Doing this with battery voltage sensing alone isn't really the right way to do it IMO. You should really have some form of power detection, something that monitors watt hours or amp hours drawn out of the battery vs put back in.

If no one else comes up with anything I'll post more details, but I tend to get into waffle mode easily

Hope that inverter is not 12 volts.
4000 watts is just not practical at 12 volts.

Well you could do 2x 1/0 awg wire depending on the round trip circuit length.

smoothJoey said:

Hope that inverter is not 12 volts.
4000 watts is just not practical at 12 volts.

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yes it is 12 volt I can't imagine getting anywhere close to 4 k more like 1 K if that ever.

4kW inverter on 12v is a fake inverter. It's just bad design and guilt by association, likely bad quality. They put such a low voltage trip because they know nobody can keep from excessive voltage drop from battery. Even if you use it at lower wattage loading the overhead current will likely be high, consuming more power. I would not be surprised if it draws at 4 to 6 amps from battery with no AC load on inverter.

Cabling and connectors for such an inverter require very high current capability. You are tripping low voltage because of voltage drop on wires/connectors.

It is hard to put a power efficiency number on such an inverter as there is so many places to have voltage drop, including items internal to unit. The unit likely doesn't even have battery cable terminals appropriate for cable size required for such high currents.

4kW / 80% effic. = 5kW from 12v input. That is 420 amps average and 650 amps peak on sinewave peak output current !

For a 6 foot run pair from battery:

0/1 gauge has 220 watt loss, 0.52v drop
0/2 gauge has 178 watt loss, 0.42v drop
0/4 gauge has 112 watt loss, 0.27v drop

That is just the wire. Connectors will add more loss, and then there is the issue can battery supply that much current without its voltage collapsing.

The more the voltage drop, the more the inverter DC current has to be to pull the same AC output wattage.

LennyM said:

yes it is 12 volt I can't imagine getting anywhere close to 4 k more like 1 K if that ever.

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Very large, 4/0 cable is needed for >1% voltage drop on a 3.3 ft round trip cable run to be able to use the abilities of the inverter on hand.

LennyM said:

Also how much capacity do I actually need to run 600 W for 6 to 8 hours

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Assume 86% efficiency on inverter at that power. Problem with large wattage inverters is they have higher overhead current used regardless of loading on inverter. If you don't need the power don't use an inverter way above need. Only need to account for starting surge current for things like refrig compressor.

600/.86 = 700 watts from inverter. At 12.3v at inverter that would require 57 amps. The voltage and therefore current depends on your cabling voltage drop.

6 hrs requires 342 AH's
8 hrs requires 456 AH's

AGM capacity 4x100 = 400 AH, should not draw more then 300 AH's if you want batteries to have any longevity.

Also the 100 AH rating on battery is likely based on 5-10 amp load. It will be less at higher current drain.

thanks for your invite

RCinFLA said:

4kW inverter on 12v is a fake inverter. It's just bad design and guilt by association, likely bad quality. They put such a low voltage trip because they know nobody can keep from excessive voltage drop from battery. Even if you use it at lower wattage loading the overhead current will likely be high, consuming more power. I would not be surprised if it draws at 4 to 6 amps from battery with no AC load on inverter.

Cabling and connectors for such an inverter require very high current capability. You are tripping low voltage because of voltage drop on wires/connectors.

It is hard to put a power efficiency number on such an inverter as there is so many places to have voltage drop, including items internal to unit. The unit likely doesn't even have battery cable terminals appropriate for cable size required for such high currents.

4kW / 80% effic. = 5kW from 12v input. That is 420 amps average and 650 amps peak on sinewave peak output current !

For a 6 foot run pair from battery:

0/1 gauge has 220 watt loss, 0.52v drop
0/2 gauge has 178 watt loss, 0.42v drop
0/4 gauge has 112 watt loss, 0.27v drop

That is just the wire. Connectors will add more loss, and then there is the issue can battery supply that much current without its voltage collapsing.

The more the voltage drop, the more the inverter DC current has to be to pull the same AC output wattage.

Click to expand...

LennyM said:

thanks for your invite

Click to expand...

LennyM said:

thanks for your invite

Click to expand...

RCinFLA said:

Assume 86% efficiency on inverter at that power. Problem with large wattage inverters is they have higher overhead current used regardless of loading on inverter. If you don't need the power don't use an inverter way above need. Only need to account for starting surge current for things like refrig compressor.

600/.86 = 700 watts from inverter. At 12.3v at inverter that would require 57 amps. The voltage and therefore current depends on your cabling voltage drop.

6 hrs requires 342 AH's
8 hrs requires 456 AH's

AGM capacity 4x100 = 400 AH, should not draw more then 300 AH's if you want batteries to have any longevity.

Also the 100 AH rating on battery is likely based on 5-10 amp load. It will be less at higher current drain.

Click to expand...

LennyM said:

thanks for your invite

Click to expand...

LennyM said:

thanks for your invite

Click to expand...

LennyM said:

thanks for your invite

Click to expand...

LennyM said:

thanks for your invite

Click to expand...

smoothJoey said:

Hope that inverter is not 12 volts.
4000 watts is just not practical at 12 volts.

Click to expand...

Thanks for all your information my inverter draws 1.4 A when idling not 4 to 6 as you suggested I do believe it is a quality inverter for the price. Perhaps the 4K Claim is a bit much

LennyM said:

Thanks for all your information my inverter draws 1.4 A when idling not 4 to 6 as you suggested I do believe it is a quality inverter for the price. Perhaps the 4K Claim is a bit much

Click to expand...

Did you solve the low voltage cut-off for your inverter?

No , thanks for asking since my system is only for emergency back up my search for a solution has taken a backseat. I so appreciate the knowledge this form has to offer. I am learning slowly. Peace