Inverter Low Voltage Cutoff--Why SO low?

[Vigilant24]

I'm planning to buy two 12V inverters: a small one (about 500W) and a bigger one (about 2000 watts). I want to protect my 2 x 105AH FLA batteries, but have been surprised to see that the low voltage cutoffs on inverters tends to be at about 9-10 VDC (often with an alarm starting at about 0.5 V above that).

Now, if I understand things right, an "at rest" nominal 12V battery will show an open circuit voltage of about 12.7V when full, about 12.10 when discharged to 50%, and about 11.5V when it has just 10% remaining. At 10 V, it would be long gone, or at least damaged.

Why do these inverters allow the battery to discharge to 10V (or lower)? Is it due to some difference between "resting" voltage and in-use voltages?
Is there a general consensus about what the inverter cutoff should be in order to protect a FLA battery?
Is there an (affordable?) 500w inverter that has an adjustable low voltage cutoff?
I looked into buying a separate low-voltage cutoff circuit board (cheap insurance at about $15), but most max out at about 10 amps which doesn't provide much wattage at just 12V.

Thanks for any assistance.

Mark

 

[rodrick]

Check out victron battery protector

 

[time2roll]

Yes the lead acid is getting pulled down 1.0 to 1.5 of voltage sag at 160 amps so the cut off is a bit low to prevent premature disconnect. Battery will recover voltage when the load is dropped. Need to manage things manually or get a monitor. The cut off on the inverter is not there to save the battery from itself.

 

[HighDesertOffgrid]

Is it due to some difference between "resting" voltage and in-use voltages?

Voltage sag is a thing, even with lithium chemistries. My experience: When an inductive load kicks on and pulls 5X amps on an appliance, even a LFP battery at 30% charge will drop voltage significantly and kill the inverter while then rising back to a safe voltage.

 

[Warpspeed]

The problem Mark, is that a perfectly healthy battery that is getting down a bit say to 11.5 volts, may drop mightily in voltage from that, under a very sudden surge load. A great many loads draw high initial current surge at turn on, and the surge may only last for a second or less.
So the voltage may briefly dip below 10 volts for an instant, then spring back up to maybe 11.2 volts.

Now if your inverter shuts down at say 10 volts, you might have very frequent shutdowns even with the battery at 11.5 volts.
The battery internal resistance is also going to be higher at low states of charge, which makes all this a lot worse.

So its fairly normal practice to set undervoltage shutdown very low, and you will find it actually does shut down long before the battery resting voltage gets anywhere near as low as that.

 

[Vigilant24]

So its fairly normal practice to set undervoltage shutdown very low, and you will find it actually does shut down long before the battery resting voltage gets anywhere near as low as that.

Thanks, Warpspeed. The examples are useful. In the case of this small inverter, my plan is to use it for low loads overnight (DW's CPAP, maybe a room fan, etc), so there won't generally be high startup loads. I'm just a bit afraid that a low (100w= approx 0.1C for a single battery), continuous (8 hour) load won't cause much of that voltage sag and that the "running" voltage of the battery won't be much lower than the resting voltage would have been, and the LVC will just come too late.
I suppose the logical thing to do is to just set things up with the real equipment and see if it is a real problem or one I've created out of nothing It wouldn't be the first time!

Present small inverters I'm looking at:
Bestek 500W Pure Sine Wave (7 watt standby, 11V low voltage cutoff, $67)
Folkma 300W Pure Sine Wave (8 watt standby, 10 V low voltage cutoff, $58. Has a nice LCD display that includes load info)
WZRELB 80W Pure Sine Wave ( 5W standby, 10.5V low voltage shutoff, $73. Not so small. Lots of watts/$ and could also run
some of my bigger loads if required)

Mark

 

[Vigilant24]

Yes the lead acid is getting pulled down 1.0 to 1.5 of voltage sag at 160 amps so the cut off is a bit low to prevent premature disconnect. Battery will recover voltage when the load is dropped. Need to manage things manually or get a monitor. The cut off on the inverter is not there to save the battery from itself.

Thanks. I guess the required monitor would be a coulombmeter? That would probably work, but seems like it's overkill for my little (emergency backup use) system. Maybe I'll go there, though.

It would seem possible for a smart circuit to do about what we'd do manually: Be cognizant of the battery capacity and type, of the recent load that could be causing voltage lag, and then make an assessment (guess?) of the likely real-world SOC, then cutoff at the user-specified SOC. Might be a good Arduino project.

Check out victron battery protector

Thanks, they look good. There's not a lot of info on just how "smart" they are (i.e. the logic being used), it appears that they do allow a user-set cutoff voltage and have a delay to allow power to continue due to a short-term voltage sag.

Voltage sag is a thing, even with lithium chemistries. My experience: When an inductive load kicks on and pulls 5X amps on an appliance, even a LFP battery at 30% charge will drop voltage significantly and kill the inverter while then rising back to a safe voltage.

Yep. In my "normal" anticipated use that inductive load (room fan startup) would be mild and just once at he beginning of an 8 hour steady low load. So, unless there's an approx 2V voltage drop due to a constant 100w load (approx 0.1C), an inverter with a 10V cutoff would see the FLA battery drop well into the "oops! overdischarged" zone.

Thanks again, this has been very useful. I'll know more when I get some real hardware and check things out.

Mark

 

[mikefitz]

You cannot use a Victron battery protect in the power feed cable to an inverter. You could use it to control a remote disable feature if the inverter has this.
The idea of using a low cost low voltage detect module could control the inverter if it has remote enable/disable, or hack Into the unit and replace the on/of switch with a relay contact.
You should have no problem with two 105 Ah batteries overnight.
Incorporating a battery monitor into the system would give you a more accurate battery SOG and actual power use.
Some types Incorporate programmable relays that can control external devices.

Mike

 

[efficientPV]

It is not what you think it is. The inverter mfg doesn't give a damn about the battery. That protect voltage is to protect the inverter, not the battery. That is the safest low voltage where there is enough drive voltage to operate the FET outputs without going into the linear region which will destroy FET.

 

[Vigilant24]

It is not what you think it is. The inverter mgg doesn't give a damn about the battery. That protect voltage is to protect the inverter, not the battery. That is the safest low voltage where there is enough drive voltage to operate the FET outputs without going into the linear region which will destroy FET.

I guess my clue >should< have been that none of the spec sheets )or advertisements) mentioned an input for differing battery types. In my defense, though, many of the product listings claimed that the low voltage cutoff would "protect the battery."
Well, maybe it will, but if so it may be just as much by coincidence as by design.

Thanks.
Mark