Inverter low voltage cutoff use and settings

[booster]

Gerry's question about inverter low voltage cutoff vs autostart, and a past discussion about the reprogramming of the low voltage cutoff of the Roadtrek frigs brings up a kind of "catch 22" question, which I also noticed on the bench as I was testing our new system.

The inverter low voltage cutout is fixed setting at one voltage, sometimes with a time at that low voltage being needed (to keep it on during high surge startups?)

In the world of smaller inverters that run small loads like many of us have had in the past, the low voltage cutoff could pretty easily be set to keep the inverter from running the batteries lower than we wanted to take them. If you have a reasonable amount of batteries (especially AGM or lithium) the voltage drop running small loads is only a couple tenths of a volt, so you could set the low voltage cutoff at your desired minimum discharge and be pretty accurate. It would trip a bit early with loads on, but not much.

Now if you take a look at the bigger inverter, bigger load, systems, everything changes. With big loads like air conditioning, you can easily lose considerably more than a volt of voltage even with full batteries, and it gets worse as they get discharged. If you want to keep your batteries above 50%, like many do, you might set the low voltage cutoff at 12 volts to assure holding the 50%. In this case you could trip the cutoff with full batteries by running a big load. If you set the low voltage cutoff low enough so you can run the big load and have it cutoff at about 50% with the load on, you will have it set so low you could have totally dead batteries if you just have low loads, or had the inverter on by mistake taking parasitic power.

It would seem that it might be a good idea for the inverter low voltage cutoff be a load modified setting (perhaps with a settable lowering of the setting with increased load-maybe .1 volt per 10amps or something like that). That way it could be set to be effective, and useful, over the entire range of use

I don't know of any inverter that currently works that way, but there may be some.

[markopolo]

Maybe like adding coulomb counting to the low voltage cutoff routine so it knows the real rate of discharge and can adjust the low voltage cutoff set point. I don't know if there's anything out there like that.

[booster]

Originally Posted by markopolo

Maybe like adding coulomb counting to the low voltage cutoff routine so it knows the real rate of discharge and can adjust the low voltage cutoff set point. I don't know if there's anything out there like that.

That is a very good point. I think they all use the low voltage cutout, maybe just do to momentum.

On a unit like the Magnum we got, which has a battery monitor as an integrated part of it, they could easily set it up to shut off based on the actual SOC of the batteries based on actual amp hours.

That would also make it so folks could much more easily predict how much longer things like the air conditioner would run before cutout.

I think we need to send Marko to work at one of the inverter/charger companies.

[gerrym51]

as long as autostart(or any companys version of it) starts recharge before the inverter reads low voltage and stops-i would say its aok.

i just asked the question because of pets and air condtioning. if no pets would not make a difference-at least to me

[booster]

Originally Posted by gerrym51

as long as autostart(or any companys version of it) starts recharge before the inverter reads low voltage and stops-i would say its aok.

i just asked the question because of pets and air condtioning. if no pets would not make a difference-at least to me

I didn't start this thread to specifically answer your case, as it was more toward the folks who don't have, or don't want to use, autostart. Autostart will cover the problem. Your case pointed out that bad things can happen with the low voltage cutout settings.

It boils down to you can have inverter set so you can run your big loads without tripping out, or you can protect yourself from killing the batteries with the inverter running small loads or just on. With current stuff, it doesn't appear you can get both.

[markopolo]

Davydd's rig has that additional protection built in. The system will disconnect the batteries based on state of charge.

It's a great idea when you think about it. If you accidentally leave a small load running his setup will disconnect the batteries before they're dead.

[booster]

Originally Posted by markopolo

Davydd's rig has that additional protection built in. The system will disconnect the batteries based on state of charge.

It's a great idea when you think about it. If you accidentally leave a small load running his setup will disconnect the batteries before they're dead.

That is a very nice feature, and an absolute must with the lithium batteries. I think the manufacturers are way behind on this kind of thing. Most of them that have a battery monitor don't even control the charging with them, much less the other cool stuff that could be done, from what I have seen. When you spring for the fancy remote and battery monitor, you should be able to control all the features off the monitor and shunt, as it is such a much better way to do it.

[wincrasher]

I think with the lithiums, state of charge is really the only way to go. The voltage holds pretty steady down to almost 0. I think Davydd is mistaken thinking he'd see 11.6 volts - I think his bank would be empty at that level.

My Onan's autostart is based only on voltage. So I am probably going to have to set that fairly high - probably 12.3 to 12.5 volts. Otherwise I'll be dead empty.

[Davydd]

I was referring to this old chart for that 11.6 volts. Normally when I am at 99% SOC the voltage is reading around 13.4. Like I said, I need to go to electrical school.

[wincrasher]

I noticed that too. Those old charts just won't work for the lithiums.

You should map a new one.

[booster]

Originally Posted by wincrasher

I think with the lithiums, state of charge is really the only way to go. The voltage holds pretty steady down to almost 0. I think Davydd is mistaken thinking he'd see 11.6 volts - I think his bank would be empty at that level.

My Onan's autostart is based only on voltage. So I am probably going to have to set that fairly high - probably 12.3 to 12.5 volts. Otherwise I'll be dead empty.

I think where you set it will have to be determined by how much your loads drop your voltage. You won't really know that until you run the various loads and see. I can tell you that 440ah of AGM batteries will drop about .5 volt with a 65 amp load, which is less than the microwave or the AC. If you set at the 12.3-12.5, you might wind up with the generator starting every time you run a moderate DC load.