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Low voltage shutoff for power inverters?

 
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I have a few solar panels that connect to a deep-cycle battery, and some inverters that come off of that. When the battery gets drained, and the voltage gets too low, the inverters emit a steady alarm — and keep drawing power. So if I'm outside, or have my headphones in, or am asleep, they'll just keep running down the battery until it needs to be resuscitated.

I'm surprised they make inverters like this. Why not just shut them  off when you have a low battery? Can someone recommend a decent, cheap inverter that automatically shuts offf? Or, is there a device I can put between the battery and the inverter that will do this?
 
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Location: Clackamas County, OR (zone 7)
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Here is one solution:

https://www.amazon.com/Universal-Voltage-Triggered-Controller-1URVTLC-1224-BSD/dp/B015Q51ZDY

You can program this $40 unit to activate a relay at a certain threshold. You would then need to wire the inverter(s) through a relay so that when the voltage dropped too low it would open the relay.

A couple issues: If you are running large currents, the DC relay you would need is going to be pretty pricey. 30amp automotive relays are cheap and readily available, so up to 300ish watts on 12v or 600watts on 24v would be really cheap and easy to implement. If you are trying to switch off a load that can pull hundreds of amps, you might end up spending a couple hundred bucks on a beefy contactor. It might be possible to scrounge a starter solenoid, but you might have to do some digging to figure out its amp rating. Contactors also pull a few watts just to keep the coil energized, so they will be a bit of a parasitic load on a small system.

Also, voltages will sag when there is a large load applied - which may cause it to trip prematurely. That unit lets you program the voltage where the unit powers back on though, so it should be possible to prevent the relay from chattering.
 
Morgan Nunan
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Thanks for the reply. I saw some units like this on ebay, but I didn't think they would work because I'm using big cables with alligator clips to connect the battery to the inverters. However, maybe I could cut the ends off an extension cord and use those wire instead, which might fit into the relay.

I'm not sure what wattage one of the inverters is, but the other is 400w and it connects to a 12v deep cycle battery. I use them to charge my phone, batteries, cordless drills and similar items. So I'm never drawing a lot of power from them.
 
Carl Nystrom
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Location: Clackamas County, OR (zone 7)
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I didn't think they would work because I'm using big cables with alligator clips to connect the battery to the inverters.



Maybe it is time to dial in your setup a little more. Get some ring terminals, splices, and spade connectors and a decent crimping tool and you could easily redo the wiring. If you have an extension cord laying around, you could harvest wire from that, but you can also buy wire so that it is the right size for what you need. Every wire that connects to a battery's positive terminal should have a fuse on it sized to interrupt the largest current that the wire can safely handle.

For a 400 watt inverter on 12v that would be about 33 amps (400W/12v = 33.33A) 10 ga wire would be ideal, although for short sections of wire in free air 12 ga or even smaller could probably still work.

If you are running only small loads, then 400 watts is probably more than you actually need. Larger inverters tend to have larger idle current draw. If you are trying to keep your system very lean, then I would suggest you get one of these:

https://www.amazon.com/Digital-Battery-Low-Voltage-Protection/dp/B07929Y5SZ/ref=sr_1_1?dchild=1&keywords=12-36V+Battery+Over+Discharge+Low+Voltage+Disconnect+Protector+Modules+DC+Output&qid=1632339115&sr=8-1

and put a 100Watt inverter on it. (note that it is only rated for 20amps, so MAX 240watts) Really, you dont want to discharge a lead acid much beyond 50% - so once you get under something like 12v you are shortening the lifespan of your battery quite significantly. At 11.5v, it maybe has 10% of its capacity left. You could even get a 2 pack of protecting circuits for a dollar more, and set them to different setpoints. Have a big inverter shut down at 12v, and then limp along on a smaller inverter until you get to 11.5v. If you cycle below 50%, you can probably only count on about 500 cycles before the battery is toast. If you routinely take it down to 10%, you will be lucky if it lasts a year.

Edit: if you really want to run a bigger inverter, here is a cheap work-around: Get the smaller protecting circuit, and wire the "load" to the coil of one of these:

https://www.amazon.com/Continuous-Solenoid-Nickel-Plating-Lucky-Seven/dp/B0859BZJNW/ref=sr_1_3?dchild=1&keywords=50+amp+solenoid&qid=1632339700&sr=8-3

Then wire a positive cable through a large fuse and then through the big posts on the solenoid to the inverter. That should let you run anything up to 3600W.


 
pollinator
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Location: North central Ontario
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Usually when an inverter has that kind of alarm it is one designed to be used in a vehicle situation. That annoying skreech is designed to start sounding at 11.5 or so so you still have power left to start the vehicle. When you use them off grid often small battery banks will go bellow 11.5 starting heavy loads and the sound starts. Usually push them further and they do have a shut off voltage, usually 10.5 to 11 volts.  As an Alternative better inverters have a programmable voltage cutoff. You would still be using its standby current but not the current that was going to the load. I like these small ones here as a fairly cheap puresine inverter: https://www.magnum-dimensions.com/product-inverter/400w-12vdc-pure-sine-inverter-0
it will shut off loads at 10.5 volts and allow them to turn back on at 11.8... its not adjustable though.
Cheers,  David
 
Morgan Nunan
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Thanks for the replies, this has all been very helpful. I will circle back once I've had a chance to try some of these solutions.
 
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Nobody is paying attention to the root cause of this problem, that you seem to be frequently draining your battery to the alarm stage.  This is really a clue that your system is too small or not the best designed.  Since grid-tie panels are getting so cheap these days, maybe it's time to revamp your system.

One thing hobbling you is sticking with 12V.  You automatically get twice as much power at 24V and four times as much at 48V.  Give us the specs on your equipment, and we could make cost effective upgrades that improve performance.

I'm running both 24V and 48V systems at my homestead, and I've never once dropped lower than 75%.  And that's even with me running my 240VAC well-pump.
 
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