Low Battery Cut Out Limits

5 Posts
Mar 15, 2010 05:25 pm
Low Battery Cut Out Limits

I have an Xantrex XW 4548 Inverter Charger and I am using a 48 volt battery bank. I noticed that the default value for the Low Battery Cut Off is 44 volts for a 48 volt system. If the battery voltage drops that low before any warning or cut off, GOODBY BATTERIES. The LBCO value can be changed but it has a maximum limit of 48 volts. At that point the batteries would have discharged to 25% capacity. Why doesn't the range allow a setting of at least 48.8 volts which would be a 50% discharge of the battery bank?
220 Posts
Mar 15, 2010 06:27 pm
Re: Low Battery Cut Out Limits

 Ain't that the truth! well said ray. it's still a mystery to me as to why they can't give us a better range of voltage adjustments on these things, especially the high end stuff!

 think about the poor newbie who reads that his equipment "protects" him from overdischarging his bank and that confidence in the equipment is the thing ruins him.

 if we go in and change things we void the warranty.

manufacturers are you listening?

 cheers, dave
65 Posts
Mar 15, 2010 06:52 pm
Re: Low Battery Cut Out Limits

Well, there is one aspect to take into consideration - a battery bank under load is different than at rest (no load). When your inverter starts drawing from the battery bank, the voltage dips very suddenly. As soon as you remove that load (the inverter stops drawing), the voltage will come back up.

The best metaphor I can think of is someone pushing a trash can 90% full of crumpled paper down with their hand til it looks like it's only 60% full. The moment they remove their hand the crumpled paper pushes back up - perhaps only to 80%. Same with batteries, the voltage bounces back after you remove the load.

That's why voltage isn't an exact way to measure how full your batteries are if they're in use. It is fairly exact if you leave them to rest for a few hours.

So the manufacturer doesn't want your inverter to prematurely cut-out. I don't know if 44V corresponds to 50% full exactly under load... but I know that under load a 44V will go up a few volts when the load is shut-off.

"Making Renewable Do-able" (tm)
1 Posts
Mar 17, 2010 11:03 am
Re: Low Battery Cut Out Limits

Sascha D has the right idea on this one.  Battery voltage under load does not correspond to rest voltage, the latter being the measure of a battery's state of charge.  If the cut-off were set at 48V or higher, you would regularly lose your inverter when you loaded up the system.  I know that Outback chargers with AGS have different setpoints for 2-minute, 2-hour, and 24-hour voltage cutoffs, so that you can differentiate between a battery that's showing low voltage under load and one that has truly reached a significant depth of discharge.   
184 Posts
Mar 18, 2010 10:39 pm
Re: Low Battery Cut Out Limits

As I see it, getting the most from my off-grid system while protecting and preserving my batteries is a balancing act.  I found myself either shutting down the system too soon (not getting the full benefit of the system), or waiting too long to turn it off (risking battery damage).  To help me more efficiently use my system, I've created a chart.  While this doesn't answer the questions posted here concerning controller settings, it might contribute to a better understanding of battery SOC under various conditions.  I hope you find this information helpful.

220 Posts
Mar 19, 2010 04:08 pm
Re: Low Battery Cut Out Limits

good topic and some informative posts here.

 as we can see there are soo many variables.. a small load and cutoff points are close to our settings, a large battery bank won't "dip" as much as a smaller one and so on.

 a good chance to thank solarjohn for his chart. john, i have been using your SOC chart and have it hanging for easy reference at several systems..each set up is a little different than the others but your chart gives folks a good "feel" for whats happening. thanks john, ..i highly recommend it to others.

 cheers, dave

ps.sj, get home soon! it's time for planting...it's strange how a place (the bay) can be soo beautiful and yet ugly at the same time. stay safe!
5 Posts
Mar 21, 2010 07:48 pm
Re: Low Battery Cut Out Limits

Thanks for all the responses to my original question. With regard to concerns over having the system shutdown when the battery voltage drops due to a current surge. If you did decide to set the cutoff voltage to a higher value, all you have to do is increase the Low Battery Cutoff Delay. This can be set for anywhere from 0 to 600 seconds which would prevent system shutdown due to voltage drops from temporary current surges. The default setting is 10 seconds.
Thanks again for all the replies!!  Ray
5 Posts
Mar 21, 2010 08:04 pm
Re: Low Battery Cut Out Limits

Forgot to mention my solution to the problem. Have decided to use the Auxiary connection on the inverter which can send out a 12 volt signal. This can be programmed to sense low battery voltages from 20 to 52 volts for a 48 volt system in .1 volt increments. The delay time before it is triggered can be set in 1 second increments up to 600 seconds. I bought a piezo electric car alarm at Radio Shack for 6 bucks and change and wired it to the auxilary output. This wont shut the inverter down, but it will warn me when my battery bank is getting low so I wont kill the batteries.
184 Posts
Mar 23, 2010 06:03 pm
Re: Low Battery Cut Out Limits


You might want to consider a voltage-controlled-switch to sense battery voltage and turn the inverter on and off.  As you probably realize from this discussion, you need to be able to controll two set-points, one to turn the inverter off at the low point, and the other to turn the inverter on when the batteries are fully charged again.  Keep in mind that if you reconnect the batteries before they have a chance to fully charge (on a regular basis), you may damage them.  My choice for a voltage-controlled-switch was the Morningstar Relay Driver.  I love it.

John D.
Mar 28, 2010 07:16 am
Re: Low Battery Cut Out Limits

...and here I was thinking all of this time that I was suppose to be sizing systems so that things such as auto low battery cut off wasn't even an issue. I mean to say, I thought automatic low voltage cut off switches were redundant to a properly sized system and a battery state of charge monitor installed in a conspicuous place. Boy, do I feel dumb. Its like they say, "you learn something new everyday."

Wait! Does this also mean that, when I begin calculations for sizing the; wire, fuse and, disconnect going from battery to inverter that, I should be dividing the inverters automatic low voltage cutoff setpoint from its continuous rated wattage? Or do I use the inverters nominal battery voltage rating?
184 Posts
Mar 28, 2010 02:33 pm
Re: Low Battery Cut Out Limits

Thomas;  I think there are a few issues here that come into play.  1. Due to budget considerations, not all of us have the battery capacity that we would like to have.  2.  Power consumption can be more than usual from time to time, draining the batteries faster than usual.  3.  Extended periods of bad weather limit the charging capacity significantly from time to time.  Forgetting to turn of a load can also drain the batteries faster than usual. For all of these reasons I'm glad to have added cutoff automation.  Without it, one screw-up might irrepairably damage my batteries.  I certainly don't want that to happen.  A battery monitor is a great accessory, but I'm not always home to monitor that.  Also, I hope to avoid sleep interruptions by something "beeping" on my system, requiring immediate attention.  My automation takes care of all of these issues. 
Regards, John
Mar 30, 2010 06:15 am
Re: Low Battery Cut Out Limits

OK ok, you got me. I was being a butthead in an attempt to drive a point. So, pry my eyes out with a spoon already.

Automatic Low Battery Voltage Cutoff Setpoint.
It speaks for it self doesn't it?

Hey, I am all for safety features. But, like seatbelts in cars, I am not going to rely on that seatbelt to save me in a crash, instead, I am going to rely on my defensive driving skills to keep me from having a crash.
Right about here one might think, "well thats different, driving and automobile and having an off grid power system" but I don't think it is. To my mind it is Analogous.

I could have, not paid, for that defensive driving course. I could have just stuck with the free (more or less) high school drivers ed. course and had a drivers license just the same as millions of others out there on Americas highways, on Our highways.
But I didn't.
 Because I believe that along with that freedom to make choices, we all cherish so much, here in the United States of America, comes the responsibility of making the right choice.
(I wonder if this is how bureaucracies got started?)

(Oh, for those of you who have lived "sheltered" lives, that thing about prying eyes out with a spoon, in some circles thats just a way of apologizing without having to apologize. Crazy huh?)
Jun 16, 2010 12:19 am
Re: Low Battery Cut Out Limits

very funny yet i find them fantastic   hahaha
2 Posts
Aug 17, 2010 07:22 pm
Re: Low Battery Cut Out Limits

There's more than one way to skin a cat, so here's another solution for a low battery voltage cutoff switch that may save some money:  I use two devices in series.  The first is the "Load Manager 1H", model #091-96-XX (substitue the xx for the voltage you want) manufactured by Kussmaul Electronics.  You can find them at www.kussmaul.com.  This gizmo has separate adjustments for disconnect and reconnect and voltage points, and they come in a few different voltage models.  I have one for my 24V system and it works great.  The limitation is that the relay is only good for 30 amps.  So, I use the Load Manager to control a 200 amp high current 24 V DC relay, which looks suspiciously like an old style starter solenoid on a car.  That relay feeds my 3KW (6KW Peak)full sine wave inverter.  I've had the system for over two years on a bank of ten, 100 Ah AGM batteries and it works fine.  Combined cost was about $130 up here in Alaska, so you should be able to do better down in the "lower 48".  If your inverter has a remote control line you'd be able to use the Load Manager by itself to provide a contact closure to the line, instead of cutting off the main battery voltage to the inverter.  That would save you $40-$60.
18 Posts
Aug 25, 2010 04:59 pm
Re: Low Battery Cut Out Limits

John, I like the relay idea you have here.  Where did you find the 200 amp relay? Conrad
18 Posts
Aug 25, 2010 05:19 pm
Re: Low Battery Cut Out Limits

Of course I found it here at altE
2 Posts
Aug 27, 2010 03:12 am
Re: Low Battery Cut Out Limits

I purchased it at ABS Alaskan, www.absak.com.  They have three stores:  one in Fairbanks, Anchorage, and in Washington state.  In the Fairbanks store, Judson can help you with this solution.  Phone number is 800-235-0689.

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