Sudden battery voltage drop off

[off-grid-geeks]

Hello everyone!
I'm hoping for some insight to something new to me.

I am away from my cabin for a couple of weeks, and have been monitoring the solar setup remotely.

The loads I left on are:
   Refrigerator running off an inverter, pulls about 14 amps at 12 volts for about 3 minutes every hour or so
   Security lighting, 2 amps at 12 volts from about 5PM till 11 PM
   wifi and cell router, 1 amp at all times
   Security camera, 0.5 amps at all times

Now the problem... we have now had 6 days of no sun in a row at that location.
Here are the charges I've gotten during that time:
  11/25   0.5kw
  11/26   0.3kw
  11/27   0.0kw
  11/28   0.1kw
  11/29   0.0kw
  11/30   0.6kw

As expected, the battery bank (12 volts @ 880Ah) has dropped over this period.
I unfortunately do not have a low voltage disconnect set up, something I will quickly rectify upon my return.
The inverter (Xantrax Prosine 1000) does have a built in LVD, but it does not kick in until 10 volts. Even when it does kick in, the only thing running on the inverter is the fridge.

Well, early this morning, it appears that the batteries crashed.
Here are today's early morning battery changes. Note what happens at 4:44AM, this is the part I am confused about:
00:00   11.4
01:40   11.3
03:00   11.2
03:36   11.1
04:15   11.0
04:35   10.8
04:44    9.3
05:00    9.2

I've attached a screen grab of this morning as a picture shows it better.

My question, any idea what caused the 1.5 voltage drop at that time?
The inverter LVD would have shut down the inverter right about then when the batteries hit 10 volts.
Nothing else in the house is on to pull that kind of load to eat up that power.

Any ideas?
Thanks!

[Vic]

Hi o-g-g.

Well,  at abut 10 V,  a 12 V battery has ZERO Capacity.   So,  almost any load would cause the battery voltage to crash.

Perhaps one of the DC loads tries to draw more than normal current when it's DC input voltage drops below its minimum input V spec (?).

Your exact loads are difficult to grasp,  but  the exact cause may not be as important,  in the scheme of things,  as the thought that your battery bank is probably toast.

What is the Charge Controller on this system?

And,  it is not clear what are the battery temperatures,  as this can have a fairly large influence on apparent battery voltage.

Guessing done for now,   GOOD LUCK!   Vic

[zoneblue]

What Vic said, batteries fall off a cliff at 0%SOC.

Now... if the system cant cope with your "away" loads, albeit winter, then something is not well balanced. You had better turn the fridge and inverter off until this is resolved.  Arriving to warm beer is a hardship i know! But low voltage events are time to reassess the design and make sure it doenst happen again. Ever.

The loads I left on are:
   Refrigerator running off an inverter, pulls about 14 amps at 12 volts for about 3 minutes every hour or so

So granted the fridge isnt being opened, but 3/60 duty cycle is impressive. Im guessing the ambient is mighty chilly.  =12*14*3/60*24= 200Wh/d. Very low.

   Security lighting, 2 amps at 12 volts from about 5PM till 11 PM
   wifi and cell router, 1 amp at all times
   Security camera, 0.5 amps at all times

=2*12*6 = 144Wh/d
= 1.5*12*24 = 430Wh/d

Then we need to account for the electronics tare.

The Prosine has something like a 22W tare (kinda high for a kW class inverter if you ask me), and 1.5W sleep mode.

If theres ANYTHING else on the inverter except the fridge, sleep mode will be defeated. Worse case scenario is:
= 22W*24h= 528Wh/d
If it is running in sleep mode on a 3/60 cycle, then lets call that a round 60Wh/d.

Finally 5W for the classic gives =5*24=120Wh/d

So grand total is betw 960Wh/day and 1430Wh/d

Now the problem... we have now had 6 days of no sun in a row at that location.
Here are the charges I've gotten during that time:
  11/25   0.5kw
  11/26   0.3kw
  11/27   0.0kw
  11/28   0.1kw
  11/29   0.0kw
  11/30   0.6kw

So right there is your problem. You cant use power that you arent producing. You need 1.0-1.4kWh/d and yet youre only producing  0.25kWh/d on average.

[off-grid-geeks]

Yes, I know the battery bank is at zero.
Yes, I know its probably toast.
And as I had already stated, the only thing on the inverter is the fridge.
And I disagree that having only one item on the inverter defeats sleep mode.
The controller is a Classic 200, unsure why that matters, how could the controller suddenly cause the battery bank to instantly drop 1.5 volts?
The battery temperature is around 48 degrees F. The temp had little change this past 6 days, don't see that causing a sudden voltage drop. There was not a sudden arctic vortex tearing through the house at 4:45AM.

No, I do not agree that the system is not well balanced, as I said in my post we just went through 6 days of zero sun. I am not about to design a system that can deal with that many days of gloom, which so far has happened once in 4 years. If I had been home I would have shut off the fridge, so yes, I need to add a Low Voltage Disconnect that works at a higher voltage than 10 volts. But if you feel a system that runs out of power after 6 days of no sun is incorrect.

It was stated that the 'problem' is that I'm only producing 0.25kw/day when I need 1.0-1.4kw/day.
That is NOT the problem. Sheesh. I already said there was no sun for 6 days. So its not really great insight to say that was the problem. I was again asking if anyone had an idea as to why there was that sudden early morning 1.5 volt drop. I wasn't asking why I ran out of power, I already explained that; 6 days of no sun. During more typical weather, I average 2.5-3kw/day.

I was not asking for a critique of my system. I was asking if anyone had an idea on the sudden early morning 1.5 volt drop on the battery bank, a drop that remained untill the sun arrived this morning.

I appreciate the suggestion that one of the loads may have caused a sudden increase in load when the voltage got down to 10 volts. That may indeed be the cause.

Thanks.

[Westbranch]

OGG, I think you may find a lot of the answer you seek if you convert all the loads you listed in watts rather than amps.

That way you can see just how many volts you need to supply those watts..  roughly W = V * A, so as the watt load is applied the Volts needed will not stay constant as the battery V drops...
maybe precipitously ? !

[zoneblue]

I only do water. Theres plenty of alternative uses for water...  like make beer?

[Vic]

Hello everyone!   ...   

   ...   
Nothing else in the house is on to pull that kind of load to eat up that power.

Any ideas?
Thanks!

Hi again o-g-g,

Not to beat on you too much again,  but  at about 10 Vdc  the batteries have NO CAPACITY,  whatsoever.  The battery is VERY HIGH IMPEADANCE.  This equates to being able to deliver almost NO POWER whatsoever,  so,  even a light load will cause the battery voltage to dive even further.

Sometimes,  we can tend to think of battery voltage as equating the amount of power available,  but the voltage verses current/power availability  is very non-linear for almost all battery chemistries.

So,  suggest that you not look too much for something that is applying a fairly large load,  that might have caused this battery voltage fall-off.

Furthermore,  with batteries being about 46 Degrees F,  the battery voltage is even lower than it your noted readings imply.   The battery bank is quite  D E A D.

The culprit might be the inverter,  as its LVD may not behave well with very high Z batteries ...   or perhaps the fridge kicked on again,  or whatever.

Just my opinion,   but,  as stated above,  in  a previous post   ...  am not too certain that just what was the thing that caused this rapid voltage fall-off,   just that having batteries this far discharged causes unpredictable results.

I was not trying to cast doubt on your CC,  just was curious what it was.   Believe that the Classics like 8 or more volts on the battery to operate ...

Think that you do know,  that almost all of us here are just volunteers.  We all are trying to help a bit,   as much as we can.

Almost irrespective of how much information is given in an initial post,   there will usually be the need for some more info.  In attempts to try to move things along,  we often need to guess a bit,  or,  in my case,  ask a few stupid questions,  but most of us really do read,  and re-read an initial post,  ponder things  some,  before writing a reply.

I did go back and read all of your previous posts,  trying to get a better picture of your system,  and where on the planet it might be located,  and so on.  Did not have the time to do this prior to my initial post.

So,  this is a friendly  Forum,  with a lot of experienced and helpful folks,  all doing a good job of trying to get along,  while conveying useful,  and fairly accurate information,  often in the absence of some data that might make conclusions a bit more straightforward.

Thanks for you patience when hearing things that you might not want to hear.    Vic
But think that you are unhappy with some analysis,  and a question or two.   But,  it believe that zoneblue's comments and analysis is accurate,   and that post took zb many minutes to write.   Am sure that that had the sole intention of being helpful.

[off-grid-geeks]

Yes, thanks everyone for taking the time to think and respond.

My frustration came at many of the replies focusing on why the batteries went dead, which I was not asking.
I know full well why they went dead.

What puzzled me was that sudden, instant drop of 1.5 volts at 4:44 AM when I know there was no major load on.
I think I got the best answer with the post about batteries at that state of depletion at that temperature do unexpected things.

Thanks again!
-Alan

[Halfcrazy]

Hello everyone!
I'm hoping for some insight to something new to me.

I am away from my cabin for a couple of weeks, and have been monitoring the solar setup remotely.

The loads I left on are:
   Refrigerator running off an inverter, pulls about 14 amps at 12 volts for about 3 minutes every hour or so
   Security lighting, 2 amps at 12 volts from about 5PM till 11 PM
   wifi and cell router, 1 amp at all times
   Security camera, 0.5 amps at all times

Now the problem... we have now had 6 days of no sun in a row at that location.
Here are the charges I've gotten during that time:
  11/25   0.5kw
  11/26   0.3kw
  11/27   0.0kw
  11/28   0.1kw
  11/29   0.0kw
  11/30   0.6kw

As expected, the battery bank (12 volts @ 880Ah) has dropped over this period.
I unfortunately do not have a low voltage disconnect set up, something I will quickly rectify upon my return.
The inverter (Xantrax Prosine 1000) does have a built in LVD, but it does not kick in until 10 volts. Even when it does kick in, the only thing running on the inverter is the fridge.

Well, early this morning, it appears that the batteries crashed.
Here are today's early morning battery changes. Note what happens at 4:44AM, this is the part I am confused about:
00:00   11.4
01:40   11.3
03:00   11.2
03:36   11.1
04:15   11.0
04:35   10.8
04:44    9.3
05:00    9.2

I've attached a screen grab of this morning as a picture shows it better.

My question, any idea what caused the 1.5 voltage drop at that time?
The inverter LVD would have shut down the inverter right about then when the batteries hit 10 volts.
Nothing else in the house is on to pull that kind of load to eat up that power.

Any ideas?
Thanks!

So my 2 cents. Remember My Midnite plots every 10 minutes. So the top of the cliff (about 10.8 volts) and the bottom of the cliff (about 9.3 volts) are each a data point. So I would say without a shadow of a doubt in my mind, The fridge kicked on and the dead battery plummeted to well below 10v thus kicking off the inverter. Then it stayed there until the sun came up and started charging again.

[Halfcrazy]

Yes, thanks everyone for taking the time to think and respond.

My frustration came at many of the replies focusing on why the batteries went dead, which I was not asking.
I know full well why they went dead.

What puzzled me was that sudden, instant drop of 1.5 volts at 4:44 AM when I know there was no major load on.
I think I got the best answer with the post about batteries at that state of depletion at that temperature do unexpected things.

Thanks again!
-Alan

We posted at the same time but it was not an "Instant" drop. Those data points are 10 minutes apart and are 5 minutes apart on the classic. So they could be up to 14 minutes apart.

[zoneblue]

Your question is kinda like: why did my airplane hit the ground when i was flying at least (about) a quarter inch off the deck? Whereas we were like: how to make that plane run sweet at 30,000 feet. Lead acid batteries 101 says they need to be kept at higher SOCs for longevity. You cant rely on LVD because thats designed to protect the inverter not the bank. By the time LVD kicks in the battery is already grinding duralloy on the tarmac.

Yes, thanks everyone for taking the time to think and respond.

My frustration came at many of the replies focusing on why the batteries went dead, which I was not asking.
I know full well why they went dead.