timematcher

When load is applied to the 12 volt battery (either through the Inverter/UPS or via a battery jerk tester) , following things start happening in a shorted cell:
Lots of bubbles start coming out of the shorted cell(s) The overall voltage falls by the factor of each of the shorted cells. So if one cell is shorted , the faulty battery's overall voltage will fall down to 10 or 9Volts immediately while the other fully charged, non-faulty battery (in a 24 volt combination) will still show 12.xVolts on load and 13.x volts without load. If you have a single battery of 12 Volt , you can not do a comparison like above but you may see lots of bubbles coming out of one or more cells on load which would tell that particular cell is shorted. You can use carbon Rods (the one extracted from regular AA Cells), hold then with some metal clamper, and use a multimeter to test each cell by dipping the carbon rod(s) into each respective cell and comparing voltage between each cell. Please use this as last confirmation test and dip only for a short period of time as the sulphuric acid in the battery may start dissolving the carbon and introduce impurities in the battery. In an ideal fully charged, battery with no short, each cell may read 2.2Volts and the whole battery should read 13.x Volts without load. On Load, the battery should read 12.X volts overall, and each individual cell should read 2.1 or so volts. If you have a battery tester such as the following (shown in a screenshot), you do not have to perform the above steps but the above steps are more fine tuned way of testing the battery and specifically which cell is shorted. Hope it helps.

I am going answer them in reverse order.
Battery cut-off voltage - At this voltage the UPS/inverter will shutdown to protect the batteries. The default setting (21V) is value is too low but changing it messes with the Axpert's software/SOC readings.
Back to grid voltage  - at this battery voltage the unit will force the load to be supplied by AC.
Back to discharge voltage - Once this voltage has been reached the unit will allow the load to be supplied by battery.
These two settings are used in conjunction with your output source priority. Since you are set with Utility priority (rightly so) they should not have any bearing until you have PV panels and that setting is either SBU or SOL. However I have seen stranger things and these may be active when they should in fact be inactive.
You need to set it to Flooded (which will then have 29.2V for bulk and 27V for float). If you set to User then you can choose the correct setting if you find that the Phoenix batteries require a slightly different bulk and float settings. Each setting you change you must click Apply on the button or it just reverts to the previous setting.

The inverter has several selectable programs and some of them allow you to implement a kind of self-consumption. This is specifically when you have solar panels. The inverter will then disconnect the grid (even though it is available) and run from the batteries and incoming PV during the day, while there is sun.
The "Back to" voltages control this. When the battery voltage hits the higher one, it disconnects the grid. When it hits the lower one, it reconnects the grid. If you have no PV, then these don't apply.
The last one, battery cut-off, is a safety feature. This is the voltage where the inverter will turn off to avoid damaging the battery. For a 12V battery this voltage is usually 10.5V, so 21V is a good voltage to use for that.

Morning Awais
I am not familiar with the Alpha P3000/24 and but it is clearly a Voltronic (or a Voltronic clone) product.
 
Three issues:
I don't like that your voltmeter readings and readings from the UPS software differ. Having been repaired once I think you internal voltage module on the UPS has been damaged and is giving inaccurate readings.  The voltronic software is notoriously poor using voltage as a determination of state of charge. Ignore the SOC in the software and look at the LEDs on the unit (if it has any). The Axpert unit (which is related to your unit) has an LED that flashes whilst charging and goes on (lights up but stops flashing) when fully charged.Alternatively you can roughly see that the batteries entering float charge when the battery bank accepts 3% of the bank's Ah rating. So 3A on a 100Ah battery bank.   The recuperation from 22.78V to 28.21V is too quick for my liking. What size are the batteries?  
Set to flooded. "Lead Acid water maintainable batteries" are "flooded batteries". Better still is if you have the datasheet for your batteries you can set battery type to user (if this is an option) and then manually set to the required bulk and absorb voltages as determined from your batteries's datasheet.
I am concerned that you are discharging your batteries too far. One should not really ever go below 24V which is roughly 50% depth of discharge (DOD).
 
 

The product looks legit, they do have a model like this one. But the copy's looks legit too.
 http://www.voltronicpower.com/oCart2/index.php?route=product/product&path=24&product_id=174
Just for interest sake. 
Manual if someone needs to check anything.
http://www.voltronicpower.com/oCart2/files/manual/ALFA 3K&5K manual 20150708.pdf

Set max charge current to 20 amps.
Can't see what the voltages should be. Tried googling for that spec sheet, only found the XP230, which appears to be flooded lead acid. So best I can do is go with generic voltages.
Set absorption voltage (aka bulk charging voltage) to 29V.

Set float voltage to 26.5V.
Battery cut-off at 21V is fine.
I would try to increase the back to grid voltage, though that depends on your load. With my load, a 50% DoD would correspond better with 24.2V (but I have AGM batteries). 23V seems a bit low. You may have to experiment.
Other forum members might disagree with the above voltages, but I think everyone will agree 28.2V is too low for absorption (aka bulk charging). More common voltages are between 28.8V and 29.6V.

Morning Awais
From looking at your settings.
Set back to discharge voltage to Full.
So it looks like your battery bank is a 150Ah bank and therefore you should not be charging at greater than 15A. Since Axpert only goes up in increments of 10A that means you are limited to 10A both max charging rate and max AC charging rate. (Please confirm that you have no solar panels installed and charging only happens via AC). At the moment you have it set to 50A which is way too high. You will damage your batteries.
Since you have light loads I would make back to grid at 24V. (Roughly a 50% depth of discharge (DOD). Anything more than this will seriously compromise your lead acid battery bank's longevity. 
What I suspect is happening is that the battery is discharging and then discharging again without every being fully charged as you back to discharge is set at float and not a higher value. These units manage to do some odd things and what you want is the unit to be in bypass mode with your batteries fully charged and to use battery power only when there is no AC power from the grid.
 

The standard settings for lead acid and a Voltronic unit is 29.2 V for bulk and 27V for float. 
Just as a comment how did a Pakistani company decide on Phoenix as a product name. I certainly would not like my batteries to be "rising from the ashes".

Victron has an inverter range called Phoenix :-) But that is because they use names from Greek Mythology. Hence Phoenix, Centaur, Lynx, Argo... The exception is their version of Linux on the CCGX which is Venus. That's from Roman mythology.
:-)