As lead acid reaches near full charge, its lead sulphate plates are finished being converted back to lead and lead dioxide respectively. After this we reach what engineeres call the "gassing stage" or "overcharge" stage or what is more commonly known as the absorbption stage, which is where the cell potential rises enough where electrolysis can begin, here oxygen gas is essentially produced at the postive elecrode/electrolyte interface and in a VRLA that gas diffuses over to the negative electrode where it recombines at the negative interface using the incoming electrons and water in a series of reactions. This maintains the cells stotiometric balance. Wheras in an open wet cell the oxygen gas at positive and even hydrogen gas at negative bubble up through the water and vent out the caps resulting in water loss. In lead acid, the cell potential or emf is direct function of the electrolyte temp and concentration of sulphuric acid, the cells amper hour capacity is directly proportional to this concentration of acid. So as you lose water you gain acidity and a higher emf (why vrla have a higher ocv), but also the electrolyte becomes more acidic, too acidic, this is one of the fundamental achilles heels of lead acid and why its is such an awful chemistry and why for example flooded nicad blows it out of the water for cycle life and calander life. Its acid attacks its electrodes and thermodynamically its a rather unstable chemistry.
UPS batteries are usually always VRLA (sla/ usually agm), which means they are sealed completley and must be sealed to remain pressurized in order to work properley, if the internal cell pressure isnt right the recombination process inside won't work well and could result in overcharge which will then lead to a loss of water and thus reduced capacity/runtime. No way to access them to top up in any practical way, even more so since these batteries are cheap mass produced shit.
The problem is the electrolyte in a vrla is in a precise starved state unlike wet cell which the electrodes are complelety flooded, its absorbed into the glass seperators and the surface of the porous lead plates, and done so in a very fine precise way that would be completley disrupted if acessed from the outside. Too much water will effect the high perf nature by ruining the chemical balance at each electrode and the dissfusivity of the oxygen as it mirates away from the positive over to the negative, a disruption here will cause a stoichiometric imbalance which will probably lead to hydrogen being formed at the negative too which will outgas and slowly ruin the battery. while too little will lead to corrosion on the pos plate in particular, however this will be very unlikely to achieve.
Some wet lead acids are even sealed up to and require a bit of work to access, they too use a similar recombination process as vrla to deal with water loss from the overcharge stage. But are mostly found in engine cranking apps.
Flooded Cell (commonly called "SLI" = Starting Lighting Ignition) are a bit different.
If your 'car battery' has caps like that, it likely is intended to be topped off with distilled water; to replace losses from evaporation and electrolysis*.
SLI can be open flooded or agm, its just an outdated term, like dual purpose, leasure battery etc.
It should be noted lead-acid battery technology is over 150 years old and really has not changed much since it was invented. The biggest improvements, IMO, is simply in the purity of the materials used to make them.
But until Man can create perfection 100% of the time, impurities are still likely and it does not take much of one to start building a bridge in the gap between the plates. And once further (and inevitable) buildup of contaminants complete the bridge spanning the gap, a short is created. And that short typically results in excessive heat buildup inside the cell - which typically results in a loss of voltage output as well as excessive pressures inside the battery case.
Incorrect. There have been huge improvements, like the high perf of vrla since the 80s, and the advent of the gel deep cycle rivaling the best flooded deep cycle in cycle life. Actually there is still much we don't know about lead acid due to budget constraints. We know quality can be improived on but its hard, the direct operational electrochemistry is quite complex unlike say li-ion.
Contaminants are not normally a problem! The main causes of failure are sulphation, active material shedding which can cause a soft short, and pos grid corrosion, shorts can happen by shedded plate material flaking off, but in lead acid the shorts dont heat the cell, they just cause a cell to self discharge rather rapidly which kills the cell. made worse by the fact most don't know how to service them and abuse them with poor charging practices.
This is commonly seen (and felt) by swelling, a bulging battery, and one that gets quite warm as it tries, but fails to fully charge. This also puts additional strain on the charging system as it has the impossible task of fully charging a battery that cannot be fully charged.
This swollen battery condition is sometimes seen in UPSs when you try to remove the batteries and discover they are stuck inside the UPS battery compartment. That should raise some serious red flags but I caution, that would be for significant swelling, nor necessarily minor.
Thats NOT how it works! swelling is largely normal in some ways, like with the ends but not the widths. The reason the cell gets warm is simply too much current flow and any exothermic reactions. Since vrla are a dry cell, they have no liquid mass to sink the heat away and the recombination gasses which carry alot of heat cant escape out side like in a open wet cell.
There is no strain on the charging system, unless you are pulling too many amps from it that causes it to get hot, in the latter stage of charging the charger will be supplying very little amps. In the eyes of the charger it will simply see a cont load which may muddle up the charge alogrithim, but many use simply cc/cv circuits and can deal with it.
Losses by electrolysis is an issue, mitigated by having catalysts in the caps (to recombine the hydrogen and oxygen)
but I don't know how recent this technology is.
FAQ What are HYDROCAPS? HYDROCAPS are a catalyst battery cap designed to replace the conventional vent cap on a lead-acid or alkaline battery.
hydrocapcorp.com
Well inside a vrla there is no direct recombination as this would require energy input (making bonds) and balance is maintained largely by oxygen recombination at the negative alone and an oversized negative plate, but inside a sealed wet cell thats another story, you still get water looses, but this is mitigted by using lower charge voltages vs open wet cell, this means the lower voltages cannot be used to give the cell a good stir up (to correct stratification) and properly equalize all cells, this is why open cells last so long. Most sealed up wet batteries can only take <14.4V really.
Be really careful playing around with lead acid batteries, all kind of things can go wrong. A couple of guys ended up having one explode in their face when I did my national service. You can also get thermal runaway in them.
Thermal runaway may be considered a 'hot topic', but in this blog, we demystify what causes thermal runaway - and how to prevent it.
www.power-sonic.com
This is a very rare event that arises from poor practice, nothing more. The explosion results from the explosive hydrogen (h2) gas that builds up from overcharge or even drawing too much current being ignited by some spark like a dodgy battery charger circuitryor some short etc in poor ventilation. In proper setups with proper care, explosions just never really happen, still when you periodically equalize a wet cell you produce much more hydrogen gas and so forced ventilation here is mandatory.
Thermal runaway is a general term that refers to a feedback loop. In lead acid it refers to a situation where excess current is allowed to flow when the cell is fully charged which means exothermic reactions producing heat, this causes a slight drop in IR, which causes more current to flow and more recombination reactions producing even more heat, which causes a further drop in IR and so on until the electrolyte boils away the cell is ruined. In a vrla the pressure will build up and the water loss will be great which means reduced amp-hours followed by a physical ruined battery with possible nasty shorts. However this is very rare overall and usually happens from a malfunctioning charger thats voltage is allowed to rise too high, it wont happen on wet cell because the cell simply cant heat up like that, the water sinks the heat away to the surroundings and much of this heat leaves with the O2 & H2 gasses, the mass of electrolyte will just boil away slowly until the person notices. But in sealed vrla (agm & gel) with their dry electrolyte, these gasses stay inside the cell.
This situation is also present in sealed Ni-Cd & Ni-MH, only much much worse, which is why these are never float charged (charged with constant-voltage circuitry) as the voltage can be allowed to climb and ruin the cell. This is why ni-cd isnt used as an automobile starter, it would make an amazing battery if it did. Ni-Cd is the back bone of industry. Instead Ni based are charged using constant-current circuitry which is fine for the laymen, but CV can be done if the charging infrastructure is well set up with fail safes. Here, fires are possibly because any nearby commbustible materials might get hot enough.
Now this contrasts with Li-Ion, where the thermal runaway can lead to fires because of the flammable electrolyte solvent, the nasty exothermic reactions between the electrodes and electrolyte under electrcial abuse, and the fact li-ion produces its own oxidizer from inside-the oxygen that gets released from the postive electrode mainly. This is where too much current flows (usually from an internal short) which causes physical breakdown of say the oxide layer on the negative electrode, which exposes it to the electrolyte and a reaction happens that adds even more heat, which causes even more breakdown of the insides and even more nasty reactions and so on. At about 90-120C is where it starts, and once TR srarts, its impossible to stop, in Pb and Ni its very slow but in some li-ion it usually happens in an split second like when you crush an 18650 cell and it goes into flames. This is why all li-ion cells are limited to run at no more than 75-80C in extreme apps, the cells do have hard thermal protection, but nothing to stop an internal short.
Normal? Total nonsense. It is NOT normal at all! Enough said. I've given your nonsensical comments more attention than they deserve. 
You sure that is an "UPS" and not just a power block? If truly an UPS, where it normally just passes the AC from the wall outlet but kicks over to battery backup automatically during a power outages, that is a fantastic price.
