Brief note on VRLA technology
|The electrode reactions in all lead acid batteries including VRLA batteries are basically identical. As the battery is discharged the lead dioxide positive active material and the spongy lead negative active material both reacts with the sulphuric acid electrolyte to form lead sulphate and water. During charge, this process is reversed. The coulomb efficiency of the charging process is less than 100% on reaching final stage of charging or under over charge conditions, the charging energy is consumed for electrolytic decomposition of water and the positive plates generate oxygen gas and the negative plates generate hydrogen gas.
Under typical charging conditions, oxygen at the positive plate occurs before hydrogen evolution at the negative. This feature is utilized in the design of VRLA Batteries. In flooded cells, the oxygen gas evolved at the positive plate bubbles upwards through the electrolyte and is released through the vents. In VRLA batteries the oxygen gas evolved at the positive instead of bubbling upwards is transported in the gas phase through the separator medium to the negative plate. The separator is a highly absorbent glass mat type with very high porosity designed to have pore volume in excess of the electrolyte volume (starved electrolyte design), due to which the oxygen gas finds an unimpeded path to the negative plate. Reaction with the spongy reduces the oxygen gas Lead at the negative plate turning a part of it into a partially discharged condition, there by effectively suppressing the hydrogen gas evolution at the negative plate. This is what is known as the oxygen recombination principle.
The part of negative plate that was partially discharged is then reverted to original spongy lead by subsequent charging. Thus a negative plate keeps equilibrium between the amount which turns into spongy lead by charging and the amount of spongy lead which turns into lead sulphate by absorbing the oxygen gas generated at the positive plate. The oxygen recombination principle can be shown by the following reaction mechanism.
from the above equation it can be seen that the reaction is reversible and based on which the lead acid battery is classified as secondary battery which can give no. of discharge and charge cycle. During discharge the lead dioxide in positive plate and spongy lead in negative plate react with sulphuric acid in the electrolyte to from lead sulphate both in positive and negative plates and water in the electrolyte. The chemical reactions for the same are shown below.
What is Shelf life of VRLA battery Consequences of prolonged storage of battery with out freshening charge. Why freshening charge is required for VRLA battery?
A VRLA battery comes in fully charged condition. For any battery Shelf discharge is common observed phenomenon. During self-discharge the active material on the plates gets a converted into sulphate that is discharge compound. This is called “sulphation” which means the formation of lead sulphate on the surface and in the pores of the active material of the plates. The reason for this is as follows.
Lead sulphation is formed as a result of local action or self-discharge of the plates. This happens by the action of the acid solution on the active material of the plates. Sulphation is a necessary part of the operation of battery and is not a source of trouble. The rate of sulphation depends on the concentration of the electrolyte and the ambient temperature.
This sulphation of plates will reduce performance of the battery drastically during service if it is not treated properly. This can easily be reduced/removed by charging the batteries at a low rate of current (say 2% of Ah capacity) for a prolonged duration of 60 hrs. If the batteries are stored for more than the specified period, it is strongly recommended that they should be charged as per the above before putting in to service.
Henceforth it is recommended that once in six months the battery shall be given freshening charge if they are connected to Load.
For freshening charge details pls. Refer O & I manual.
External factors effect on Life of V R L A battery
The life of the VRLA batteries like any other battery depends on various parameters like depth of discharge, charging voltage, ripple content, voltage regulation, operating temperature, nature of application, monitoring procedure followed etc., The effect of each of the above parameters have been briefly described below.
|Conventional Batteries Vs VRLA batteries
|Other intangible benefits include in VRLA batteries:
|Comparison between SMF- VRLA and SMF-Automotive Batteries
|Life -Even in ideal start-light-ignition the primary application SLI batteries are intended for life is less than 36 months.|
Comparison of Ni-Cd Batteries Vs VRLA Batteries
Heat generation from battery bank during Float & Boost mode -calculation
Boost mode the heat generation in Watt-Hrs for 2V cell.
|Amount of Hydrogen gas that will evolve from the battery during trickle mode.
The H2 gas evolved form a battery bank =
A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%.
Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about 1.5 times as long as if it is cycled to 80% DOD. If cycled only 20% DOD, it will last about 2 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the positive plates rather in an even film.
AGM batteries have several advantages over both Gelled and Flooded, at about the same cost as Gelled
Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.
Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.
The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents.
AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Power stack batteries can be almost fully recharged (95% or better) even after 3 days of being totally discharged.
AGM's do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in AGM's are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.
Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs these features.
Why Aging factor for Battery sizing calculation
The performance of any lead acid battery is relatively stable throughout most of its life, but begins to decline with increasing rapidity in its latter stages. The decline will be very drastic once the capacity drops to 80% of its rated capacity and there will be little life to be gained by allowing operation beyond this point.
In order to ensure that the battery meets the given duty cycle even at the end of its life (i.e. at 80% performance level) it is a prudent practice to consider a factor of 1.25, which is normally referred to as ‘Aging Factor’ or ‘Life Factor’ . But it is not necessary that the battery be replaced only when its capacity reaches 80%, and it can be done even at higher values of, say 85% or 90%. In such cases the aging factors to be considered will be 1.17 or 1.10 etc. respectively.
The International standard IEEE Ltd., 450-1995 states that, “The recommended practice is to replace the battery if its capacity as determined in 6.5 is below 80% of the manufacturer’s rating if the battery was sized using a 1.25 aging factor. If a lesser aging factor was used, battery replacement will be required before 80% capacity is reached to ensure that the load can be served (consult the battery manufacturer).”
The timing of the replacement is a function of the design/sizing Criteria utilised and the capacity margin available, as compared to the load requirements. A capacity of 80 % shows that the battery rate of deterioration is increasing even if this is ample capacity to meet the load requirements of the DC system.
Ageing factor one should consider while sizing the battery Ah capacity depending upon the end of life capacity specified by the end user.
Monitoring chart for VRLA batteries Maintenance recommendations for VRLA battery.
How do I get optimum life of VRLA battery?
What is the maintenance that has to carry on VRLA batteries.
|Note: Maintain the Battery records with out fail. Follow the O&M manual instructions on further details and instructions.
|AC ripple voltage and current effects on battery performance.
The achievement of optimum life form a VRLA battery system can also be related to the quality of the DC output voltage of the charger. The output should be as pure DC as is practical for the DC output voltage of the charger. If the output contains a significant AC component can cause additional heating of the battery. If the AC component is sufficiently large, during a portion of the waveform the charging voltage could actually dip below the battery OCV and slightly discharge the battery thus affecting the battery active materials. An excessive AC ripple effect would be, while the DC helps the battery plates for conversion of the active materials through the main reaction, the AC component (i.e. the ripple content) leads to side reactions. One of the major side reaction is hydrolysis of water thereby liberating hydrogen and oxygen gases in addition to the hydrogen and oxygen gases liberated from the main reaction. The gases thus liberated from the main reaction recombine to form back as water in a VRLA battery due to the oxygen recombination principle. The gases liberated from the side reactions increase the cell internal pressure increases beyond allowable pressure value the `safety valve` opens and releases these excess gases into the atmosphere. Thus the batteries are subjected to loss of water, eventually results in premature capacity loss.
Further, due to the availability of abundant quantities of nascent oxygen gas near the grid structure, the rate of corrosion of the grid increases drastically, thereby resulting in reduction of the service life of the batteries.
For best results, the AC ripple voltage on the charger output should be less than 2% p-p (peak to peak) of the battery DC charging voltage to ensure that the battery will not be “cycled”.
The AC ripple voltage will induce an AC ripple current and the value of this current will be related to the value of the voltage and the relatively low impedance of the battery (I=E/R). This AC ripple current will cause additional heating of the battery, which could affect the battery life, if significant. The AC ripple current should be limited to 0.05C for best results. For example, a 100 ampere-hour capacity © battery should experience less than 5 AC amperes ripple current for best results.
C10 Capacity Test Procedure
10 hr capacity discharge testing method:
The procedure has been prepared by considering with power stack modules, the same can be used for Amaron QuantaTM batteries.
The following tools are required to test the batteries.
|Battery to be replaced when ever it fails to deliver less than 80% of the rated capacity.|