A COMPARISION OF AGM AND GEL SEALED LEAD ACID BATTERIES(zhuan)

There are several ways to make recombinant type sealed lead acid VRLA batteries.  The following focuses on the two most popular constructions : Gelled electrolyte and AGM type, Valve Regulated Sealed Lead Acid batteries.

The battery industry is moving away from the term “sealed lead acid” battery because it implies “totally sealed”.  This is not strictly true because all of the so-called SLA batteries have a one-way valve or vent usually one in every cell to relieve excess internal pressure.  The vent also serves to pressurize the cell to a present pressure, usually 1-5 psi, to aid the recombination process.  In order to better describe the battery and how it works, the industry is adopting the term “valve-regulated” or VRLA when talking about this type of battery.

Whilst discussing AGM and GEL batteries, we use the term “recombination”.  Recombination is the process whereby the gasses that are generated by the battery during the normal charging process are “recombined’ within the cell and are not released to the environment.  The oxygen gas generated at the positive plate migrates to form water.  Of course this means almost no gassing and no need to add water over the life of the battery.  The recombination process is not 100% efficient, therefore all sealed VRLA batteries do emit some gasses, however if generally is only about 1% the amount of an equivalent AH flooded cell.  For this reason, a sealed battery can never be used in a totally unventilated environment.   However, normal room ventilation is more than enough to safely use recombinant batteries.  No special ventilation or rooms are required.

AGM means Absorbent Glass Mat.  AGM batteries are also known as “starved electrolyte” batteries because, by design, the amount of acid in the cell is limited to provide a dry battery and recombinant characteristics.

AGM batteries are constructed by placing a piece of the sponge-like AGM material between each of the positive and negative plates of the cell.  This AGM material consists of a mat of very fine glass fibres that is about 95% by volume void space.  This mat is then about 95% saturated, with the residual 5% allowing oxygen gas migration paths to allow recombination to occur.

This 95% saturation also ensured a dry battery i.e.  a battery that has no free liquid to spill in the event of a case rupture.  The AGM mat also serves as the separator between the plates.

As far as the float service life of batteries are concerned,  the primary determinants are positive grid thickness and alloy composition.  As long as there are similar between the Gel and AGM batteries,  the float lives should be similar.

Cycling of batteries is another important area of comparison between Gel and AGM batteries.  Contrary to popular belief,  AGM batteries, when the plates are vertical., do experience a stratification effect.  As the acid is liquid, the high specific gravity acid coming out of the plate during recharge will tend to flow to the bottom of the cell creating stratification.  Equalization will not de-stratify an AGM battery.  Time for diffusion to occur will de-stratify the battery, however this will be a very long time.  Gel batteries do not experience any stratification whatsoever.  Because of the structure of the gel,  it absorbs the acid produced during recharge and allows it to diffuse throughout the Gel matrix.

Stratification or lack thereof, is not the only important factor in cycling of batteries.  Some other important factors are grid alloy composition and additive.  A calcium alloy AGM battery will have a relatively poor cycle life because the positive active material adhesion to the grid is relatively poor under cycling conditions.  The stress of deep discharging the battery, due to formation of lead sulphate which has a larger volume than lead dioxide and consequently tries to expand the grid, breaks the bond between the grid and the active mass, electrically isolating the active material causing a capacity loss.  Addition of antimony to the positive grid alloy substantially improves the bond strength and deep cycle performance at the expense of shorter float life due to greater grid corrosion.  In a Gel battery, the addition of a certain amount of phosphoric acid to the fully formed battery during the Gel fill process improves cycle life.  The phosphoric acid reacts as the grid / positive active material interface dramatically improving the material adhesion and thus the cycle capabilities of  the battery.  So far it has not been found possible to improve the cycle life of AGM batteries along similar lines using phosphoric acid.

Modular batteries, because of their long  rectangular shape are much less susceptible to thermal runaway because they have a relatively high surface area to volume ratio.  Because they can be stacked on their sides and use a proprietary lead tin alloy, their cycle life tends to be good.  The horizontal mounting of the element minimizes stratification effects and promotes good cycle life.

The industry is moving toward the use of valve-regulated recombinant batteries because of their low gassing and low maintenance requirement characteristics.  Each type of lead acid battery that exists: flooded, AGM or Gel, has its own unique characteristics that make them well suited for a particular application.  The one point always stressed is that there is no such thing as a universal battery.  This means that a single type of battery cannot cover the broad range of applications that exist in the real world.  Choosing the best battery for a particular application is usually not a difficult thing to do, but it can be an expensive mistake if the wrong battery is used.  Having the above information will make it easier to make the right decision.


GAS PRODUCTION IN Gel Battery

The question has been asked “can the Gel battery be used in a sealed enclosure” the answer is a very definite no.

Gel batteries in normal use give off a small amount of gas however in abusive condition i.e. if the batteries were to be charged at an excessively high voltage the gas will greatly increase.  For this reason the Gel battery should never be put in a sealed enclosure or a confined space.

An engineering test program was requested.  To determine the amount of gas evolved in a room from the Gel battery during a float charge of 2.30 vpc 13.8 volts at 20 C.

The results are as follows :

12-44 11.53 Litres /Year
12-60 18.47 Litres/Year
12-80 26.86 Litres/Year
12-100 31.54 Litres/Year
12-110 35.62 Litres/Year
12-200 65.70 Litres/Year


If the batteries were to be cycled or floated at a different voltage the gas evolution would change.

For a charge voltage of 2.35 VPC, 14.1 volts per battery at 20C the volume of gas evolved would be :-

12-44 23.06 Litres /Year
12-55 36.94 Litres/Year
12-80 53.72 Litres/Year
12-100 63.08 Litres/Year
12-110 71.24 Litres/Year
12-200 131.40 Litres/Year


GelCOLD TEMPERATURE TESTING

By means of an example to highlight the use of Gel in cold conditions below is details of the US Geological Survey Organization.  They have been using gelled batteries for remote survey equipment in the Arctic and Antarctic for quite a while with very good results.

They were using a 5 year design life 12-100 Gel battery 10 year design life.  The batteries were recharged using solar panels and during the winter were simply left outside in a snow bank until the following spring.

1. The cold temperature that the battery encounters will not harm the battery at all.  It will result in a reduced capacity so the battery will have to be sized properly.  The low temperature will not harm the battery even if it freezes, because the Gel has enough space available to accommodate the expansion caused by freezing, without that space the expansion will occur causing physical harm to the battery plates and case.  If a flooded battery is frozen, it typically is torn apart by the freezing / expanding process.  AGM type batteries can also be frozen with no harm for the same reason as Gel batteries.

2. Gel batteries are very well suited to low temperature applications.  Tests were carried out on a cold battery tested at –500F or –460C The battery performed very well actually providing more than 25% of rated capacity, substantially above the expected capacity of 15 to 20%.  Recharge was reasonably successful and no damage occurred to the battery.


The cold test of the above cell was performed for the US Government in order to qualify the battery to be used in Dew Line Installations in Northern Canada.  Gel batteries of this type are presently installed at these installations.

ADVANTAGES OF GEL BATTERIES
IN CYCLING APPLICATIONS

Many companies are wisely turning to batteries that have fewer problems and a longer life span.  Gel batteries are cost effective when used in cycling applications.  Following are technical details to justify this statement.

The Gel series is rated at 500 cycles to an 80% Depth of Discharge, 3 times greater than most AGM PV batteries.  Therefore, in theory, if the Gel battery were 3 times greater in cost, the cost per cycle would work out the same.  In practice the price is comparable and as such translates into a lower overall cost.

A very important point to consider is cell shorting.  Cycling batteries, particularly PV batteries tend to spend a fair amount of time operating in a partial state of charge condition.  This means that lead sulphate will exist on the battery plates for long periods of time.  Lead sulphate has the nasty tendency to “Harden” with time and results in a permanent loss of capacity.  This situation can happen with any type of  battery, but if this sulphation is allowed to continue for a long time, the sulphate can grow through the separator and contact the other plate causing a “sulphation short” Gel batteries, due to the presence of the gel and a separator resist this type of failure mode exceptionally well.

The Gel battery is also particularly good at recovery from deep discharge.  AGM and most flooded batteries can be over discharged to the point where it is almost impossible to recharge them.  This is because the electrolyte can be driven to almost pure water by the discharge and this leaves no ions in the solution to conduct current.  When the battery is placed on the charger, no charge current can flow through the battery to recharge the plates. The gel battery due to the tight structure of the gel, always has some ions legs to conduct charge current resulting in excellent recovery form deep discharge characteristics.

Another important advantage of the gel battery is that it is much more tolerant to overcharge abuse than AGM batteries.  Almost any loss of water in an AGM battery results in a loss of performance due to the “starved electrolyte” method of construction.  In a gelled battery there is commonly an excess of electrolyte that migrates into the plate area providing some water reserve.   In an abusive situation this reserve can help extend the lift of the battery.

Stratification is also an issue that needs to be addressed.  Contrary to popular belief, AGM batteries do stratify.  This is one of the reasons that AGM batteries do not cycle as well as a comparable Gelled battery.  Gel batteries do not stratify at all due to the hydrogen bonded structure of the gel.  The acid that is released from the plate during the charge process is absorbed by the Gel and not allowed to run down to the bottom of the cell.  No stratification results in a longer cycle life and lower self discharge.

A minor, but nonetheless important, feature of the Gel battery is that it has calcium alloy grids.  Calcium alloy batteries require less overcharge to become fully charged than antimony alloy batteries.  For recharge,  this can be a big plus.  A typical antimony alloy battery will require only about 4-7% overcharge and for shallow D.O.D’s even less.  Another significant point that shows the superiority of the Gel battery in cycling applications.

In conclusion gelled batteries cycle up to 3 times more that AGM batteries.  Gelled batteries resist sulphation better the AGM batteries, gelled batteries more easily recover from deep discharge gelled batteries are able to withstand overcharge better than AGM batteries, gelled batteries do not stratify which results in longer cycle life and lower self-discharge.  Gelled batteries, with calcium grids, have lower self-discharge rates than AGM batteries with antimony plates.  Gelled batteries are more able to withstand higher temperatures due to more electrolyte being available.

At present gelled technology is being used by most of major wheelchair manufactures on their wheelchairs.  Several of the handicapped “scooter” manufacturers are using gelled batteries.  The advantages are good cycle life, safety and low maintenance.  Other portable power applications include golf caddy (motorized golf cart) portable manlift, and portable lighting equipment.


GEL VS AGM BATTERIES IN
CYCLING APPLICATION

CYCLE LIFE  
The present small mono-block batteries using AGM technology, Absorbed Glass Matt, have a cycling life of approximately 100 to 150 cycles to 80% DOD, Depth of Discharge.  The Gel series in contrast is rated at 400 cycles to 80% DOD a factor 3 times greater.  Therefore in life cycle cost terms the Gel battery works out at a lower overall cost for the customer.

CELL SHORTING
Another important point to consider is cell shorting.  Cycling batteries, particularly PV batteries, tend to spend a fair amount of time operating in a partial state of charge condition.  This means that lead sulphate will exist on the battery plates for long periods of time.  Lead sulphate has the nasty tendency to “harden” with time and result in a permanent loss of capacity.  This situation can happen with any type of battery but what can happen if this sulphation is allowed to continue for a long time is that the sulphate “lump” can grow through the separator and contact the other plate causing what is called a sulphation short.  Gel batteries, due to the presence of the gel and a separator, resist this type of failure mode exceptionally well.

RECOVERY FROM DEEP DISCHARGE
The Gel battery is also particularly good at recovery from deep discharge.  AGM batteries can be over-discharged to the point where it is almost impossible to recharge them.  This is because the electrolyte can be driven to almost pure water by the discharge and this leaves no ions in the solution to conduct current.

When the battery is connected to the charger, no charger current can flow through the battery to recharge the plates.  The Gel battery due to the tight structure of the Gel always has some ions left to conduct charge current resulting in the excellent recovery form deep discharge characteristics.

BATTERY ABUSE
Another important advantage of the Gel battery is that it is more tolerant to overcharge abuse than AGM batteries.  Almost any loss of water in an AGM battery results in a loss of performance due to the so-called starved electrolyte method of construction.  In a gelled battery, there is commonly an excess of electrolyte that migrates into the plate area providing some water reserve.  In an abusive situation, this reserve can help extend the life of the battery.


CALCIUM ALLOY GRIDS
Another feature of the Gel battery is that it has calcium alloy grids.  Calcium alloy batteries require less overcharge to become fully charged than antimony alloy batteries.  For PV applications where there is limited time and energy available for recharge, this can be a big plus.  A typical antimony alloy battery will require about a 10%, and sometimes more, overcharge to get fully charged.  A Gel battery will require only about 4-7% overcharge and for shallow DOD’s even less.  A significant point in that it shows the superiority of the Gel in certain applications.

STRATIFICATION
One of the reasons that AGM batteries do not cycle as well as a comparable gelled battery is that AGM does stratify.  Gel batteries do not stratify at all due to the hydrogen bonded structure of the Gel.  The acid that is released from the plate during the charge process is absorbed by the Gel and not allowed to run down to the bottom of the cell.  No stratification results in a longer cycle life and a lower self-discharge.