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Real World Effects on VRLA Batteries in Float ApplicationsCo-authored by Joe Jergl, Bruce Cole, and StevePurcell

GNB Technologies, Lombard, IL 60148

ABSTRACTThis paper explores some effects that todays operatingenvironments have on valve-regulated, lead-acid (VRLA)batteries and gives suggestions for maximizing VRLAbattery life in these environments. Specifically, this paperdiscusses he effects of temperature and float voltage onfloat current and positive plate and relates these effects tothe expected ife of the battery.

1. INTRODUCTIONThe lead-acid battery is an electrochemical plant thatfunctions by slowly consuming itself. Whether the battery isflooded or valve regulated (VRLA), proper operatingconditions are paramount to maximizing battery life.However, VRLA batteries have characteristics that makethem even more sensitive to proper operating conditionsthan flooded batteries. In fact, several failure modes exist

which are unique to VRLA batteries relative to floodeddesigns.

Theseunique failure modes nclude negative strap corrosion,dry-out, (due to water loss via vent operation, normalpositive grid corrosion, improper charging, and water vaporpermeability of containers), and loss of compression n theabsorbent glass mat (AGM) separator material. Othertechnical papers presented in the past have discussed thefirst two of these failure modes and they are fairly wellunderstood. The AGM compression oss is a relatively newdiscovery and has been covered by GNB in other papersandworkshops. We will continue to report our findings horn

our ongoing investigating of this issue.

While VRLA batteries have some unique failure modes,they also offer significant benefits versus looded batteries:l they give off much less hydrogen gas. somecan be installed in any orientation. they do not require any regular electrolyte maintenance.

The good news s that, owing to these unique characteristics,VRLA batteries can be used in environments where floodedbatteries ypically cannot be used such as Huts, CEVs, anddistributed cabinets. The bad news is that theseenvironmentsoften presentmuch harsher conditions than the

central office where flooded batteries are used.

The central office environment is characterized bycontrolled temperature, controlled float voltage, filteredcharging current, and regular maintenance. In these newenvironments, temperature extremes and variability,uncontrolled humidity, dust, dirt, and lack of ventilation,can all conspire o shorten he life of a VRLA battery.

The combination of VRLAs additional failure modes andthe harsh environments in which they are used presentseveral new problems for battery designers. Specifically,striking a balance between the need to prevent thermalrunaway, the need for proper positive plate polarization, andthe need to avoid sulfation of the negative plate, is a keyissue which must be carefully considered in order tomaximize VRLA design ife.

At the same ime, it is essential for the user community tounderstand the impact of these new environments. Bymaking sure that the operating conditions are as close toidea1as possible, the user can increase he useful life of thebatteries.

The key variables which must be controlled are temperatureand float voltage. This paper discusses he effect of thesetwo variables on float current and plate polarization and howto balance contradictory needs n order to maximize the lifeof VIUA batteries.

2. PURPOSE OF FLOAT VOLTAGEFirst of all, lets define the importance of proper floatvoltage in general terms. A constant voltage float charge sapplied to a stand-by battery in order to ensure that thebattery is at full state-of-charge SOC) capacity when it isneeded.

SOC is defined as the batterys actual ability to deliver

current at a particular point in time relative to its ratedcapacity. For example, a 90% SOC means a battery willdeliver 90% of its rated capacity if it is dischargedwhile atthat SOC.

So the main purpose of the float voltage is to keep thebattery at a 100% SOC so it will deliver thedesired amountof current or power, for the desired period of time, to aspecified end voltage.

However, as stated earlier, a battery functions by slowlydestroying itself. Specifically, the normal wear-out modefor a lead-acid battery is positive grid corrosion which

occurs during battery charging.

So the key to proper float voltage is keeping the battery fullycharged (which is function of float current) whileminimizing grid corrosion (which is a function of positiveplate polarization) in order to maximize the health and lifeof the VIUA battery.

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3. RESPONSE TO CHARGING CURRENTVRLA batteries respond differently to a charging currentthan do their flooded counterparts. Specifically, when acharging current is applied to a flooded cell, the followingeventsoccur (see Figure 1):. cell voltage increases;. positive plate potential increases;. negative plate potential increases.

oltageI

Cel

Figure 1: Simplified Tafel Curves for Flooded Cells

The responseof a VRLA cell to a charging current is similarexcept in one important aspect. As shown in Figure 2, thecell voltage and positive plate potential both increaseas withflooded batteries, but the negative plate potential remainsdepresseduntil the charging current value becomes verylarge.

r: oltage

+mV

0.C.L-mL

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Figure 2: Simplified Tafel Curves for VRLA Cells

To understandwhy this is important, we need to talk aboutopen circuit voltage (OCV). The OCV of a battery isdefined as the voltage across the terminals of the batterywhen nothing is connected o it.

At open circuit, the plates have a potential which is afunction of the specific gravity of the electrolyte in the

battery. The OCV of the cell is the sum of the individupotentials of the positive and negative plates.

When the charging current applied to the cell reaches certain level, the positive and negative plates begin tobecome polarized relative to their open circuit potentialThe polarization does not occur immediately because hinitial current goes to replace current lost due to sedischarge.

These polarizations cause he voltage of the cell to rise suchthat it is equal to the cells OCV plus the positive annegative polarization levels.

For example, say a flooded cell has an OCV of 2.06 and 2.17 float charge voltage is applied to the cell. The cevoltage is now 2.17 so there is a total of 1lOmV opolarization between the positive and negative plates. Fothe sake of discussion, lets say that the positive platpolarization is 65mV so the negative plate polarization 45mV (refer to Figure 1 to see hat this is reasonable).

Recall that, for VRLA cells (see Figure 2), the negative plapolarization remains low until the charging current valbecomesvery large. To relate this to the example aboveassume hat a VRLA cell has an OCV of 2.15 and a 2.2float voltage is applied. Like before, there is 110 mV totpolarization so the individual polarizations still have to aup to 11OmV.

Referring to Figure 2, assume hat the negative polarizatiois only 5mV instead of 45mV. This means hat the positivplate polarization must now be 105mV which is much high

than the 65mV we saw n the flooded example.The point is that, relative to the negative plate polarizatiothe positive plate polarization is much higher in VRLbatteries than in flooded batteries. As we shall see laterpositive plate polarization is directly related to the rate which the positive grid corrodes.

But before we get there, lets take a closer look at whahappens nside a VRLA battery on float charge.

Besides eplacing current taken out of the battery during andischarges, he charging (float) current also supports severa

competing chemical and electrochemical eactions.Specifically, the float current must also be high enough treplace current lost due to self-discharge. At the same imethe normal corrosion of the positive grid and the oxygerecombination processalso consumecurrent.

So the charging current must be high enough to compensatfor all three other reactions (besidescharging the battery) inorder for the battery to remain at a full SOC.

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At an optimum constant float voltage and temperature, heVRLA battery will draw as much current as it needs tosupport all of these internal reactions and keep the batterycharged.

But float voltage and temperature are not always wellcontrolled so the current actually drawn by the battery willvary and can becomedetrimental to the batterys health andlife outside of certain limits.

The trick is defming certain limits for both float voltageand temperature. The answer is neither obvious nor simplesince they both affect plate polarization. They also bothaffect float current which has a direct impact on thermalmanagement ssues. The following sections explore theindividual impacts of float voltage and positive platepolarization and then their combined effects.

4. OPTIMUM FLOAT VOLTAGEThe optimum float voltage for a VRLA battery is that which:. allows for a float current which will compensate or all

side reactions. results in a positive plate polarization value at the

minimum positive grid corrosion rate

4.1. Float Voltage Effects on Float CurrentFigure 3 shows an example of float currents which wouldresult for a GNB Absolyte IIP for various float voltages at25C. It is clear in this example that at 2.23 VPC andabove, the float current is sufficient to overcome self-discharge and still provide the current required by thepositive grid corrosion and oxygen recombination processes.

Figure 3: Float Voltage Effects of Float Current at a Fixed Temperature

2.21 2.23 2.25 2.27float voaape (VPC)

2.30 2.35

The chart also shows, or this example, hat at 2.2 1 VPC, thefloat current is not sufficient to support all of thesereactions. Since the grid corrosion and oxygenrecombination processeswill draw as much current as isavailable, there will not be enough current left to overcomeself-dischargeand the battery will never reach a full SOC.

At the other end of the scale, a too-high float voltage of 2.35will provide much more current than is needed. The excess

current will go toward the oxygen recombination processwhich is an exothermic reaction. The result is that theexcess current will be converted to heat and the internaltemperature of the battery will continue to increaseunchecked see hermal runaway discussionbelow).

4.2. Float Voltage Effects on Positive Plate PolarizationThe detailed dynamics of plate polarization are quitecomplex and beyond the scope of this paper. However, adiscussion of positive plate polarization (PPP) is necessary

to understand he effects of float voltage and temperatureonbattery life. For our purposes, the reason that PPP isimportant is that it has a direct impact on positive gridcorrosion. Figure 4 shows he correlation betweenPPP anda grid corrosion rate acceleration factor which is ameasure of how fast the grid will corrode relative to anoptimal PPP evel which has a factor of 1.0.

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I i.500 ----:-----I .-~---:-----:-----:--~ 4

. , I / 1 / /.4.000

3.500

3.000

2.*00

2.000

1.500

1 ow0 50 too 15.0 200 250 loo

PPP (ml,

Figure 4: Positive Grid Corros ion Acceleration vs. (+) Plate Polarization

One should observe that there is a range around an optimalPPP level (which is at the bottom of the curve) where grid

corrosion is not significantly accelerated.

The exact optimal level varies depending on specific designdetails but, for the sake of this discussion, we will assumethat 75mV is optimal and a range of 40mV to 120mV isacceptable. The most important thing to recognize fromFigure 4 is that once the PPP level deviates beyond thisrange - either higher or lower - the rate of corrosionaccelerates apidly.

So, given that proper PPP is important becauseof its effecton grid corrosion, how does float voltage affect PPP?Figure 5 (next page) shows an example of PPP levels for

various float voltages at four different temperature evels.At a float voltage of 2.25 VPC, we see that the PPP levelsfor all four temperaturesare within the acceptable ange.

At the low end of the float voltage range, we see that webegin to reach he low end of the acceptablePPP range. It isalso clear that, in this example, a float voltage above 2.27causes he PPP o quickly exceed he acceptable ange at thelowest temperature.

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-1SC -2OC -25.C-m-WC 7x--45c

Float Voltage [V)

gure 5: Example of Float Voltage Effects on Positive Plate Polarization

This is why battery manufacturers specify an acceptablefloat voltage range and a recommended alue at a particulartemperature. For example the GNB Absolyte IIP family hasa recommended loat voltage of 2.25 volts per cell (VPC) at25C with an acceptable range of 2.23VPC to 2.27VPC.

The acceptable ange \;indow reflects an expected accuracyof the rectifier voltage control at *I% of the setting.

5. OPTIMUM TEMPERATURELead-acid battery manufacturers use a temperaturespecification which is normally 25C (77F) or 20C (68OF).All electrical ratings and performance specifications arebased on this specified standard emperature. Furthermore,the design life of the batteries is specified at this standardtemperatureand the design life is cut approximately in halffor every 10C (1YF) that the ambient operatingtemperature ises above hat optimal temperature.

Unfortunately, increasingly few places where lead-acidbatteries are used are controlled to exactly 25C. Mostbattery manufacturers ealize this and generally accept anddesign or seasonal emperaturevariations to somedegree.

15-c 2-z-c 2x WC WC WC 4scTemperature

Figure 6: TemperatureEffects On Floa t Current at a Fixed Float Voltage

Problems for VRLA batteries can occur when the averagetemperature s higher or lower than 25C, if the temperaturevaries by more than +lOC from the average, and/or if thetemperaturevariations do not follow a sine wave over time.These problems are exasperated if float voltage is nottemperature compensated. The temperature/float voltagerelationship will be discussed ater inthis paper. But first,we need o take a closer look at the effects of temperatureonfloat current and positive plate polarization.

5.1. Temperature Effects on Float CurrentFigure 6 shows he effect of temperatureon float current at afixed float voltage. It is clear that increase n temperatureleads o a dramatic increase n float current and, conversely,as temperaturedrops, float current drops off rapidly.

For example, at 45C we see that there is a great deal ofexcess current going into the battery. While the currentdemands of self-discharge and positive grid corrosion areincreasing, there is more than enough current to satisfy theirneeds and all the excess current goes into the oxygenrecombination process.

As discussedabove, this process s exothermic so the excesscurrent results in the generation of significant heat. Thisheat causes the temperature of the battery to rise evenfurther so the battery draws even more current whichgeneratesmore heat which causes he battery to draw morecurrent . . . (see hermal runaway discussion below).

At the other end of the scale, we see that a temperatureof15C results in a float current which is insufficient tosupport the chemical and electrochemical processes

necessary or the battery to function.The result is that the battery will never reach a full SOC andthe plates will eventually sulfate to the point that the cell isno longer useful for its intended purpose.

5.2. Effects on Positive Plate PolarizationFigure 7 (next page) shows the effect of temperature onpositive plate polarization (PPP) at various float voltages.

Observe that at the optimal temperature of 25C the PPPlevels are within the acceptable range at all of the floatvoltages shown.

At the extremes of the temperature range, we see that,depending on the float voltage, we begin to push the PPPlevels to the edges of the acceptable range so that positivegrid corrosion is accelerated.

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0 1115C 2o'c 25C 3oC xx 45%

T*mpar.tre-2.21 +223 -225 -227 -2.30 -23

Figure 7: Example of Temperature Effects On Positive Plate Polarization

6. COMBINED EFFECTSSo far we have discussed he effects that variations in floatvoltage and temperature have on float current and positiveplate polarization individually. We now need to look attheir combined effects and then answer the next logicalquestion: What does his all mean n terms of battery life?

After all, thats what really counts.

For the purposes of this section, Figure 8 shows expectedlife of a GNB Absolyte IIP battery for various float voltagesand temperatures. The following paragraphs explain theeffect on life of several different temperature-float voltagescenarios.

It must be emphasizedhere that this chart assumes hat thefloat voltage and temperature are heldconstant throughoutthe life of the battery. Although this is not a real-worldassumption, he chart still serves o illustrate the theoreticaleffects of temperatureand float voltage on life.

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I-i*1vFc**23vFc -2&R: -&227"kEL3omj35-C

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Figurel: GNB Absolyte IIP Life VS Temperatures & Float Voltage

6.1. Low Float Voltage & Low TemperatureEffect on Positive Plate PolarizationLow float voltage decreases positive plate polarization(PPP) while low temperature ncreases t. Although it wouldseem easonable o assume hat these wo effects cancel eachother out, this is not the case.

In fact, a small percentagedecrease n float voltage leads oa fairly large decrease in PPP while a large percentagedecrease n temperature eads to a relatively small increasein PPP.

The result is that, although the low temperature partiallyoffsets the low float voltage, you still end up with significant decrease n PPP. The risk is that, if this decreasein PPP s too large, it can lead to acceleratedgrid corrosion.

Effect on Float CurrentLow float voltage and low temperatureboth cause ow floatcurrent. This means he battery will not have enough currentavailable to it to support its internal reactions so it will nevereach its ml1 state of charge. Also, the negative plates willsulfate leading to a shortened ife.

Effect on LifeReferring to the GNB Absolyte TIP example in Figure 8, wsee that at the lowest float voltages shown (2.21 and 2.23VPC) and the lowest temperatureshown (1 SC), the life is ata minimum. Low float voltage combined with lowtemperature s very detrimental to VRLA battery life.

6.2. Low Float Voltage & High TemperatureEffect on Positive Plate PolarizationBoth of these conditions cause educed PPP which can leadto acceleratedgrid corrosion.

E#ect on Float CurrentLow float voltage decreases float current while hightemperature ncreases t. A lower float voltage can partiallycompensate or the higher temperature (or vice-versa) tohelp keep float current within an acceptable ange. This isone of the fundamental principles behind temperaturecompensation see Section 6.5).

EfSect on LifeAlthough the combination of low float voltage and hightemperature s not ideal, it will actually result in longer lifthan if both variables are low (see Section 6.1 above andSection 6.5 on temperaturecompensation).

This is shown in Figure 8. For example, by examining the2.21 VPC line, it can be seen hat life expectancy s greaterat 30C than it is at 25C. However, it should also be notedthat as soon as the temperature goes above 30C life

expectancy begins to drop off rapidly. This says hat thereare limits to temperaturecompensation see Section 6.5).

6.3. High Float Voltage & Low TemperatureEffect on Positive Plate PolarizationThese conditions will both increasePPP which could lead toacceleratedgrid growth.

Effect on Float CurrentHigh float voltage increases float current while lowtemperaturedecreases t. Similar to the situation in 6.2, the

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higher float voltage can partially offset the reduction in floatcurrent due to low temperature (see temperaturecompensationdiscussion n Section 6.5).

Effect on L$eAgain, although this combination is not ideal, it is betterthan if both variables are either high or low. The is shownin Figure 8 where we see hat the higher float voltage lines(2.27 VPC for example) result in longer life at lowertemperatureshan they do at higher temperatures.

At the same time, it can be seen that at low temperature(15C for example), relatively longer life expectancies areachieved at higher float voltages than they are at lower ones(see Section 6.5).

6.4. High Float Voltage & High TemperatureEffect on Positive Plate PolarizationThe high float voltage will increase PPP while the lowtemperature will reduce PPP. However, the high floatvoltage increases PPP more than the high temperaturedecreases t. The net effect will be increased PPP andaccelerated rid corrosion.

Effect on Float CurrentThe more concerning aspect of these conditions occurringtogether is that they both increase loat current. The excessfloat current will generate heat which raises temperatureeven further (see hermal runaway discussion below).

Effect on LifeLike the combination of low float voltage/low temperature,the combination of high float voltage/high temperatureresults n very low life expectancies seeFigure 8).

However, the combination of high float voltage and hightemperature s even worse because t could eventually leadto thermal runaway and the rapid destruction of the battery.

6.5. Temperature CompensationIn applications where the temperature s not well controlled,benefit can be gained by adjusting the float voltage basedontemperature.

For example, if the operating ambient is expected toconsistently be below 25C a higher float voltage will offsetsomeof the negative effects of lower temperature see 6.3).

Conversely, if the operating ambient is higher than 25C, alower float voltage may be appropriate o help keep the floatcurrent within an acceptable ange.

There are, however, limits to temperature compensation.For instance, if the temperature is very low, you cannotsimply raise the float voltage to a very high level tocompensate for the low temperature because the floatcurrent will go too high. This will result in excessive

gassing and possible dry-out which will lead to prematureloss of capacity.

At the same ime, you cannot not lower the float voltage to avery low level to compensate or very high temperaturesThis will result in insufficient polarization of the platewhich will lead to acceleratedgrid corrosion and shortenedlife o f the battery.

2.500

2 400

2 200 .~

Figure 9: TemperatureCompensationExample

Figure 9 shows a typical temperaturecompensationcurveof float voltage versus temperature. In the example shown,temperature compensation can be done betweenapproximately 0C and 35C. Outside of this range, thefloat voltage must be held at a constant level to preventgassing (at the low end of the temperature scale) orinsufficient plate polarization (at the high end of thetemperaturescale).

7. THERMAL STABILITYA detailed analysis of thermal runaway is beyond the scopeof this paper, but a brief explanation may be helpful ingaining a full understanding of the risks associatedwith highfloat current and the benefits of temperaturecompensation.

It can be determined, for any battery, how much heat thebattery can physically dissipate. For example, assume aparticular battery design can dissipate x watts for each1C rise in temperature.

Assume urther that the float current goes up due to a higherfloat voltage or an increase in ambient temperature. Anycurrent in excess of that needed by the recharge or gridcorrosion process or to overcome self discharge, will gotoward the oxygen recombination process which generatesheat.

Recall that the battery can only dissipate x Watts per 1Ctemperature gradient. If the recombination processgenerates x + y watts, the y watts cannot be dissipatedso it will raise the temperatureof the battery.

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This increase n the temperaturewill cause he float currentto rise even higher generating more heat which cannot bedissipated. The battery temperature rises causing floatcurrent to rise and, eventually, the battery goes nto thermalrunaway.

It should be noted that there are many factors which cancontribute to thermal runaway besides emperatureand float

voltage (e.g. shorted cells, heat conduction characteristicsofthe battery, ground faults, float behavior of the battery withage, and poorly matched battery strings). But, by carefullycontrolling the float voltage and ambient temperature seenby the battery, the user can eliminate two of the mostinfluential factors.

Float voltage control is obviously a function of the rectifierso it is important to select a rectifier which has voltagecontrol accuracy of +l% and temperature compensationcapabilities.

When assessing he temperature the battery will see, it isimportant to consider solar exposure, air movement, andproximity of other heat generatorssuch as electronics.

8. SUMMARYAlthough GNB and other battery manufacturesare activelyresearching ways to increase battery life in uncontrolledenvironments, it is critical for the user community tounderstand the impact of these environments on VRLAbatteries.

Specifically, the users need to make every possible attemptto control temperature and float voltage to as close aspossible to the standard conditions specified by themanufacturer.

Temperaturecompensationshould also be utilized whereverpossible to help offset the negative impacts of these harshenvironments.

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