Experimental analysis of battery charge regulation in photovoltaic systems

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS

Prog. Photovolt: Res. Appl. 2003; 11:481–493 (DOI: 10.1002/pip.509)

Experimental Analysis ofBattery Charge Regulationin Photovoltaic SystemsPablo Dıaz*,y and Miguel Angel EgidoInstituto de Energıa Solar-ETSI Telecomunicacion, Ciudad Universitaria, 28040 Madrid, Spain

The influence of charge regulation on batteries for stand-alone photovoltaic systems

is analysed in relation to two factors: battery lifetime and the daily energy service

supplied. The regulation thresholds adjusted in the charge controller (for disconnec-

tion and reconnection), in overcharge and deep discharge conditions, determine the

whole system operation. Laboratory testing procedures are proposed and applied to

different components of the photovoltaic rural electrification market. Finally, techni-

cal recommendations for charge regulation of lead–acid batteries are presented.

Copyright # 2003 John Wiley & Sons, Ltd.

key words: regulation; batteries; charge controllers

INTRODUCTION

Batteries are commonly referred to as one of the weakest components in stand-alone photovoltaic (PV)

systems in the field.1–3 Battery lifetimes well below expectation, and the consequent need for regular

replacement increase their overall cost by up to 40% of the total.4 Together with this high cost, a major

point is their influence through poor energy supply. Even in cases of long battery lifetimes, with reports of more

than 7–8 years of field operation, either the real capacity after that time was not measured,5,6 or, when it was,

very low values were obtained. A representative example with batteries still in operation after 8 years in the

Bolivian High Plateau, showed, when they were measured, capacities between 3 and 50% of their initial value.7

In these conditions, with the storage capacity much reduced, the system is not working as expected. Other rea-

sons, such as real electricity consumption being lower than estimated or a desire to save money, may be respon-

sible for the delay in replacement.

Different factors determine the battery operation in PV systems:4,8,9 variability of operating conditions, type

and characteristics of the battery, and the charge regulation applied during its lifetime.

Variable and unpredictable operating conditions, due to the randomness of solar radiation, and the personal

consumption pattern of each PV user, that also changes over the years, strongly influencing battery perfor-

mance. These variable conditions experienced by each individual battery, combined the great differences

between one place and another, make it difficult to design an optimum and universal battery for PVapplications.

Furthermore, the cost and availability of common types of commercial lead–acid batteries used for PV systems

(mainly SLI automotive, tubular stationary, solar modified and VRLA) are the definitive decision factor, some-

times not for the batteries installed initially, but in subsequent replacements. This fact has to be considered in

the design and analysis of every PV rural electrification program.

Received 1 May 2003

Copyright # 2003 John Wiley & Sons, Ltd. Revised 18 July 2003

* Correspondence to: Pablo Dıaz, Instituto de Energıa Solar-ETSI Telecomunicacion, Ciudad Universitaria, 28040 Madrid, Spain.yE-mail: [email protected]

Applications

Once in operation, any battery has to work within a controlled and limited range in order to deliver the expected

long lifetime. Excessive overcharge and deep discharge conditions must be avoided by appropriate charge reg-

ulation, according to the battery type and characteristics. Irregular battery operation causes a variety of degrada-

tion mechanisms:4,10–12 excessive gassing; corrosion;13 sulfation; loss of water; and active mass and stratification.

In this sense, compatibility between battery requirements and the associated charge controller seems to be, and is

in practice, a decisive point to extend battery lifetime. However, this is not always taken into account when

designing PV systems. At present, it is very frequent in PV solar handbooks and technical specifications14 linked

to PV rural electrification programs, that the charge regulator and batteries are specifed separately, including set-

point voltages. Battery and charge controller combined performance is still not well coordinated, with important

consequences not only on battery duration, but also on the short-term energy supply to users.

Assuming that technical quality and reliability of all PV system components is crucial and must be strictly

controlled, a technical standard15 was elaborated in 1998 at the Instituto de Energıa Solar and widely dissemi-

nated.16 The aim of this work from the start has been its applicability to PV rural electrification programs, which

are mainly undertaken in developing countries. In order to implement quality controls in such regions, subse-

quent testing procedures were developed, and applied, avoiding the need for sophisticated instrumentation or

long time-consuming tests.17 Within that framework, previous tests were performed on batteries and charge

controllers of the actual PV rural market.18 Battery cycling tests were not considered, because of the long time

required, which reduces their applicability, and also, because of the great disparity in real operating conditions.

Different testing procedures for battery cycling are reviewed in the literature.19

As a follow-up, this article analyzes specific battery charge regulation issues from simple but useful labora-

tory tests. Testing procedures for battery charge regulation are presented, for both battery and charge control-

lers. Tests are then performed on a given battery and different charge controllers available on the PV rural

market, and the results are discussed under two main aspects: battery lifetime and daily energy supply. Tech-

nical recommendations are then proposed, based on test results and previous knowledge of battery performance.

All recommendations, tests and comments are referred to small 12 V systems, representative of a typical appli-

cation for rural electrification, the solar home system. However, the main results can be extrapolated to larger

stand-alone installations. Other non-photovoltaic battery applications are beyond the scope of this work.

TESTING PROCEDURES FOR BATTERY CHARGE REGULATION ANALYSIS

In order to analyze in detail the effect of the charge controller on the battery, it is useful to perform a series of

tests where the battery and the charge controller are implemented together. In this way, it is possible to inves-

tigate not only the battery performance, but also the electrical service supplied to the users. These tests take into

account the disconnection thresholds of the generator and load lines adjusted to each unit, but also the recon-

nection values, an important influence on system operation.

The electrical scheme for battery–regulator testing is very simple, as shown in Figure 1.

Together with specimens of the battery and charge controller that are to be tested, the instrumentation needed

is limited to the following:

Figure 1. Electrical scheme for battery-charge controller testing

482 P. DIAZ AND M. A. EGIDO

Copyright # 2003 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2003; 11:481–493

* current source (used as PV module) with a maximum current intensity above the PV generator short-circuitcurrent Isc;

* DC lamp with a power such that the current intensity is equal to the maximum load current of the system;* calibrated shunt with a nominal current similar to the nominal PV system current;* voltmeter or data acquisition system (DAS);* wiring, so that voltage drops between battery and charge controller, �VRB at maximum current are lower

than 1% of the nominal voltage of the system.

Concerning the maximum wiring voltage drop allowed, Figure 2 shows its strong influence on battery reg-

ulation. From measurements taking here as an example, battery voltage VB in discharge and two different wiring

voltage drop situations for the same low voltage disconnection (LVD) point are represented. A lower voltage

drop limit would need thicker wires, which are more expensive and not always available. In addition, higher

limits would affect the previously designed regulation.

Testing procedure: high charge

Using the current source as a PV generator, with a voltage limitation similar to its open-circuit voltage Voc,

charge the battery (initially discharged) through the charge controller at I’ Isc up to the end-of-charge thresh-

old. Then, maintain charging for at least 48 h after beginning the test. After the recharge test, discharge the

battery to 10�8 V (for 12 V systems), without charge controller, in order to obtain the total energy available after

charging, by means of the equation:

Estored;x ¼T

3600

XIi 8 Vi510�8 V ðfor 12 V systemsÞ ð1Þ

where Estored,x is the total energy stored in the battery (in A h) and available for a subsequent x-hour discharge, T

is the measurement period (in seconds) and Ii is the measured current (in amps). During the test, register the

current and the battery voltage periodically (for example, every minute, T¼ 60 s).

Testing procedure: deep discharge

In order to test the combined operation of the battery and charge controller close to the deep discharge zone, the

following test sequence can be applied, while registering battery current and voltage periodically (for example,

every minute):

* discharge the initially charged battery (Icharge¼ 0; Idischarge> 0 A, for example a 20-h discharge current I20)through a lamp, down to the automatic load disconnection threshold (LVD);

Figure 2. Influence of wiring voltage drops in discharge protection

BATTERY CHARGE REGULATION 483


* wait until open-circuit battery voltage is stabilized;* recharge the battery (Icharge> 0, for example I20) until load reconnection threshold is reached and loads

switch on again automatically.

TEST RESULTS AND DISCUSSION

In order to understand the influence of the selection of a specific charge controller on battery performance, but

more, on the system energy supply (and lack of supply), five different models commonly utilized in the PV rural

electrification market have been tested with the same battery. All five charge controllers can be defined as

‘good’ ones, according to the previously mentioned technical standard. They are widely installed thoughout

the world. Likewise, the selected battery is a common 12 V solar model for small PV applications, with

100 A h at 100-h rating, and recommended for the charge controllers tested. Makes and models of specimens

used in the tests have not been identified. It is not the intention of this work to perform a wide market compar-

ison, but to extend its usefulness to other models, characterized by their own regulation strategy, which can be

redesigned.

Regulation in high-charge phase

During sunny periods, batteries can easily reach the end-of-charge voltage allowed by the charge controller

(either with ON/OFF or PWM types). However, commonly the full battery recharge is not achieved the first

time that this end-of-charge threshold is reached on a given day. Actually, the battery needs to stay for a longer

period of generator disconnection–reconnection cycles (or pulses in PWM) to achieve or, at least, increase its

charge. This is justified by the particular physical and chemical processes taking place inside the battery. By

allowing higher battery voltages, although faster recharges are assured, some types of degradation phenomena

are also accelarated (excessive gassing, loss of water3 and corrosion) and battery life would be reduced. In this

context, recharging time is, in a sense, sacrificed to achieve longer battery lifetimes.

The aptitude of a battery to recharge, for the same current, is then determined by several factors that have to

be considered together:

* end-of-charge voltage;* reconnection voltage (in ON/OFF controllers);* regulation strategy.

The proposed procedures have been applied to batteries and associated charge controllers, with a constant

charging current intensity of 3 A. Technical specifications of each component and main results are summarized

in Table I.

Before the discussion of these results, it is worth mentioning that the energy stored after recharge (fifth col-

umn of Table I), is not the total energy entering the battery during the recharge test. A part of this energy is not

dedicated to recharging the battery, but to other loss processes such as gassing. So, the real energy stored is the

value obtained in a complete discharge test performed after charging. Furthermore, it should be clarified that

Table I. Results of battery–regulator tests in high charge zone

Charge controller Battery C20¼ 93A h

End-of-charge threshold (V) Generator reconnection Energy % of total capacity

Type (generator disconnection) threshold (V) stored (A h) recharged (SOC)

A ON/OFF 14�6 13�7 89�6 96

B PWM 14�4 — 88 95

C ON/OFF 13�8 13�5 74�1 80

D ON/OFF 14�0 12�9 71�2 77

E ON/OFF 14�4 (boost) 13�6 81�3 87

13�8 (floating)



intermediate capacity measurements were done after each test, in order to assure that the battery had not suf-

fered any degradation with associated capacity loss during the development of the whole testing campaign.

The results reflect, as a general conclusion, the great influence that selection of the charge controller has on

battery operation in recharge. Charge controllers designed for the same PV system type and size, and connected

to the same battery, may offer in practice a quite different performance. Disparities in regulation strategies and

adjusted thresholds between apparently similar units have effects on the whole system.

To enlarge on this point, particular issues of each test are exposed in detail. This, as well as showing the effects

(and defects) of each individual regulator, helps to explain the reasons for a better or worse operation, depending

on the components included in the PV system. Battery voltage evolution graphics are shown in Figure 3.

Figure 3. Battery voltage evolution in recharge, with the five different charge controllers connected



Charge controller A

An acceptable battery recharge is obtained with this ON/OFF charge controller (96% of battery capacity),

mainly due to the high end-of-charge voltage (14�6 V). However, because of the high reconnection voltage

of the generator line (13�7 V), the number of cycles (reconnection–disconnection) is probably excessive, with

a very short duration and a small amount of energy supplied to the battery in each cycle. This fast oscillation

could cause damage to the switching device (MOSFET) if it is not well designed for that performance. In the

same way, the battery voltage, ranging between those two values, is actually maintained for long periods at high

values, where excessive gassing and corrosion would be damaging. A lower end-of-charge threshold (14�4 V,

for example) and a wider gap, with a reconnection below the maximum value specified (13�5 V), can still assure

a good recharge without excessive overvoltage.

Charge controller B

Battery recharge with this PWM charge controller is also satisfactory (95%). However, as PWM control strategy

maintains a constant battery voltage once the end-of-charge threshold is reached, it is important to avoid high

values to reduce water losses and corrosion. A long time at 14�4 V, with low charging currents after the first

phase, could lead to such phenomena, decreasing battery lifetime. Lower voltages, as high as 14�1 V for

PWM regulators, would comply with the compromise between adequately recharging the battery and limiting

overcharge.

Charge controller C

In this case, the useful energy recharged is only 80% of the maximum, which is certainly not enough. The end-

of-charge threshold is too low (13�8 V), so the amount of energy recharged until it is reached for the first time is

lower than expected. Also, the narrow gap between disconnection and reconnection leads to a high number of

fast cycles, but with small gain in each, because the battery does not have enough time to recover, and internal

resistance is still higher than if some delay is permitted. The adjustment of end-of-charge voltage should then be

modified (increased) while maintaining the same reconnection point for the generator line.

Charge controller D

The end-of-charge voltage of this model is not quite wrong, although values above 14�1 V are recommended.

But it is the reconnection threshold where the importance of common battery and charge controller tests is more

evident. In this way, after the first disconnection of the generator line due to high voltage, battery voltage

decreases (because current is now interrupted). However, even after a long time, battery voltage does not reach

the reconnection point set by this charge controller (12�9 against 13�0 V for the open-circuit battery voltage) if

there are no loads, which is normal during the day. In these conditions, the energy stored in the battery and

available for later consumption is not enough (77%) and an important part of PV module potential during sunny

hours is then being wasted. As a conclusion, the reconnection voltage of the generator line should be always

above the open-circuit voltage at high state-of-charge conditions, following basic requirements.

Charge controller E

This is an intermediate example, with 87% of maximum energy stored. It is a charge controller with a right

floating range (13�6–13�8 V), however, since the battery stays at moderate voltages during long periods of high

radiation, the boosting or gassing threshold should be higher for the battery tested (14�7 V, for example), to

ensure an appropriate recharge and also to reduce stratification processes.

As a summary, attention is again drawn to the significance of combined design and selection of the battery

and its charge regulation, and also the need to consider not only the end-of-charge point, but the reconnection

one and the complete regulation strategy.

Regulation in deep discharge phase

With separate battery and charge controller tests it is possible to estimate the maximum depth of discharge that

would be permitted in a PV system, at a given current. Earlier tests performed at the Instituto de Energıa Solar

laboratory18 showed that a significant fraction of the charge controllers on the market lead to very deep battery



discharges, which implies lower battery lifetimes than expected, mainly due to stratification, corrosion or sulfa-

tion processes. However, there was a positive trend in new models as it can be observed here.

Now, together with the review of the deep discharge protection given by different charge controllers con-

nected to the same battery, the aim is to analyze and compare their behavior concerning load reconnection.

As mentioned before, this point determines the number of hours (or days) that a PV system has to wait without

energy supply until consumption is again allowed by the charge controller, owing to a certain amount of battery

recovery. Beyond technical issues about battery lifetime, this non-operational period would determine the users’

perception and opinion about their system.

Despite its practical relevance, it is commonly forgotten in charge controller standards,14 with very few refer-

ences in the literature.20 As with many other aspects, this is not a very complex technical problem, but one that

involves considering the real final objective: to supply a correct energy service.

Following the previously proposed testing procedures for deep discharge conditions, and with a current of

3 A, the same charge controllers were tested together with the same battery. The main specifications and test

results are shown in Table II.

The fourth column shows the maximum depth of discharge DODmax allowed by each charge controller, as a

percentage of the total battery capacity. Meanwhile, the fifth column gives the energy required by the battery

(with a 3 A charge) to reach the load reconnection threshold for each charge controller. It is expressed by Erec, in

A h. Therefore, if that amount of energy is not supplied by the PV modules to the battery, no electricity con-

sumption is permitted in the system. The last column presents the recovery energy required in each case, as a

percentage of the useful battery capacity CU with the same charge controller. This ratio can be calculated by

means of the DODmax as follows:

Erec

CU

ð%Þ ¼ Erec

C20ðDODmaxð%Þ=100Þ � 100 ð2Þ

The useful capacity is the maximum available energy for consumption storable in the battery with each

charge controller.

Some global conclusions can be extracted from these tests. First, the depth of discharge allowed ranges from

60% to almost 80%, depending on the charge controller selected, for the same battery and the same consump-

tion profile. This point establishes a remarkable difference between systems concerning the expected battery

lifetime (longer with lower DODmax), but also the available energy and, from this, the number of hours of elec-

tricity consumption permitted (greater with higher DODmax).

Second, once the charge controller has disconnected the load line because of low battery voltage (or state of

charge), a certain, but different, amount of energy recovery is needed, depending on the charge controller

selected. Values from 0 A h to more than 20 A h present significant disparities. Therefore, the non-operational

time and also the time that the battery stays at low states of charge vary strongly with the battery and charge

controller installed.

Both items are now analyze in detail by reviewing each particular case, so, that causes of good or bad per-

formance can be studied more precisely. Battery evolution during tests is shown in Figure 4.

Table II. Results of battery–regulator tests in deep discharge zone

Charge controller Battery C20¼ 93A h

Load disconnection Load reconnection DODmax(%) Erec (A h) Erec/CU (%)

voltage (V) voltage (V)

A 11�7 12�95 60 16�4 29

B 11�6 12�5 68 4�2 7

C 11�5 13�0 70 21�9 34

D 11�3 12�5 78 5 7

E 11�4 12�0 73 0 0



Charge controller A

In this first case, the maximum depth of discharge allowed (60%) is an acceptable value for the type of battery

utilized (solar modified). Concerning the load reconnection, the adjusted set-point (close to 13 V) would require

a long battery recovery time to recharge more than 16 A h (30% of useful capacity) before consumption is again

allowed. In these conditions, although the battery is well protected against low state of charge degradation, the

non-operational time could be considered as excessively long. During periods with low radiation (where load

disconnection voltages are more commonly reached) several days and nights without electricity supply would

probably occur. This situation is the starting point for bypassing the charge controller, so the battery stays with-

out any protection, as is widely referred to in the literature.1,7,21,22 Lower load reconnection voltages (no higher

Figure 4. Battery voltage evolution in deep discharge, with the five different charge controllers connected



than 12�8 V) are then recommended, so that the system can provide electricity earlier, assuming that the battery

is still at low states of charge, and consumption should be moderate.

Charge controller B

With this charge controller the maximum depth of discharge is slightly higher than in the first case. The 68% is

still acceptable, but as a limit. It is the load reconnection where significant differences appear. Here, the recov-

ery energy needed is only 4 A h, which is 7% of the useful battery capacity (higher than in the first case). In this

situation, the time spent by the system without consumption permission is much lower (probably few hours), so

the users’ perception would be much more positive. However, this low value, if the users are not sufficiently

aware of the energy limitations of the PV systems and the battery performance, can lead to too frequent load

disconnections.

Charge controller C

In this case the battery could be discharged down to 70% of its total energy storage, which is a slightly high

value for the associated battery, but not for other types, such as the tubular stationary battery. But it is about the

load reconnection threshold that requires attention. It is similar to case A, but even more exaggerated, with

almost 22 A h of recharge required until consumption is allowed (34% of the useful capacity). The number

of hours (or days, in this case) without electricity supply would be very high, which is a negative point for

any stand-alone system, and this is not sufficiently balanced by the longer battery lifetime that this system could

have. The lack of supply, together with a higher risk of charge controller bypass, are problems that must be

considered. Again, load disconnection set-points lower than 12�8 V are recommended.

Charge controller D

This is similar to case B concerning the amount of energy required to permit consumption, however, it presents

some special features. Given that the allowed depth of discharge is greater (78%, which is too much for solar

modified batteries), the internal resistance of the battery after load disconnection is also higher. So, during the

subsequent recharge (with current circulation), the reconnection voltage is reached earlier with this charge con-

troller, even though the gap between disconnection and reconnection is 1�2 V in case D and only 0�9 V in case B.

Charge controller E

A maximum depth of discharge of 73% is allowed with this charge controller and the battery tested, which is

slightly high for common recommendations. However, it is at load reconnection where this charge controller is

worth analyzing. As shown in Table II, there is scarcely any amount of recharging energy needed to reconnect

the load line. Load reconnection voltage is just a little higher than the open-circuit battery voltage after load

disconnection (12�0 against 11�9 V). This implies that, just when the battery recharge starts (Icharge> 0 A), the

regulator reconnects the load line, but the battery has still not recovered. If the user tries to switch in a load at

that moment (because the charge controller signal apparently allows that), then the battery voltage would decay

suddenly to the load disconnection point again. These set-points provide an unsatisfactory energy service and

do not avoid long periods at low states of charge. It can be observed that the gap between load disconnection and

reconnection is only 0�7 V in this unit, which is not sufficient as can be concluded from these tests.

As a summary of this group of tests, it is important to note the relevance of the selection and testing of bat-

teries and charge controllers in order to provide a good energy supply, together with the energy discharge lim-

itations required by each type of battery.

TECHNICAL RECOMMENDATIONS FOR CHARGE REGULATION

Following these laboratory tests, particular requirements for batteries and charge controllers related only to reg-

ulation issues are now presented, as a review of the already mentioned technical standard for solar home sys-

tems. As can be observed, some have been modified from the original version.



Batteries

Based on manufacturers recommendations, commonly included in technical brochures, battery discharge in PV

systems should be limited to a certain value in order to assure long lifetimes. In fact, the total energy discharged

by the battery during its cyling life can be expressed as a function of the number of cycles and the maximum

depth of discharge of those cycles, as shown in the example of Figure 5. However, it should be noted that in

practice daily cycling would not generally reach DODmax, so the number of cycles differs from laboratory

estimations.

This type of information and field experience in PV system operation permits us to establish a general recom-

mendation for batery discharge:

* the maximum depth of discharge, (referred to the 20-h nominal battery capacity) should not exceed thevalues proposed in Table III.

From this basic requirement, the complete battery regulation function, both against overcharge and deep dis-

charge, actually corresponds to the charge controller. The adjustment of the regulation voltages should be done

according to the associated battery, because battery voltage evolution with state of charge (or deep of discharge)

varies from one battery to another, depending on its type and internal constitution. In previous studies, complete

discharging tests were performed on different batteries on the PV market, following standard procedures,18 at

the same current. Battery voltages for the recommended depth of discharge values are shown Figure 6.

However, many charge controllers on the market do not permit modification of the manufacturer’s voltage

set-points. In a recently published23 survey, 45% of the charge controllers were found to have fixed end-of-

charge setpoints. And 40% of them do not allow readjustment of the deep discharge regulation. The intention

is to avoid inadequate modifications that could affect battery operation, which is something that happens in the

field.1

Charge controllers

The main regulation requirements for charge controllers to be installed in stand-alone PV systems for rural elec-

trification are proposed. Appropriate battery protection, together with sufficient energy delivery to the users,

should be the objective of any regulation strategy.

Table III. Requirements for maximum battery depth of discharge

Battery type DODmax (%)

SLI automotive 50

Solar modified 60

Tubular stationary 80

VRLA gel 30

Figure 5. Total A h discharged by the battery during its entire life and optimum DODmax



Deep-discharge protection

This group of recommendations gives the basic terms for protection against excessive battery discharge, accord-

ing to the associated battery and the global PV system function. They cover load disconnection, reconnection

and warning, with sufficient precision required by normal battery voltage evolution with its state of charge.

These are the requirements:

* deep discharge protection must be included;* ‘load disconnection’ voltages should correspond to the maximum depth of discharge values defined in

Table III, when the discharge current, in A, is equal to the daily load consumption, in A h, divided by 5;* load reconnection voltage should be between 0�9 and 1�2 V higher than the load-disconnection voltage (for

12 V systems). This requirement was modified from the original version, where a 0�5 V gap wasrecommended;

* load disconnection, load reconnection and warning voltages should be accurate to within � 0�5% (� 60 mV,for 12 V battery) and remain constant over the full range of possible ambient temperatures. In this case, theoriginal standard required �120 mV. As shown in Figure 7, small variations in battery voltage correspond tosignificant state of charge changes;

* warning voltage should be 0�2 V higher than the load disconnection voltage (for 12 V systems).

Overcharge protection

Requirements for overcharge protection allow a wider range of operation than for deep discharge, given that its

relation to battery lifetime is not so clearly defined, apart from some general trends. Therefore, the effect on

Figure 6. Battery voltage at recommended DODmax for 15 different batteries available on the PV rural electrification market

Figure 7. Effect of regulation set-point accuracy on battery discharge



short-term energy supply is lower, because the battery is well charged at such moments. Basic recommenda-

tions are the following:

* end-of-charge voltage should lie in the range 13�8–14�4 V (for 12 V systems) at 25�C;* in the case of PWM charge regulators, the end-of-charge voltage should lie in the range 13�8–14�1 V (for

12 V systems) at 25�C;* end-of-charge voltage should correspond to a recharge factor between 0�95 and 1, at a constant current equal

to the short-circuit current of the PV generator at the STC;* in the case of two-step controllers, the reposition voltage should lie in the range 13�0–13�5 V (for 12 V

systems) at 25�C;* end-of-charge and reposition voltages should be accurate to within � 0�5% (� 60 mV for 12 V battery).

A temperature correction of �4 to �5 mV/�C per cell should be applied to the end-of-charge and reposition

voltage ranges mentioned above. This requirement is compulsory only if ambient (indoor) temperatures around

the controller are expected to vary significantly during the year, say by more than �10�C. Otherwise, tempera-

ture compensation circuitry is not really needed.

CONCLUSIONS

The influence of charge regulation, not only on the battery itself, but also on the electrical supply to users is

clear. In this context, common tests performed on batteries and charge controllers to be selected or designed to

work together, show their usefulness. It has to be remarked that all tests were performed at more or less constant

currents, while real values in PV systems vary continuously. However, for the purposes of this work, apart from

the analysis of a specific system, some general conclusions can be drawn.

Test results show the effects that the inclusion of different models of the actual PV rural electrification market

can have on the system. Some issues need to be taken into account:

* in overcharge regulation: end-of-charge voltage, and also the gap with the reconnection of the generator linehave to be strictly considered to achieve adequate battery recharge with low degradation damages;

* in deep discharge regulation: load-disconnection set-points have to follow battery discharge recommenda-tions, which is not commonly the case. Together with this, more attention has to be paid to load reconnectionvoltages, where the disparity of strategies can definitely determine the system energy supply to users inperiods of low solar radiation.

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Experimental analysis of battery charge regulation in photovoltaic systems

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