Inverse charge variation: Float charging effect on unbalanced battery strings in photovoltaic applications Emmanuel O. Ogunniyi#, Christo Pienaar* Electronic Engineering Department, Vaal University of Technology, Andries Potgieter Blvd, Vanderbijlpark, 1900
[email protected] *
Telkom Center of Excellence, Vaal University of Technology, Vanderbijlpark, 1900 South Africa
[email protected]
Abstract―Most solar charge controllers used in photovoltaic applications charge batteries in three stages- bulk, absorption and float. Float charging, which is often applied after the bulk and absorption charging enables the batteries to be charged and maintained at a constant voltage charge level especially when the batteries are being used in standby power applications. However, when there is a charge imbalance among the batteries in any string on float, a charge drop in one of the batteries in a string―which could be due to internal or external factors―concurrently results in the same amount charge gain on the others, thus resulting in extreme charging conditions and hence reduction in states of health (SOH) of the batteries. The experimental analysis in this study, based on three strings of battery pairs placed on a float charging in a photovoltaic system reveals the proportion of this inverse charge variation and the impact it has on the strings if equalization charge is not applied. Keywords― Float charging, three-stage charging, battery charge imbalance, solar charge controller, inverse charge variation, SOH, equalization charge.
I. INTRODUCTION Solar charge controllers and most modern battery smart chargers allow batteries to be charged in three stages: Bulk, Absorption and Float charging. Battery strings/banks used in standby power applications such as emergency lighting, uninterruptible power supply (UPS) system, photovoltaic applications etc. are often kept on float charging when they are not in use in order to supplements the charge loss due to selfdischarges. However when there is an imbalance in the charge levels of the batteries in a string/bank, the batteries would float in different charge ratios, the proportion of which depends on the charge imbalance among them. During floating, charge imbalance among the batteries in a string was observed to be an inverse charge variation. That is when there is a potential drop in one battery in a string either due to some internal or external factors or both, there is a contributory charge gain on the other(s) within the same string. The amount of this gain is observed to be inversely related to the amount of charge drop from the weak battery in order for the entire batteries to leverage with the constant floating voltage. This in result exposes these batteries to extreme charging conditions of undercharging and overcharging at the same time. The experimental analysis in this study, based on three strings of battery pairs placed on a float charging in a photovoltaic system reveals this inverse charge variation and
how its impact would affect the SOH of batteries in a string. This investigation more so is a further proof for the need for charge equalization among batteries in a string/bank. II. BATTERIES FOR PV APPLICATIONS The current applicable batteries used in PV applications are the lead acid, nickel-cadmium, nickel metal hydride and lithium ion batteries[1]. 1) Lead acid: Lead acid batteries are the most common batteries used in PV application due to their low cost and little maintenance requirement. They are manufactured in two categories of flooded (wet) and sealed lead acid batteries (SLA). The two types of SLA batteries are absorbent glass mat (AGM)-valve regulated lead acid (VRLA) and gel-VRLA batteries. However, they have limitations of short service life, low energy density and a resulting large footprint [2]. 2) Nickel-cadmium: Nickel-cadmium (NiCd) battery systems have been proven as better alternatives to lead acid batteries due to their longer service life, higher energy densities, excellent low temperature characteristics and low maintenance [2, 3]. However, NiCd batteries contain toxic heavy metal which makes them environmentally unfriendly. More so, their high self-discharge rate and memory effect [4] have limited their advancement for use in various applications. 3) Nickel Metal Hydride: Nickel metal hydride (NiMH) batteries are feasible alternatives to NiCd batteries as they are environmentally friendly having no poisonous substance such as cadmium or lead. More so, they contain high cycle life especially in energy turnover and high energy density which is about 25-30% better than high performance NiCd batteries [5, 6]. The major setbacks to NiMH batteries are the instability of metal hydride, high self-discharge and a considerable high cost of operation [1, 3]. 4) Lithium ion: Lithium ion (Li-ion) batteries currently have top application usage in portable electronics such as laptop computers, cell phones, camcorders, cameras, power tools etc. [7]. The advantages of this technology include high energy density compared to lead acid, NiCd or NiMH batteries. More so, they have longer cycle life and can attain up to 100% energy storage efficiency [2, 8]. These unique combination of advantages of Li-ion battery have attracted investors in developing the batteries for applications such as photovoltaic, power utility, electric vehicles, submersible and marine sectors. Currently, these technology are already being
implemented in these applications but the high cost of production and operation are limiting their market entry. Also, lithium ion batteries in general require the use of a sophisticated management systems [2].
about one hour by the charge controller, while the current gradually tapers off due to an increased internal resistance of the battery as it charges up. This allows the battery to be charged up to 98% SOC [12, 13].
Table I shows the basic characteristics of the common batteries used in PV applications
3) Float: During the float charging stage, the charging voltage is dropped to a level which is below the absorption charge voltage, usually between 13.4 to 13.7 V for lead acid batteries, and the batteries draw a very minimal maintenance current continuously. This stage finally brings the battery to a 100% state of charge and maintains this charge level continuously to compensate for the battery self-discharge [12, 13].
TABLE I BASIC CHARACTERISTICS OF BATTERIES FOR PV APPLICATIONS
High Energy density Stable voltage Low self-discharge High Cycle life Memory Effect High cost Environmentally Unfriendly
○ ● ● ●
○
○
● ●
○
○ ● ● ●
Li-ion
Gel
AGM
SLA
NiMH
Nickel NiCd
lead acid Flooded
● Yes ○ Borderline No
[9]
A typical three-stage charging curve is shown in Fig.1.
●
● ● ● ●
●
●
Charging Started
Bulk stage
Absorption stage Float stage
DC Voltage Constant Voltage
● DC Current
A. Battery Standards Battery standards are rating types set up by both national and international standards organizations, which are assigned to rechargeable batteries based on different standard conditions in order to indicate their compatibility, quality assurance, safety and reliability for different applications. Some of these standards include Ah (Ampere-hour), RC (Reserve Capacity), CCA (Cold Cranking Amps), SAE (Society of Automotive Engineers) rating, DIN (Deutsches Institut für Normung) based on German Institute for Standardisation, IEC (International Electrotechnical Commission) number, ETN (European Type Number), Yuasa Number based on British Battery Manufacturers Society (BBMS) standard in the United Kingdom and JIS (Japanese Industrial Standard) number [10, 11]. While any of these standards could be assigned to different battery types, the batteries with CCA and SAE ratings are meant to be used only for automotive application purposes for maximum optimization [10]. B. Three-Stage Charging of PV Battery The common three stages of battery charging, either by a smart charger or solar charge controller, that ensure that rechargeable batteries attain 100% state of charge (SOC) are bulk, absorption and float charging [12, 13]. 1) Bulk: During the bulk stage, a relatively high charging current is held constant by the charge controller against the rising internal resistance of the battery and hence the voltage rapidly rises to about 80 to 90% SOC, attaining between 14.4 to 14.6 V for Lead acid batteries [12, 13]. 2) Absorption: During the absorption stage, the battery charge voltage is held constant at the ‘bulk’ voltage level for
Time
Figure 1: Three-stage charging curve of rechargeable batteries [13]
C. Battery Charge Imbalance and Categories Charge imbalance is the disparity that occurs among the operating characteristics of batteries in a string or bank, thus making them have different SOC levels during a charging or discharging process. Charge imbalance among series connected batteries has become the most prominent challenge in recent times and this is often instigated either through an external load mismatch on the battery bank or through inherent differences in the chemical properties of the batteries. Majorly, charge imbalance among batteries can be broadly categorized into two; the internal sources and the external sources [14, 15]. 1) The internal sources: The internal sources are the differences in the electrochemical characteristics of the batteries which consists of manufacturing variance in physical volume, variation in internal impedance, different states of health (SOH) and different self-discharge rate [14, 16]. 2) The external source: The external sources are due to an unequal draining of charge ‘externally’ from some individual batteries that constitute the bank through a load, protection ICs, or different thermal variation across the pack [15, 17, 18]. These result in different self-discharge rates of the batteries across the battery bank. D. Effects of Battery Charge Imbalance Charge imbalance among battery string or bank results in either overcharging or undercharging of the batteries. Overcharging leads to battery gassing and explosion while the
effects of battery undercharging include sulfation and acid stratification [19] and either of these could result in reduced SOH of the battery. 1) Gassing and Explosion: Battery gassing is the excessive emanation of hydrogen gas during a continuous electrolysis of water in the battery electrolyte, caused during a severe overcharging of the battery. If this is not controlled early, it could result in battery explosion, damaging the battery [19]. 2) Sulfation: Sulfation of a battery occurs when the amorphous lead sulfate is converted to a stable crystal deposit on the negative plate of the battery due to a prolonged state of undercharging. The formed crystals become an insulator which further prevents the battery from attaining a full charge, resulting in the short service life of the battery [19].
through the use of external circuits which actively transport energy from the overcharging batteries to the undercharged batteries while the charging continues or just after the charging process is completed [16, 20, 23]. III. EXPERIMENTAL SET UP A block diagram of a typical battery management system (BMS) in a photovoltaic application as employed in this study is shown in Fig. 2. Solar charge controller
Load
3) Stratification: Stratification occurs when the concentrated electrolyte of a battery sinks to the bottom of the battery, leaving the water at the upper half of the cell. This as well occur when the battery is left at low SOC, below 80%, for a prolonged time., since the electrolyte at the lower part of the plate is only used for conduction, the battery capacity becomes reduced, while there is an oxidation at the upper part of the plate leading to grid corrosion. The combined effects of this continuous overcharging and high temperatures significantly increase the rate of grid corrosion and water loss resulting in short battery life [19].
Monitor and logger
PV Array
4) Reduction in battery state of health (SOH): A continued exposure of battery to extreme charging condition of undercharging or overcharging over time results in a reduction in its percentage SOH level. A battery SOH is the measure of the remaining charge-retaining-capacity of a battery as compared to its rated capacity according to a supported battery standard rating.
Battery String I
B1
B2
Battery String II
B3
B4
Battery String III
B3
B5
2 x 12V batteries in series Figure 2: Block diagram of the experimental set up
E. Battery Charge Equalization This is a method of bringing two or more series connected unbalanced batteries to an equal state of charge (SOC) in order to enhance uniformity among their cells, improve their SOHs and service life. There are different techniques of battery charge equalization and these can be categorized broadly into three; battery selection, passive methods and active methods [16, 20].
The equipment adopted for the experimental set up and analysis are sunmodule SW 225 polycrystalline solar panels with maximum power point voltage (Vmp) of 29.5 V and maximum power point current (Imp) of 7.63 A under standard test conditions [24], FLEXmax programmable solar charge controller, 100 Ah AGM-VRLA batteries, 12 V inverter, 100 W lamp load and CoDeSys data monitoring and logging software as preinstalled on a computer.
Battery selection involves grouping of batteries with similar electrochemical properties for charging, but this method is not efficient due to the variations in self-discharge of the batteries in the string, as this could reduce the life time of batteries in a bank [21].
IV. RESULTS AND DISCUSSION
Passive method balances through forced overcharge and/or dissipation of excess energy through a bypass resistor across each battery [16, 22]; this method however wastes much energy in the form of heat and thus reduces the efficiency of the battery system. Active method is more preferred to other methods as it yields higher efficiency and it is applicable to batteries of different types. Active method equalizes battery charge
1)
Results
SOH determination
The precise definition of battery state-of-health (SOH) depends on the application in which the battery is being used. For example, a battery SOH could be estimated from a number of high current discharge pulses that may be delivered by the battery when used for portable defibrillator, whereas available capacity is taken into consideration for SOH determination of batteries used in UPS or photovoltaic applications [25].
* 100%
(1)
The SOH is hence estimated from equation 1 when the battery is at its full SOC, using the equation 2 below. SOH =
*100% (2)
A battery full charge point is known when the battery has become steady drawing a very minimal current from the charging current. At this point the battery is said to be at 100% SOC but that does not imply it is as well at 100% SOH. For experimental analysis, the available charge capacities at full charge of the batteries used in the experiment were determined using a Midtronics MDX-300 battery conductance and electrical system tester. Also, all the battery adopted are the same type of nominal 12 V, 100 AH valve regulated lead acid battery applicable for PV applications. All measurements Table II IEC and SOH measurement of battery 1 and battery 2 Battery String I Battery 1 Battery 2
IEC Rating (A) Rated Available capacity capacity 355 293 355 289
SOH Calculated SOH SOH variation 82.53% 1.13% 81.40%
Tables II- IV show the data derived from five batteries used for the analysis. For battery string I at the third stage of the experiment, battery 3 was re-paired with battery 5 to derive a wide SOH variation for analysis, since there is a wide gap difference between the SOHs of both batteries. The results, as graphically shown in Fig. 4a and Fig. 4b, indicate that at the float charging stage, the battery string I with 1.13% SOH variation has a very minimal charge imbalance while the string II with 19.15% SOH variation displays a noticeable charge imbalance among its batteries. Fig. 4c shows that the imbalance was more pronounced at the string III as the SOH variation among its batteries was 80%. In order to ascertain the charge variation proportion among these battery pairs, a deliberate state of charge imbalance was introduced to a battery pair floating at approximate the same voltage (14.0 V). Here, one battery was deliberately discharged through a load for about one hour while still floating with the second pair, the effect of this discharge led to instantaneous voltage gain on the other with equal amount of voltage drop on the first, thus exposing it to extreme overcharging conditions (see Fig 4d).
Battery String I
16 15
Floating stage
14 13
Battery1
12
Battery2
11 6:00 6:20 6:40 7:00 7:20 7:40 8:00 8:20 8:40 9:00 9:20 9:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00 12:20 12:40 13:00
SOC =
were recorded in Amperes following the IEC battery standards, and the recorded IEC values of the batteries at full charge were compared with their IEC rated capacity, as estimated from their datasheet, for SOH calculation.
Charge voltage (V)
The common three categories of techniques for SOH determination are coulomb counting, voltage recovery and impedance measurements [25]. In this paper, coulomb counting was adopted for SOH estimation because it is the simplest technique that can apply to all common battery systems (such as lead-acid, NiCd, NiMH and Lithium batteries) used in PV applications [26]. In coulomb counting method, battery SOC is first determined by monitoring the charge flowing into and out of the battery. The relationship between the battery’s rated and the available charge capacity as shown in equation 1 [27] thus gives the SOC. This information is then used to estimate the SOH of the battery.
Time
Table III IEC and SOH measurement of battery 2 and battery 3 SOH Calculated SOH SOH variation 97.46% 19.15% 78.31%
Table IV IEC and SOH measurement of battery 3 and battery 5 Battery String III Battery 3 Battery 5
IEC Rating (A) Rated Available capacity capacity 355 346 355 62
SOH Calculated SOH SOH variation 97.46% 80.0% 17.46%
Battery String II 15
Floating stage
14 13
Battery3
12
Battery4
11 6:00 6:20 6:40 7:00 7:20 7:40 8:00 8:20 8:40 9:00 9:20 9:40 10:00 10:20 10:40 11:00 11:20 11:40 12:00 12:20 12:40 13:00
Battery 3 Battery 4
IEC Rating (A) Rated Available capacity capacity 355 346 355 278
Charge voltage (V)
Battery String II
Fig. 4a
Fig. 4b
Time
Battery3 Battery5
17 15 13 11 9
6:00 6:19 6:38 6:58 7:17 7:36 7:55 8:15 8:34 8:53 9:13 9:32 9:51 10:10 10:29 10:48 11:07 11:26 11:45 12:04 12:23 12:42
Charge Voltgae (V)
Battery String III
Time
Fig. 4c
Battery1 Battery2
Floating stage
When a constant voltage supply (Vs) is applied across series-connected loads in a closed loop such as shown in Fig. 6a, the algebraic sum of the potential difference (VR) across each load must be equal to the total voltage supplied. This is Kirchhoff voltage law (KVL) for a closed loop circuit. In a similar way, series connected batteries placed on constant voltage supply (float charging) from a charge controller form a closed loop with the voltage source. Therefore, as shown in Fig. 6b, the sum of the individual voltage (VB1 and VB2) across each of the two series-connected batteries on floating would be equal in value to the supplied float voltage (Vf) from the charge controller.
VS R1
R3
10:10 10:10 10:13 10:13 10:16 10:16 10:19 10:19 10:22 10:22 10:25 10:25 10:28 10:28 10:31 10:31 10:34 10:34 10:37 10:37 10:40 10:40 10:43 10:43 10:46 10:46 10:49 10:49 10:52 10:52 10:55 10:55 10:58 10:58 11:01 11:01 11:04 11:04 11:07 11:07 11:10 11:10 11:13 11:13
Charge voltage (V) Charge voltage (V)
1616 15.5 1515 14.5 1414 13.5 1313 12.5 1212
2) Discussion
Time Time
Fig. 4d
R2 Figure 6a: Vs = VR1+VR2+VR3
Figure 4: a. battery string with close states of health SOH. b. battery string with relatively high states of health SOH difference c. battery string with very large SOH difference. d. Inverse charge variation among floating batteries with different SOC
VB1
Effect of Inverse charge variation
The effect of this inverse charge variation was observed on the battery string3 to have led to a reduction in SOH value of the battery unit that was being exposed to continuous overcharging state as a result of voltage drop on the other in the same string.
IEC measurement (A)
Fig. 5 shows a continuous drop in SOH value of the battery over the period of four weeks. 350 345 340 335 330 325 320 315 310 305
346 336 326
week 1
week 2
week 3
Vf
322
week 4
Period (weeks) Figure 5: SOH reduction due to inverse charge effect on Battery 3, String III
VB2
Figure 6b: Vf = VB1 + VB2
However, each of the series connected batteries often absorbs different quantities of charge from a charge controller during floating, the proportion of which depends on their SOHs or internal resistance within their cells. The effect of this would be a different voltage values across each battery which in essence leads to charge imbalance within the battery string. When there is a charge imbalance among the batteries in a series string placed on float charging, it was observed that a charge drop on one battery would create an equal charge gain on the other batteries within the string. The neighboring battery in the string would in essence become more excessively charged above their set floating value due to this charge gain. This inverse charge variation results in order for the total voltage across the batteries in the string to be equal to the set floating voltage value from the charge controller-in support of KVL for a closed loop. Meanwhile, when this variation occurs in a battery string, it exposes the entire string to extreme charging conditions which could eventually lead to fast degradation and damage of the entire string/bank.
V. SUMMARY OF TESTS This study considers how charge imbalance affects the charging condition of batteries at floating stage. The analysis was done in three categories; one with battery pair of close percentage SOHs, the second with battery pairs of a little difference in percentage SOH while the third pair has a wide variation between their SOH levels. The data obtained and analyzed graphically displayed the conflicts that occur within the battery pairs with more inverse charge variation effect on the battery pair consisting of wider gap difference between their SOH levels. It was observed that the effect of a charge drop in one battery among a battery pair on a float charging, results in a contributory charge gain on the other within the same string. More so, the extent of this inverse variation depends on the proportion of the imbalance and/or SOH difference among the batteries constituting the string. VI. CONCLUSION Most standby power application systems such as emergency lighting, UPS, inverter systems, energy storage system in communication industries and renewable energy (photovoltaic) applications etc. are often dependent on battery strings/banks which are usually charged and kept on float charging as long as they are idle. The effect of float charging when there is an imbalance, either slight or large, among battery strings is investigated in this paper. Experimental analysis from the work reveals an inverse charge variation when there is a charge imbalance among battery strings placed on float charging. The detrimental result of this effect to battery life include fast capacity degradation and SOH reduction. This result is another indication for charge equalization need for battery strings and banks especially when they are being used in standby power applications system that often require them to be placed on float charging. REFERENCES [1] [2] [3] [4]
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