Research on the Charging Regime of Intelligent Charger of Automotive Valve‐ regulated Lead‐acid Batt

Vehicle Engineering (VE), Volume 3, 2015 doi: 10.14355/ve.2015.03.005

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Research on the Charging Regime of Intelligent Charger of Automotive Valve‐ regulated Lead‐acid Battery Suyun Luo*1, Peng Jia2, Yangchun Wu3, Lei Tang4 Shanghai University of Engineering Science, No.333, Longteng Road, Songjiang District, Shanghai, China, 201620 *1

luosuyun@sues.edu.cn; 2122100352@qq.com; 31060156185@qq.com; 4tommy9106@sohu.com

Abstract Charger has a great impact on the performance and service life of battery and charging regime is significant for intelligent charger. Researchers usually combine two or three charging methods to get a good performance charging regime, but the effect is still less than satisfactory and cannot charge and renovate the battery at the same time. This article studies on charging regime of intelligent charger based on normal charging methods character analysis of VRLA battery and references to some charger products regimes, and gives a seven phases regime including Resonant composite pulse charging, soft start, bulk charge, absorption, battery test, recondition and float. From the result shown in experiments, this charging regime resolves the problem of failing to charge and renovate batteries simultaneously, implementing a high performance charging.

Keywords Valve Regulated lead‐acid battery; Charging Regime; Intelligent Charging; Resonant Composite Pulse Charging

Introduction Lead‐acid batteries were invented more than a century ago and now they remain the most widely used type of rechargeable battery and are used in many applications such as automotive, traction and stationary applications, uninterruptible power supplies (UPS), solar traffic lights, telecommunications and energy storage devices for renewable energy. Rechargeable batteries used for automotive are divided into starter battery and power battery.

Starter batteries used in normal gasoline engine and diesel engine automobile are usually Lead‐acid batteries, while power batteries applied in pure electric vehicle and hybrid electric vehicle are typed into lead‐acid battery, NI‐MH battery, Ni‐Cd battery, sodium‐sulfur battery and Lithium‐ion battery etc.

Although with the disadvantages of low energy and low power density, lead‐acid batteries are still ranked an important position in automobile field for good price, convenient manufacture and maintenance. The performance and service life of battery are affected greatly by charging regime and so do the performance of vehicle. This article will discuss the intelligent charger charging regimes of valve regulated lead‐acid battery.

Types and Charging Methods of Lead-Acid Batteries Applied in Vehicle Types of Lead‐Acid Batteries Applied in Vehicle Lead‐acid batteries are classified into flooded (or wet) batteries and VRLA (Valve‐regulated lead‐acid) batteries according to whether gas emission is permitted during operation.VRLA batterie is developed base on traditional flooded batteries. It has better performances on service life and maintenance. In order to extend the service life and minimize maintenance, immobiliezed electrolyte was used in VRLA battery. With the immobilized electrolyte, the oxygen generated during the charge is captured and recombined in the battery, in the so‐called “oxygen recombination cycle”, and gases are allowed to escape only if the internal pressure exceeds a certain level by means of pressure‐release valves so that water loss during battery operation is minimized.

There are two distinctly different types of technologies for immobilizing the electrolyte in VRLA batteries. One is the absorptive glass mat (AGM) technology. For AGM batteries, the sulfuric acid electrolyte is captured in an absorptive glass mat separator. The other is gel technology. In GEL batteries the sulfuric acid solution is mixed

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with a gelling agent to form a gelled electrolyte. They apply different methods to stabilize or capture electrolyte, both sealing the battery based on cathode absorption principle, but providing different aisles for oxygen produced by positive electrode to escape to negative electrode. For AGM batteries, most of the electrolyte is kept in glass mat separator, but it must be left about 10% glass mat separator keeping away from electrolyte to allow the emission of hydrogen and oxygen during charging. Gel agent in GEL batteries keeps electrolyte by forming three‐dimensional porous mesh structure with SiO particle as skeleton. When silica sol turn to be silica gel after it is filled in battery, the skeleton shrink further, causing crack throughout positive electrode and negative electrode and providing a channel for oxygen.

When a percentage of calcium was added in battery grid frame, the battery was called calcium lead‐acid battery, the intensity of which is enhanced by the process.

Factors Influencing VRLA Batteries Service Life Charging current and voltage, discharging degree, discharging speed and frequency all affect lead‐acid battery service life. Overcharge would bring about excessive oxygen evolution during the end of charging. Excessive oxygen brushes positive plates, causing the active material loosening, falling off and softening. Long‐term undercharge would form large particle crystal which is difficult to restore, causing battery sulphation and failure.

Temperature is another main factor that affects the service life of batteries[1][2]. High temperature accelerates positive plate grid corrosion and brings battery irreversible damage. Low temperature reduces electrolyte fluxility, slowers ionic diffusion, increases battery resistance, and lowers battery capacity and the active material availability of negative plate. Wrong charging method would rise sharply battery temperature and lower its service life.

Main Charging Methods for Automotive VRLA Batteries Appropriate charging regimes could recover battery active material as much as it could and high up the battery state of charge (SOC) to 100% without any unfavorable temperature affection and overcharge, avoiding the scouring to battery plates done by the oxygen evolution during the end of charging, assuring the state of health of battery and lengthening its service of life.

Common VRLA charging methods can be summarized as constant current method, constant voltage method, pulse method etc., while the realistic charging regime adopted should bound these methods according to battery SOC, temperature etc. to take full advantage of different charging methods and avoid their deficiencies.

Recently, new charge regimes have been proposed to overcome the intrinsic problems associated with float charging so as to prolong the service life of the VRLA battery. The primary charge and discharge reactions in a battery determine the charge performance of a charge regime, while hydrogen evolution, grid corrosion and sulfation determine the aging effects.

1) Constant Current Charging (Ci) In the constant current charging process, charging current is kept at a stable amount. It is usually divided into two stages; in the first stage, charging current is kept at 1/10C20or 1/15C20 ( C20, 20 hour rated capacity) until the voltage of a single cell is up to 2.4V; in the second stage charging current is decreased to half current of the first stage till the cell voltage increase to maintain 2.7V, then charging should be ended. The process is shown in FIG. 1. It will cost about 20 to 30 hours to finish the charging if constant current charging method was adopted.

To cut down charging time, sometimes battery is charged with higher charging current (0.4C20).

FIG. 1 VOLTAGE CURVES OF CONSTANT CURRENT CHARGE

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In constant current charge regime, there is no charge voltage regulation. The battery is charged by a high charge voltage up to 2.6–2.8Vper cell, which is determined by the charging current. The positive grid corrosion rate and the hydrogen evolution rate at the negative electrode of the battery are accelerated by such a high charge voltage. If charging current is not switched to a small float current, adverse overcharging may occur with consequent damage to battery.

2) Constant Voltage Charging (Cv) In constant voltage charging, charging voltage is maintained at certain value. Usually the voltage is 2.23V to 2.3V for a cell, while charging current is different according to battery discharging state and rated capacity; the higher the depth of discharge and rated capacity is, the higher the charging current is. To a fully discharged battery, charging current maybe go up to 10C20in the beginning of constant voltage charging. Too high charging current has no good to battery capacity recovery and would increase battery temperature rapidly, giving rise to the water evaporation of electrolyte. Along with constant voltage charging continues, charging current decreases constantly until it is about 0.002C20. The current curve of constant voltage charging is shown in FIG. 2.

FIG. 2 CURRENT CURVES OF CONSTANT VOLTAGE CHARGE

3) Pulse Quick Charging It will cost a long time to charge if constant current or constant voltage charging method was adopted, but situation would be different if pulse quick charging method is used. In pulse quick charging, at the first stage, charging current high up to 0.8C20‐1C20 is employed, maintaining battery capacity to 50%‐60% and cell voltage increasing to 2.4V and then transferring to the second stage: first stopping charge (stop charging for 25‐40ms), and reversing pulse charge (with discharging current up to 1.2C20‐3C20), second stopping charge (stop charging for 25ms), and pulse charge (with charging current up to 0.8C20‐1C20). The second stage repeated until the end. Pulse quick charging curve is shown in FIG. 3.

FIG. 3 PULSE QUICK CHARGE

4) Resonant Composite Pulse Charging Resonant composite pulse charging has desulphate function for battery grid. In the charging process, a certain

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high frequent composite pulse is adopted to generate resonant with restitute crystals to active material and [5].

crystals, opening

lattice,

Resonant frequency relates to crystal size, the larger the size, the lower the frequency. Charging with steep rise pulse will generate abundant harmonic waves with bigger amplitude for lower frequency waves and smaller amplitude for higher frequency waves, supply bigger crystals more energy while smaller less energy and dissolve the crystals easily.

In resonant composite pulse charging, pulse current and its frequency are controlled to serve to battery. Suitable pulse current and comparatively small current density provided to positive grid basically will not do harm to the grid. To VRLA, transient charging voltage makes oxygen easily to be absorbed by negative grid through its circulation, which prevents the generation of water, differs this charging method as a non�loss desulphation charging. The charging voltage curve could is shown in FIG. 4.

FIG. 4 RESONANT COMPOSITE PULSE CHARGING

Charging Regime for Automotive VRLA Constant Current Constant Voltage Charging (CICV) In CI charging, if battery is in low SOC, charging current is stable in control; when charging continues, charging voltage increases gradually and has sharp increasing up to 3.0V in the end[3]. The escaping of amount of gas, scours active material of grid and makes active meterial easy to fall off, will easily precipitates battery fault.

In CV charging, low SOC battery is receptive to charging current in the beginning of charge and charging current would be very high, which makes battery temperature rise quickly, spurring the water evaporation of electrolyte and doing damage to battery; then with the rising of SOC, charging current drops slowly.

Combined with the characteristics of the two kinds of charging methods, here comes constant current constant voltage charging (CICV), which first employs constant current method at the first stage avoiding the high constant current in the beginning of charge occurred for low SOC battery and then adopts constant voltage charging for the second stage to forestall too high charging voltage in the end of charge, decreasing hydrogen precipitation and minimizing grid scouring.

For CICV, in the first CI stage, charging current usually is about 0.4C20, when cell voltage rises to 2.35�2.45V, transfers to the second CV charge and the charging voltage keeps the same as the cell voltage when the CI stage ends. The CV charge continues until charging current keeps at about 0.0015C20 and almost no longer descends.

The current and voltage curves are shown in FIG. 5.

FIG. 5 CONSTANT CURRENT CONSTANT VOLATGE CHARGE

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Combined CICV Multiple Stages Charging Under a more comprehensive consideration of battery characteristic in different states of discharge (SOD), CICICVCV is employed combined with charging regime, which can deeply recover the battery and decrease gas evolution and damage in the end of charging as far as possible.

For over discharged battery, adopting high constant current at first stage would cause thermal runaway, so it is better to charge with low constant current at this time which adequately activates the surface and inner of the grid avoiding active material falling off, prolonging the service life of battery. So before high constant current charging, low constant current charging is employed; what’s more, float charging is added to the end of charge to prevent gas evolution and its scour to grid active material and compensate the battery self‐discharge, keeping battery in full capacity. The process of combined CICV multiple stages charging is shown in FIG. 6.

FIG. 6 COMBINED CICV MULTIPLE STAGES CHARGING

Seven Stages Charging On condition with the features of multiple stages charging, resonant composite pulse charging and the SOC of battery, a seven stage charging regime was designed in this article. It comprises desulphation resonant composite pulse charging (desulphation), soft CI (soft start), normal CI (bulk charge), CV (absorption), battery testing (analysis), small CI (recondition) and small CV (float) seven stages, which realize the completely and reliable charging.

The detail for seven stages charging is shown in table 1.

TABLE

1

SEVEN STAGES CHARGING REGIME FOR INTELLIGENT CHARGER

stage

Stage name

Charging current/voltage

Time limited

1

Desul‐phation

Charging current controlled resonant composite pulse charging, end at battery voltage rise to

——

2

Soft Start

3

Bulk Charge

4

Absorp‐tion

5

Analy‐sis

Stop charging for 3 minutes then transfer to recondition stage if battery voltage decreased to under 12V, otherwise transfer to float charging.

6

Recondition

Constant 1.5A current charging continue for 4 hours, limit charging voltage at 14.1V for GEL battery, 14.4 for AGM battery, 14.7V for WET/CALCIUM battery

7

Float Charge

13.8V

Usually with 5A constant current, end at battery voltage rise to Usually with 10A constant current, end at battery voltage rise to (GEL), (AGM), (WET/CALCIUM) Constant voltage

(GEL),

(AGM),

(WET/CALCIUM)until

In 5 hours In 20 hours In 12 hours

charging current decrease to 1.5A

Continue for 4 hours

Desulphation Stage: If the voltage of a VRLA battery cell is lower than 1.75 V, desulphation charging is carried out with resonant composite pulse provided to battery until the voltage rise to .

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Soft Start Stage: If the voltage of a VRLA battery cell is lower than 2.0 V, charger supplies 5A constant charging current to battery. The stage ends at battery voltage rising to or after the stage continues for 5 hours.

Bulk Charge: Charger supplies a 10A constant current to battery at this stage. The stage ends at battery voltage rising to for GEL battery, for AGM battery, for WET or CALCIUM battery, or after it continues for 20 hours.

Absorption: After constant current bulk charge stage, charger transfers to absorption charging. It is a constant voltage stage with Constant voltage of for GEL battery, for AGM battery, for WET or CALCIUM battery until charging current decreases to 1.5A. Time for this stage is limited in 12 hours.

Analysis: charging is stopped for 3 minutes to test how fast battery’s voltage drops. The purpose of this process is to distinguish if the battery was good or not. If battery voltage decreased to under 12V in 3 minutes, it illustrates it’s an abnormal battery and needs soft recover charging, so the charging stage will transfer to recondition stage otherwise transfer to float charging.

Recondition: In this stage, constant 1.5A current is continuously provided to battery but charging voltage has to be lower than 14.1V for GEL battery, 14.4 for AGM battery, 14.7V for WET or CALCIUM battery. The stage lasts for 4 hours.

Float Charge: Float charge is the final stage. In this stage charging voltage is kept at 13.8V. Charging current drops constantly till nearly to zero. This stage assures battery recover to 100% capacity in the end.

Conclusions No matter it is electric vehicle, other new energy vehicles or traditional gasoline and diesel engine vehicles that domain the future vehicle field, VRLA battery will dominate the vehicle battery industry in the future. Suitable charging regime is important to enhance battery capacity, improve the performance and extend service life, so the research on VRLA battery charging regime has a realistic meaning. This article discussed the charging regimes of VRLA battery based on character analysis of several charging methods which were normally used. Optimizing and combining these methods, a seven‐stage charging regime with desulphation and recondition function is designed, which realizes the renovation, fully charging and effective protection at the same time.

ACKNOWLEDGMENT

This article was written based on research project of Intelligent charger development of automotive lead‐acid battery. The charging regime research was greatly helped by professor Yuanming Gong and the testing charger hardware set was helped by teacher Jianpeng Zhou. Here, we would like to extend our sincere gratitude to them and other people who helped us.

REFERENCES

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S. Schaeck, A.O. Stoermer, E. Hockgeiger. Micro‐hybrid electric vehicle application ofvalve‐regulated lead–acid batteries in absorbent glass mat technology: Testing apartial‐state‐of‐charge operation strategy. Journal of Power Sources, 2009(190) : 173–183.

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Luo Suyun Songjiang District, Shanghai, China. MasterĘšs degree, transport information engineering and control, Wuhan University of Technology, Wuhan, China, 2003.

She has worked in automotive engineering department of Shanghai University of Engineering Science, China for about 12 years just after graduated from Wuhan University of Technology, now is a vice dean, associate professor. Published SCI and EI articles: Phases PID Controller of Common�rail Pressure for Diesel Engine Electronic Injector Test Bench, SCI: 000301003800004; Measurement and Control System Development of Performance Research Test Bench for Diesel Engine Injector based on Labview, EI: 20121314910172. Major field is automotive electric control technology.

Peng Jia, Yangchun Wu, Lei Tang Postgraduate students of Shanghai University of Engineering Science, China.

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