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1 Shaken not stirred besttechnology 75 The advent of the stop-start technology for fuel saving in automobiles has created a headache for the lead-acid battery industry. Micro-hybrid cycling at partial state of charge produces serious acid stratification in a very short time period leading to major failure rates in conventional flooded batteries. In this article, battery consultant Bernhard Rose explains how insertion of a small, passive device from iq Power Licensing, into a flooded battery, can use the forces created by normal driving patterns to mechanically create the perfect electrolyte cocktail. Carbon additives have increased the cyclic capability of enhanced flooded batteries (EFB) substantially. However, EFB batteries still suffer from the negative effects of acid stratification. Anti-stratification devices such as passive mixing elements eliminate these drawbacks by keeping the electrolyte homogeneous, providing ideal conditions for the batteries chemical processes. The only truly effective system to accomplish this mixing has been developed in Germany and brought to market by iq Power Licensing, an independent battery technology company. The technology developed is a patented 360 circular electrolyte mixing system which is already in use in over a million SLI car batteries both aftermarket and OEM. The effect of such electrolyte mixing devices is to increase the cycle stability of both conventional flooded batteries as well as EFB by a factor of two or more. Using this technique, EFBs will be able to catch up to the cycle capabilities of AGM batteries and become a serious choice for stop-start applications in motor vehicles. The system also works in large truck batteries, which have a different battery cell layout. Causes of electrolyte stratification in stop-start applications With the increasing number of electrical devices in vehicles and the introduction of maintenance free calcium-based SLI batteries, a phenomenon came to light, which until recently had received little attention acid stratification. Although acid stratification has been recognised since the beginning of battery production as a performance inhibitor, it has not been a major source of SLI failure until the advent of the start-stop technology. A non-stratified, fully charged battery has an acid density between 1.26kg/l and 1.28kg/l, depending on climate and application. On discharging, sulfuric acid is consumed and the density drops. During recharging, the process is reversed. Concentrated sulfuric acid of up to 1.80kg/l is released near the plate surfaces. As this acid density is higher, it settles to the bottom of the battery, where it forms a vertical density gradient in the electrolyte over time.

2 76 besttechnology Stratification is the result of deep or shallow discharges and inadequate recharges that do not create gassing to stir the electrolyte. The reactions for discharge and charge are given below: Discharge: Pb + PbSO 4 + H 2 SO 4 2PbSO 4 + H 2 O In this case, water is produced with an SG of 1.00, which dilutes the acid immediately adjacent to both the positive and negative plates. This dilute acid will tend to rise due to its lower density. Without the stirring action caused by gassing, it is only the concentration gradient between the denser, higher SG acid at the bottom of the cell and the lower SG acid at the top of cell, which is the driving force available to equalise the acid concentration between the top and bottom of the cell. This is a very slow process and without stirring is an ineffective mechanism for density equalisation. On recharge the reverse happens: Charge: 2PbSO 4 + H 2 O Pb + PbSO 4 + H 2 SO 4 In this case the products of the reaction are the formed plates and sulfuric acid, which, within the pores of the active material where it is formed, will have an SG of 1.84, which is considerably denser than the acid density of the electrolyte. As the concentrated acid is pushed into the bulk electrolyte adjacent to the plate surface it creates a high-density zone that sinks to the bottom of the cell. Because in a modern automotive car the charging is carefully controlled to prevent gassing and thereby avoid loss of water from electrolysis, there is no stirring action from the production of gas bubbles (hydrogen and oxygen) at the electrodes. Without this stirring action, the separation of acid density in this way will lead to the effect known as stratification. Since the advent of stop-start or microhybrid cycling, the number of daily discharges Fig 1: Progressive separation of acid density from discharge to charge Discharge Charge and recharges which the battery experiences goes from several to scores or even hundreds per day. Under these conditions it is not surprising that stratification has become a serious problem. iq Power has found that in both field and cycle tests, vehicle acid densities of >1.35kg/l have been detected at the bottom of the battery cells. The high acid density on the bottom, over time reduces plate capacity and can lead to (sulfation) creating a high internal cell resistance, with an irreversible loss of chargeability of the active material. The high resistance and lack of charging results in a capacity loss and a drop in cold cranking capability of the battery. In addition, the active material is damaged by both high and low SG acid. The higher density acid at the bottom will cause paste softening and shedding whilst the low acid density will create hard gritty active material in the negative. All of which, depending upon the severity of the effect, will seriously reduce battery performance and life. Keeping the electrolyte homogenous over the entire surface of the plate creates optimal conditions for the chemical processes (eqns 1 and 2) to react efficiently. This provides a better performance and cycle life. Fig 2 shows a comparison of lead plates of two identical standard batteries. The plate on the left (1) was equipped with an electrolyte-mixing device achieving 460 cycles before failure. The plate on the right (2) is without mixing devices

3 besttechnology 77 Comparison of lead plates of a normal battery with the electrolyte mixing device achieving 460 cycles (left, 1) and without the mixing, achieving 210 cycles (right, 2). Fig 2: Comparison of plates from two batteries cycled with and without the iq Power mixing device. and failed at 210 cycles. Cycling was performed according to industry standard VW , 17.5% DoD continuous cycling with no equalisation charge. It should be noted that testing with equalisation charge creates aggressive gassing that eliminates much of the negative aspects of stratification at the cost of accelerating grid corrosion. Acid stratification is especially prevalent in maintenance-free batteries, mainly due to the low voltage charging profiles which do not create gas bubbles. This is enhanced by application profiles using many electrical devices to provide heavier discharge loads, in stop-start applications, in taxi operations and in on road commercial vehicles. It has been proven that even shallow micro-cycles of only 2.5% DoD, combined with normal SLI activity, lead to increasing acid density at the plate bottom within the first few cycles. To overcome the disadvantages of acid stratification in starter batteries, AGM (Absorbent Glass Mat) batteries were developed, in which the electrolyte is held Fig 3: Dynamic forces acting on a moving automobile Aerodynamic drag Roll Pitch Braking force stationary in a compressed glass mat, hindering acid stratification significantly. Because the battery is designed to fully absorb the acid in a glass mat separator and the saturation level is less than 100%, typically 95-97%, the electrolyte is effectively held in a suspension adsorbed onto the surface of the fibres within the mat. The acid has no free movement. Without free movement of acid it should not be able to rise or fall according Longitudinal axis M Yaw Vertical force Vertical axis Lateral force to its density. However, although the effect is small, some researchers have found that stratification can occur in AGM batteries after many cycles. Due to their higher lead weight, their conductive properties and less stratification, AGM batteries have a higher cycle life than conventional flooded batteries. To meet ever more stringent emissions standards, the automotive industry is producing vehicles with ISS (idle-stopstart) engines, even in smaller vehicles. This is increasing the demand for starter batteries with high cycle stability at competitive pricing. Due to higher production cost and the sensitivity to extreme temperatures of the AGM batteries, the car industry has increased interest in more robust, higher capacity, flooded batteries or EFBs. Use of carbon and/or other additives, in addition to other improvements Vertical vibration Braking force Slide M Motive force Lateral force Vertical force Transverse axis

4 78 besttechnology in EFB type batteries, has reduced the gap between flooded batteries and AGM cycle life. The enabling feature is a passive electrolyte-mixing device. This is an inexpensive uncomplicated but wellengineered plastic part, which can be simply inserted into the battery during manufacture. The device uses the gravitational forces of a driving vehicle (accelerating, Back Back Fig 4: Effect of braking or acceleration forces on electrolyte Original electrolyte level Electrolyte movement due to braking force Mixing device horizontal and vertical parts Battery plate iq Power Patent No EP B1 Fig 5: Return of denser acid into the bulk electrolyte Original electrolyte level Mixing device horizontal and vertical parts Electrolyte recirculation Battery plate iq Power Patent No EP B1 braking, cornering), to provide the force necessary to move the acid from the bottom to the top of a battery cell. This principle removes the need for moving parts or gas injections into the battery and relies on the natural forces created in all directions from accelerating, braking and cornering as shown in Fig 3. From this it is evident that the driving forces will be acting in all directions of the battery so that the orientation of the battery Electrolyte dissipation path Wave created by braking force Front Front (transverse, lateral, front, rear) when fitted in the vehicle, will not affect the operation of the mixing device. Principle of operation The innovation of the 360 circular electrolyte mixing is that it is a passive working system, using no moving parts. The functioning principle is that of a hydrostatic pump. The design uses the forces from driving dynamics which create a small pressure difference between the lower and upper part of the electrolyte, causing a flow from the bottom of the battery through a side channel to the top of the mixing device. The acid is then returned via the top part of the mixing device, back into the upper level of electrolyte of the battery cell, thus providing a full 360 electrolyte circulation. Fig 4 is a schematic of the side view of a battery under a braking force in an automobile. The view is from the edge and is internal, the end plate is shown with a grid pattern, the sides of the battery container are labelled front and back related to the installed orientation. The mixing device principle is shown with a horizontal and vertical part in this early patent description of iq Power. The electrolyte is represented by a wavy line with arrows indicating the direction of the force exerted by the electrolyte under braking conditions. The horizontal dashed line is the original electrolyte level when the battery is at rest or not under any acceleration force. In this condition, the force exerted by the electrolyte provides

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7 besttechnology 81 Fig 6: The iq Power mixing device fitted into a flooded battery sufficient impetus to push the bottom level of the liquid up the narrow channel created by the mixing device and the cell wall. This creates a small wave above the horizontal section, which washes down along the section to be released back into the bulk electrolyte near the centre of the battery. Once the small wave of electrolyte is returned to the bulk liquid it will tend to sink if the SG is higher than the acid near the surface (Fig 5). The small wave, shown in Fig 5, is dissipated and follows the path shown in red with potentially more recirculation in the direction indicated by the arrow. Considerable work has been done by iq Power to maximise the mixing effect by enabling this recirculation process to occur with forces from all directions. Fig 7: Upper part of the mixing device showing acid flow and return Inlet iq Power Patent No EP B1 This means that despite the orientation of the battery in the automobile, the acceleration and deceleration of the car plus the cornering forces at 90 to the plane of motion will all contribute to the mixing effect. It should be mentioned, just for completion, that riding along on a bumpy road alone shaking the battery up and down does not show any mixing effect to the electrolyte, neither with mixing parts incorporated nor without. It is not vertical forces that create the movement of the electrolyte as shown in Fig 4 it is the horizontal forces resulting from acceleration, braking and cornering. Fig 7 shows a diagram of the top part of an older version (IQP 1) of the mixing device and the direction of the acid flow from the inlet channels to the outlets. The other features are holes for the electrolyte filling and terminal poles respectively. The latest design of the mixing device, which iq Power calls generation IQP-2, has been improved substantially compared to its predecessor Electrolyte filling hole Outlet Terminal holes version IQP-1 and has also been optimised for universal immediate use in all standard battery boxes (DIN, JIS, BCI). It also operates successfully independent of the electrolyte level and so is unaffected by varying levels caused by water consumption of the battery. The IQP-2 design has been on the market for well over a year and is already in service in a million of SLI batteries. It has been patented in over 60 countries including China. The mixing elements consist of polypropylene plastic parts, the same as used for battery cases and covers and are completely recyclable with no changes in the recycling process. Also, the parts require no modification to the battery plastic box or lead elements design. The parts are currently in use in battery containers of DIN, JIS, and BCI standard batteries. The passive mixing elements work equally effectively and efficiently when the battery is placed in different positions or orientations in the car, i.e. in the front (bonnet) or rear (boot), Outlet Inlet

8 82 besttechnology transverse or longitudinal, and require only one to two hours of normal driving for a complete circulation of the entire volume of electrolyte. No extra investment is needed from the battery factory, as iq Power supplies the ready-toinstall parts on an order-by-order basis. The parts also function on any electrolyte level above the minimum level designated by each manufacturer. The installation allows for battery production speeds of up to five batteries per minute per production line. The parts can be inserted manually of by robots. Passive electrolyte mixing boosts EFB cycle life by at least a factor of two The iq Power anti-stratification device not only eliminates early battery failure caused by acid stratification but also improves the cycle life of EFBs to near that of AGM batteries in the stop-start market. The cycle life improvement due to this mixing feature is immediate and does not require changes to Fig 8: Cycle test for standard flooded batteries with and without a mixing device 12.5 Fig 9: Cycle test for EFB batteries with and without a mixing device U/V U/V cycle test 17.5% DoD: L3 (71Ah nom. cap.) cycles Comparison of cycle life of a standard flooded battery without / with mixing elements. cycle test 17.5% DoD: L3 (72Ah nom. cap.) U - with electrolyte mixing U - no mixing U - with electrolyte mixing U - no mixing cycles Comparison of cycle life of an EFB battery without / with mixing elements. the battery or plate design. By eliminating acid stratification, laboratory tests have shown a doubling of the cycle life for batteries with the device when compared to the same type of battery without it. iq Power, using many different batteries from different suppliers from different regions, have demonstrated that a cycle life improvement is possible for all types of flooded vehicle batteries. Whilst the amount of improvement was dependent upon the general quality of the base battery, a doubling of the cycle life was the average result. Figs 8 and 9 show the increase in the cycle life of two different flooded batteries: conventional (9) and EFB (10). Both figures feature the same type of battery without mixing parts and with mixing parts. A motion simulator, especially designed to simulate normal driving conditions with an acceleration of 2.2m/sec 2 (0.23g), was used for the simulation of driving forces. The values were determined from long term vehicle measurements in combined city and highway travel. Fig 8 shows two conventional flooded batteries (L3, 71Ah) from the same manufacturer. The battery made without the addition of the mixing parts gave 193 cycles compared to the same type of battery with the passive mixing parts, which gave 372 cycles. Cycle test measurements were conducted according to industry standard VW , 17,5% DoD.

9 besttechnology 83 Fig 9 shows the difference between two EFB type batteries (L3, 72Ah) from a different manufacturer, one with and one without mixing. The battery without mixing gave 896 cycles whilst the same type of battery with the passive mixing parts gave 1786 cycles. The same test procedure was used as for the tests in Fig 8. It is well known that a constant homogeneous electrolyte maintains the performance of flooded batteries over a very long period of time and that this has a very positive effect on the service life of flooded batteries. A study by Volkswagen published in Chemie Ingenieurtechnik 2011, 83, No 11, confirms the positive effect of a passive electrolyte mixing device in flooded batteries with respect to cycle life and charge acceptance. Market presence and experiences Starter batteries with the patented iq Power 360 electrolyte mixing device are already in the market in areas as diverse as Canada, USA, the Middle East and Asia. The technology has also been accepted by an OEM car manufacturer and is in use in an EFB battery for stop-start engines since January Earlier this year, iq Power received the GreenTech award for environmentallyfriendly innovation, sponsored in conjunction with the German Association of the Automotive Industry (VDA). The 360 electrolyte mixing system is the only battery technology to also be honoured with the Frankfurt Automechanika Innovation Award. For further information please visit: