Although lead-acid batteries are ubiquitous in automotive starter-lighting-ignition (SLI) applications, they have, with a few exceptions, been bypassed in favor of NiMH for current hybrid applications and li-ion for applications to come.
There are a long list of reasons for that decision: battery weight, limited cycle life in heavy use, loss of capacity from sulfation, corrosion of electrodes, degradation of active material, maintenance requirements, short life at high temperatures and a long charge time, among others. But developers of new lead-acid batteries argue that the problem is not with the fundamental chemistry, but with the battery design to date—a limitation they are trying to rectify.
|Nickel price over five years. Click to enlarge.|
There arguments are also being supported by an external, non-technology factor—the rapid rise in the price of nickel. (See chart at right.)
At the recent Advanced Automotive Battery Conference 2007, a panel session discussed the requirements for lead-acid batteries in hybrid applications, and explored some of the emerging lead acid technologies.
Eckhard Karden of Ford Research and Advanced Engineering Group in Aachen, Germany, noted that today’s absorptive glass-fiber mat (AGM) lead acid batteries already can meet the demands of micro-hybrid vehicles with limited regenerative braking capability. BMW’s use of AGM batteries to support its start-stop and regenerative braking systems in the 1- and 5-Series models and now in the MINI is a case in point.
Karden outlined the requirements for hybrid electric vehicle (HEV) applications, which involve a fundamental shift away from the traditional SLI requirements.
Robustness and reliability. HEV traction batteries are required to meet six-sigma (<12 ppm failures) over an operational life of 10 years or 240,000 km (150,000 miles). By contrast, current production SLI batteries not only do not perform to this standard, but are seen as a “wear-out” component to be replaced several times during vehicle life.
Shallow-cycle life. Cyclic wear has been a dominant cause of SLI battery failure in high-demand applications such as taxis. Hybridization increase cyclic battery use significantly, with quantitative throughput demands from micro- to full-hybrids varying by a factor of around 30, according to Karden.
Service life in partial state of charge operation. To support regenerative braking, HEV batteries operate at a partial state of charge (PSOC) to provide significant pulse-charge acceptance. Classical SLI batteries, however, are generally continuously charged at alternator output voltage, and are not optimized for PSOC operation. (BMW has developed a work-around for this with its battery management software that will be described below.)
A significant fraction of [lead-acid] battery capacity might be lost early during service life due to sulfation, particularly in the lower part of the negative plates. At higher discharge and charge rates (high-rate partial state of charge—HRPSOC), as they would be typically applied to traction batteries in mild HEVs, lead-acid (AGM) batteries tend to show equally detrimental sulfation...
Ensuring robust PSOC operation is, hence, a key challenge for the application of lead-acid batteries in advanced applications, and requires careful joint optimization of battery design and operating strategy of the battery system and vehicle.
Dynamic charge acceptance. HEV applications require good charge acceptance in a dynamic discharge/charge microcycling operation—dynamic charge acceptance (DCA). This is in contrast to the recovery form deep discharge in traditional SLI battery applications. In lead-acid batteries, DCA capability is extremely sensitive to the short-term previous charge/discharge exposure of the battery.
Battery management. Energy management and hybridization require precise monitoring and active control of the battery.
Karden also outlined the limitations of lead-acid electrochemistry:
The high molar mass of lead restricts the gravimetric energy and power density.
The electrode reaction is a true chemical conversion of lead and lead dioxide into lead sulphate and vice versa. The microscopic electrode structure is thus destroyed and rebuilt during each charge/discharge cycle, eventually leading to gradual disintegration of the porous electrode structure.
Water in the aqueous electrolyte and lead in the positive current collectors are thermodynamically unstable at the equilibrium cell voltage, leading to side reaction s such as water loss, hydrogen evolution and grid corrosion.
The electrolyte is not inert, but is consumed in the discharge reaction. This leads to a variety of issues such as transport limitation and acid concentration gradients that in turn lead to an inhomogeneous current distribution. That in turn can lead to localized overcharge or undercharge—the latter being one of the causes for sulfation during shallow cycling at partial SOC.
So far, in most HEV applications, lead-acid batteries could not meet the performance and life requirements. Instead, mostly advanced battery technologies have been chosen, namely nickel-cadmium, nickel metal hydride, lithium-ion or supercapacitors. Nevertheless, there are multiple efforts, both at established battery manufacturers and at technology-driven start-up companies to develop lead-acid based battery systems that meet the requirements of at least certain types of hybrid electric vehicles.—Eckhard Karden
BMW and AGM for micro-hybrid functions.. Earlier this ear, BMW launched two micro-hybrid functions applied in the 1- and 5-Series, and now in the MINI: Brake Energy Regeneration and Auto Start Stop Function. (Earlier post.) Both are based on today’s 14-volt vehicle electrical system and current series components.
BMW is using valve-regulated AGM batteries to support both functions. AGM batteries use absorptive glass-fiber mats soaked with electrolyte to fill the distance between plates in a lead acid battery. AGM batteries slow down structural disintegration and cyclic wear; reduce the build-up of vertical electrolyte stratification that, in conjunction with a poor charge balance, can lead to sulfation; and can inhibit acid spillage in case of mechanical battery destruction.
BMW’s battery operates at a partial state of charge to be able to accept the recuperated energy from brake regen. The battery management system monitors the state of charge of the battery at all times, and maintains a minimum state of charge to preserve cranking ability. As soon as the SOC falls below a threshold value, a switch-on request is triggered, restarting the engine (absent any constraints preventing restart). There is a switch-off prevention level, below which the start stop will not function. The amount of charge reserved for cranking varies with temperature—e.g., in cold weather, the battery reserves more charge for cranking.
Using the valve-regulated AGM battery increases the “ability of cyclisation by about three times in comparison to a conventional lead acid battery,” according to Christian Diegelmann, Development Engineer, Department Electrical Energy Storage Systems, BMW Group.
|Effpower’s design. Click to enlarge.|
Effpower bipolar lead acid batteries. Effpower, founded in 1999 by Volvo and Gylling Optima Batteries AB, is commercializing ceramic bipolar lead-acid batteries. Bi-polar designs have advantages for high-power batteries, including homogeneous current distribution; low resistance; and high current throughput.
Earlier attempts to commercialize bipolar lead-acid batteries failed primarily due to corrosion and electrolyte leakage. Lead is not inert enough to form a stable substrate for a bipolar plate. Other problems included the sealing between the cells; poor adhesion in the interface between the positive active mass (PAM) and the bipolar plate; shedding of PAM; and gas venting due to high internal gas pressure during operation.
In Effpower’s bipolar design the partitioning walls are made out of porous, lead-infiltrated ceramic (LIC) plates. The Effpower bipolar plates have shown high corrosion resistance, and the lead surface on the bipolar plate enables good contact to the active material in the same way as common lead acid technology.
A limiting factor in the use of the battery for hybrid applications had been charge acceptance, according to Bengt Wahlqvist, Chief Technical Officer for Effpower. The company had used graphite additives in the negative mass, however, with significant improvement in recharge.
|Effpower Insight Fuel Consumption Test|
Effpower has installed an Effpower battery pack in a Honda Insight for 15,000 km if testing in real traffic. The vehicle underwent four different drive cycles in Göteborg: city driving in rush hour traffic; combined highway and city; a city bus cycle; and an Effpower designed acceleration/braking test cycle.
The Honda Insight with the Effpower lead-acid battery pack outperformed the OEM NiMH pack in terms of fuel consumption. (See chart at right.)
Effpower currently has formed a partnership with Banner Batterien in Austria for the manufacturing of Effpower batteries. High volume serial production is planned for mid-2008. Effpower also has been testing the Advanced lead Acid Battery Consortium’s work with negative graphite paste with promising results.
Advanced Lead-Acid Battery Consortium. The Advanced Lead-Acid Battery Consortium (ALABC) is focused on two primary design modifications to enable the application of lead-acid batteries in HEVs: the provision of a grid design that allows the battery plates to accept the high charge rates required; and the incorporation of elevated concentrations of carbon in the negative active mass to alleviate sulfation.
|Configuration of the Ultra Battery.|
Patrick Moseley of the ALABC also noted work being done on the “Ultra Battery”: the combination of a supercapacitor in parallel with a lead-acid battery to cope with the high rate partial SOC cycling.
In the Ultra battery, a carbon capacitor plate is attached to the negative plate and enclosed within a single battery casing. Prototype batteries of this design meet or exceed the FreedomCAR targets for power, available energy, cold cranking and self-discharge, for both minimum and maximum power assist systems, according to Moseley. The battery is now in road test.
Firefly Energy and graphite foam plates. The Firefly battery replaces the conventional lead plates in a lead-acid battery with a lightweight carbon or graphite foam to which the chemically active material—in the form of a paste or slurry—has been applied. Firefly contends it can deliver lead-acid battery performance comparable to NiMH, but at about one-fifth the cost, and with greatly reduced weight compared to traditional lead-acid batteries. (Earlier post.)
According to Kurt Kelley, Firefly Energy’s CTO, the micro-cellular structure of the foam enables much greater utilization of the chemistry. The Firefly 3D battery features higher power, fast recharge capability, 15-20% increased capacity at slow discharge, better deep discharge recovery than competitive lead-acid batteries, and better capacity retention at high discharge rates.
Firefly is currently developing a newer, lighter-weight, higher-rate battery technology.