AGM battery takes primary role for idle stop-start in microhybrids

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The AGM battery has 3-4 times the cycle life of a conventional flooded-cell lead-acid battery.

Although much of battery research and development has focused on hybrids, plug-ins, and electric vehicles (EVs), with lithium-ion cells at the forefront, there is an important additional application—the so-called microhybrid with idle stop-start. This feature, widely used in Europe since 2007, is coming to the U.S., and the AGM (absorbent glass mat) type lead-acid is its battery of choice.

That’s despite the fact it costs twice as much as a conventional lead-acid battery, and U.S. EPA test cycles have few stops of duration long enough to really improve the fuel-economy numbers that automakers report. But real-world numbers seen in Europe have shown it can be worth the price premium to motorists.

There, as a result of regulation-driven carbon-dioxide emissions limits, battery maker Johnson Controls Inc. (JCI) estimates that some 70% of vehicles produced in Europe will have idle stop-start by 2015. This includes hybrids of all types, from medium to high voltage, but the key market is the internal-combustion engine, in a microhybrid with a 12-V starting/charging system.

Although 12-V idle stop-start systems require robust starters or other sophisticated restart systems to operate on this voltage, the battery itself is the primary key to effectiveness. The less capable the battery, the fewer the number of times the engine can be stopped—it’s that simple. The conventional flooded-cell lead-acid battery (FCB) can perform the function perhaps for weeks.

Fuel-economy improvements 

The two premium lead-acid alternatives are what are being used in Europe: an enhanced-flooded-cell battery (EFB) or the much more expensive but more capable AGM. The fuel-economy improvements vary according to car maker, but a BMW study estimates up to 4% overall for current systems, with the potential for 10% if a higher charge-acceptance-rate battery (over 100 A) were available. The present AGM charge acceptance rate is about 25 A, the exact rate depending on operating conditions, according to JCI, which estimates fuel-economy improvement of 5-10%.

The EFB is reasonably capable for idle stop-start in economy cars with low feature content, such as the European versions of Toyota Yaris, Fiat 500, and Ford Fiesta ECOnetic. It has thicker plates and includes a polyester scrim plate that holds additional lead alloy and applies light pressure in the cell to minimize lead flaking losses. It has a cycling life that is double that of a conventional FCB, according to JCI.

The AGM has 3-4 times the cycling life of an FCB, so despite its 100% higher cost, the AGM dominates. It takes more than 70% of the European idle stop-start market, with the majority supplied by JCI’s Varta brand, which also is the source for the 12-V battery in the Chevrolet Volt. The Kia Rio’s optional idle stop-start, first microhybrid in the U.S., has an AGM sourced in Korea, from Sebang Global Battery.

The AGM is the only type known by AEI to be used in 12-V stop-start systems planned for the U.S. market. JCI also owns the OPTIMA brand, a spiral-cell AGM used on Corvettes, among other performance applications.

Battery construction

JCI’s AGM has a positive plate with a glass “fleece” separator holding the electrolyte, and it exerts a high contact pressure in the cell. During charging, oxygen leaving the positive plate recombines with hydrogen from the negative plate to form water, which maintains the water content of the acid within the battery cell plate set. This is called recombinant operation.

The AGM, basically a sealed battery, also is called a VRLA type (valve-regulated lead-acid) because it does have a pressure-relief valve. However, the valve is intended to open only in the event of overpressure (typically in the range of 1-4 psi/7-28 kPa), which would be an abnormal condition—primarily a charging system malfunction.

Although more sensitive to under-hood heat than a conventional lead-acid, the AGM is the most efficient and flexible of the lead-acid types. Lead and acid, the key ingredients, are of ultrahigh purity, so the charging effect is superior—more amps floating in are transferred to electrical charge than rejected to gas or heat. It converts up to 96% of the charge into electrical energy vs. 80-85% for the conventional flooded cell and a maximum of 90% for the gel cell. This latter type holds the electrolyte in a silicone gel, giving it recombinant characteristics similar to those of an AGM but with shorter life and even greater thermal sensitivity.

Holding the electrolyte in the AGM “fleece” separator or a gel means relatively uniform acid distribution vs. an FCB in which the moving electrolyte results in density stratification, greater at the bottom of the cell. This can lead to corrosion, affecting both battery performance and service life.

The AGM design has low internal resistance, and in the time it has been in European service is on track to match the 4- to 5-year service life of a conventional lead-acid while meeting the added challenge of stop-start, said Craig Rigby, Vice President of Global Product Engineering for JCI’s Power Solutions. “There are no unusual warranty issues, which shows it is robust, and no reports of diminished function,” he told AEI.

Idle stop-start strategy 

The AGM can discharge quickly, a plus for restarting. Rigby said JCI's goal is even faster recharge—improvement in “dynamic charge acceptance.” This would make it more effective for regenerative braking and allow a more aggressive idle stop-start strategy. Battery makers are looking for improvements in both the chemistry and the materials for plate and separator designs to further reduce internal resistance to improve AGM performance.

Idle stop-start strategy is based on what the battery can tolerate, how far it can be discharged, and, of course, the need to maintain a normal service life. Under favorable conditions, the AGM can be allowed to drop to as low as 20% state of charge (SOC) from repeated stop-starts. However, the bottom line is maintaining adequate battery SOC in the two situations of a stop-start.

One is the length of the stop, and this is more than just a matter of waiting until traffic breaks. With greater battery capacity and the possibility of more regenerative braking to recharge, the engine can be stopped earlier, as when coasting to a stop. A related factor, of course, is the engine-off electrical load, which with a 12-V system does not include electric air-conditioning, and the microhybrid is unlikely to add it (thermal cold storage in the evaporator is a more likely approach for A/C).

The second factor is time needed to recharge so that the typical idle stop-start is disabled if the car is crawling in heavy stop-and-go traffic and stopping repeatedly. In addition, idle stop-start can only be enabled if the ambient temperature conditions permit. At very low ambient, when the battery cannot deliver the same amount of energy as at moderate ambient, and when cranking loads may be higher, the idle stop-start will be limited—if permitted at all. BMW says that maintaining an adequate SOC is an issue when ambient temperature drops below 0°C (32°F). High ambient temperatures (29-32°C/84-90°F), when A/C operation must be maintained, also result in cancellation of idle stop-start.

These AGM and EFB versions of the lead-acid battery offer acceptable functionality, and there are extra-cost ways to improve the start-stop—even with present levels of technology, such as a second battery to maintain accessory load during an idle stop. The Volvo DRIVe and Mercedes-Benz BlueEFFICIENCY microhybrids are examples cited by JCI. Also under consideration is addition of ultracapacitors, which can be charged quickly and provide a burst of power for a restart.

Battery alternatives

BMW Group has been conducting research with Axion Power International. This is a U.S. research and development company that has proposed a lead-carbon battery it claims will have performance advantages in terms of power delivery and recharge rates, charge acceptance, and long life cycles in deep-discharge use.

Another potential design is the nickel-zinc battery (Ni-Zn) from PowerGenix. This is another U.S. company, and it says its solution can provide sustained, high-charge acceptance over a long life span, at lower cost, and with half the weight of VRLA batteries. The claimed performance characteristic would enable an aggressive idle stop strategy, minimizing the cancellation of stop-start in heavy stop-and-go traffic.

Over 85 years ago, inventor Sakichi Toyoda, whose family founded Toyota Motors, offered a cash prize for a battery that had greater energy density than gasoline. There is nothing in the research laboratories that is approaching even the 10% level, but gradual improvement of the fuel-economy payback from idle stop-start can help keep the gasoline engine a moving-forward target.

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