随着北美汽车市场对启停系统的接受程度日益增加，各家厂商开始考虑借助锂离子电池、超级电容、48V“轻混”系统等一系列技术，应对相当车辆频繁启停，通常每天多达超过20次的工况。简单来说，启停系统的工作原理是通过减少发动机空转，达到降低车辆燃料消耗和排放的目的。

即使是最不可撼动的铅酸电池也开始进化了。如今，过去几十年来最常用的传统电池正逐渐被支持深度循环(deep-cycling)功能的增强型富液式蓄电池（Enhanced Flooded Battery，下简称EFB）和吸附式玻璃纤维隔板（Absorbent Glass Mat，下简称AGM）电池所取代。虽然电子元件的快速普及也是一个因素，但造成这一趋势的关键原因还是在于启停系统的崛起，而启停系统会对电池的快充功能和使用寿命提出更高要求。

“一般而言，较大的12V AGM电池的使用寿命是传统电池的4倍，这对汽车启停系统目前的推广非常重要。”通用汽车(GM)启停系统全球总监Kathi Walker表示，“锂离子电池的应用可在近期成为现实。”

虽然锂离子电池可能终有一日会成大气候，但目前由于其过于高昂的价格，并不适合用作车辆启动、照明和打火供电的主要电源。不过，锂离子电池仍然可以在动力系统之外发挥作用。

“如果要综合考量性能与成本，铅酸蓄电池和高级铅酸电池仍是世界上最适合内燃机汽车的电源技术，而且这一情况在未来10年内不会改变。”全球最大的铅酸蓄电池供应商江森自控(Johnson Controls)动力解决方案部高级市场&技术战略师Craig Rigby表示，“这就是说，我们可以通过新增锂离子电池等技术，提高铅酸电池的燃料经济性，以支持制动动能回收等更多的车内电动功能。”

目前，启停系统正是驱动汽车电池领域转型的主要推力。美国阿岗国家实验室曾在2015年发布了一份研究报告，分析启停系统对车辆启动系统元件寿命的影响，其中也包括电池。研究显示，传统富液式铅酸启动器电池的使用寿命与很多因素有关，其中受车辆启动次数的影响最小。事实上，真正影响电池使用寿命的因素，是发动机频繁启停时不能充分为蓄电池充电，以及发动机在停止状态下的过度放电。由于放电深度的影响，每次发动机熄火时，车内辅助设备的耗电对电池使用寿命的影响最大，也最直接。如果电池可以在发动机两次启动之间充分充电，上述的耗电对电池的影响几乎可以忽略不计，甚至可以认为根本就不会产生影响。

与此形成对比的是，阿岗国家实验室的研究结果显示，无论发动机的启动次数如何，如果车辆的电池一直无法达到满充状态，电池就会更快发生故障。研究同时发现，由于交流发电机的功率输出较低，发动机怠速时并不能为电池进行有效充电，而驾驶过程中的充电效果最佳。（详见以下链接http://www.anl.gov/energy-systems/publication/stop-and-restart-effects-modern-vehicle-starting-system-components。）

对于电气系统设计师而言，为频繁重新启动的发动机提供电力仅是需要面对的挑战之一。为了让消费者接受带有启停系统的车型，保证车内音响和照明功能处于恒定状态非常重要，这就需要12V电池所扮演的“缓冲”角色。

“在启动时，车辆的电系会经历一次电压骤降，从而导致车内照明变暗等状况。”Walker表示，“为了保证车内照明等功能的稳定性，车辆可以采用多种技术，保护车内元件不受电压骤降的影响，从而在车辆启动过程中保证消费者的满意度。这些技术包括双电池设计、DC/DC转换器和超级电容等。”

目前，市面上已经出现了多款采用双电池系统的车型，这种设计可以支持车内大量的耗电功能与应用。由于如今的车辆不仅采用了很多复杂的节能技术，而且还应用了多种多样的安全功能，这种双电池的设计也将日益普遍。

举例而言，2016款2.5L雪佛兰迈锐宝的启停系统正是采用了双电池设计：一块12V电池位于发动机盖之下，另一块则安装在车尾。系统（为了加快启动速度而搭配了一个串联螺线圈启动器）经过特别校准，当车辆内燃机停止工作时，第二块电池可以接收信号立刻接管车内的辅助系统供电负载，比如照明、空调、音响、车窗和门锁的功能。此外，即使发动机停止工作，车辆的空调此时仍然可以正常运行，只不过会转换为低功率模式。

“双电池系统可以在多个方面体现自己的价值，未来这种设计将越来越普遍，”Rigby表示，“两块电池可以在车辆的启停过程中提供更稳定的电压，还能为高级安全系统提供高级别的可靠性冗余。此外，这种双电池、双化学系统的设计也是性能和成本之间的最佳平衡，可以为汽车生产商和消费者提供最大的价值。”

通常来说，车辆的第二块电池多为48V系统。系统的电压更高，驱动停车摄像头和整合式传感器等功能就越容易。虽然48V系统优势多多，但专家称汽车行业未来可能仍会沿用12V系统。不过，起亚(Kia)公司曾在2016年芝加哥车展上发布了2017款Niro混动多功能车，这款CUV并未搭配12V铅酸电池，而是直接通过48V锂电池组为车辆头灯、挡风窗雨刷等功能供电，而这些功能通常是由传统的12V铅酸电池支持的（见http://articles.sae.org/14614/）。

“更高的电压可以降低系统的峰值电流。”德州仪器(Texas Instruments)商业发展经理Stefano Zanella表示，“电池对其在特定安培小时内释放的容量和负载电流非常敏感，充分利用电池的这一特点，可以有效延长电池使用寿命、减少电缆尺寸，从而降低成本，并减轻重量。”

Zanella解释说，48V系统必须搭配DC/DC转换器或第二块12V电池工作，这是由于目前的汽车采用了大量成本非常低的12V元件，汽车生产商一时很难将这些元件全部替换为48V元件。

除了48V系统，一些开发者还选择使用超级电容为车辆的启停系统供电。这些超级电容可以在发动机关闭期间提供电力，不会给电池造成任何负担。

“由于超级电容在放电时并不涉及化学反应，因此放电速度非常快。”通用汽车的Walker表示，“2016款凯迪拉克ATS和CTS均采用了超级电容的设计，可以在车辆启动时提供更快的能量提升。这种能量流可以加快发动机的启动速度，有助于为驾驶员提供更加流畅的再启动体验。”

这种设计可以延长铅酸电池的使用寿命。在凯迪拉克系统的设计过程中，通用公司与大陆集团(Continental)合作，采用了麦克斯威科技公司(Maxwell Technologies)的超级电容，有力补充了车辆的电池电源，但目前高性能版ATS-V和CTS-V尚未采用超级电容的设计。

“增加一个超级电容可以显著降低电池的启动电流，从而延缓电池化学材料的损耗。”麦克斯威高级产品营销经理Jens Keiser表示，“此外，这种设计还能降低电池的峰值电流需求。”

作者：Terry Costlow
来源：SAE 《汽车工程杂志》
翻译：SAE 上海办公室

Powering up the new stop-start systems

As stop-start systems gain acceptance in North America, a range of technologies including lithium ion batteries, ultracapacitors, and 48-V "mild hybrid" systems are under consideration to handle the aggressive start cycles, typically more than 20 per day, that are required of these systems. Stop-start is aimed at reducing vehicle fuel consumption and emissions by reducing engine idling.

Even the venerable lead acid battery is evolving. Enhanced flooded batteries and absorbent glass mat (AGM) technologies with deep-cycling capability are slowly displacing batteries used for several decades. While the rapid expansion of electronics overall is a factor, a key reason is the rise of stop-start, which requires quick recharging and long lifetimes.

“Larger 12-volt AGM batteries, which deliver up to four times the typical life cycle of a conventional battery, are important to the current implementation of stop-start,” said Kathi Walker, GM Global Engineering Lead for Stop-Start Systems. “Lithium-ion batteries could be used in the near future.”

While Li-ion batteries may someday take over, they’re currently too expensive to displace the primary storage source for starting, lighting and ignition. However, Li-ion may expand beyond its role in electrified powertrains.

“Lead-acid and advanced lead-acid batteries continue to be the best technology for internal combustion vehicles in terms of performance and cost, and this will continue well into the next decade,” said Craig Rigby, Advanced Market & Technology Strategist at Johnson Controls Power Solutions, the largest global supplier of lead-acid batteries. “That said, fuel efficiency can be found by supplementing the lead-acid battery with other technologies such as Li-ion to deliver brake regeneration and support more electrified functions in the vehicle.”

At present, stop-start is the driving force for these changes. Research published in 2015 by Argonne National Laboratory examined the impact of stop-start systems on vehicle starter system component life, including the battery. The study revealed that the expected lifespan of a conventional flooded lead-acid starter battery is impacted minimally from the number of starting events. Rather, battery life is mostly impacted by limited charge times between frequent engine start events and from excessive discharge during engine-off events from accessory loads. The length of and the cumulative accessory power draw during each engine shutdown event has a direct and strong effect on battery longevity because of the depth of discharge. If the battery is returned to a full charge between engine starts, the effect on battery life is negligible or nonexistent.

Conversely, the ANL research showed that battery failure will occur more quickly if a full charge is never reached regardless of the number of engine start cycles. Also, idling was determined to not be an effective method of recharging the battery because of low alternator power output; driving is best. (See http://www.anl.gov/energy-systems/publication/stop-and-restart-effects-modern-vehicle-starting-system-components.)

Simply providing the power to regularly restart engines is just one challenge for electrical system designers. Keeping radios and interior lights at constant levels—a "buffering" role played by the 12V battery—is a critical factor for consumer acceptance of stop-start vehicles.

“During an auto start, there is a voltage dip in the vehicle’s electrical system,” Walker said. “In order to maintain functionality of cabin systems such as interior lights, there are technologies that can be added to a vehicle to protect the components against a voltage dip and maintain customer satisfaction during an auto start. These technologies include dual batteries, DC/DC converters and ultracapacitors.”

A handful of vehicles already use dual battery systems to support large numbers of power-hungry features and functions. It may become more common as more safety critical technologies are combined on vehicles that use complex fuel-saving techniques.

The 2016 Chevrolet Malibu 2.5-L's stop-start system, for example, uses two batteries: a 12-V battery under the hood and a second one mounted in the rear. The system (equipped with a tandem-solenoid starter for faster starts) is calibrated so that when the ICE shuts off, the second battery is signaled to handle the car's hotel loads—i.e., power the lights, climate control, audio, windows and door locks. Also, with the engine off the car's climate control remains on, albeit operating on reduced power.

“Dual battery systems will become more common as they provide value in several ways,” Rigby said. “Having two batteries provides better voltage stability during start-stop events as well as redundancy to support the high degree of reliability necessary with advanced safety systems. In addition, a dual battery, dual chemistry system allows the optimization of performance and cost to deliver the best value for automakers and consumers.”

Often, the second battery is a 48-V system. Engineering higher-voltage systems can make it simpler to power functions such as parking cameras and integrated sensors. Though going to 48V is beneficial, experts said the industry trend has been to retain the 12V systems. But at the 2016 Chicago auto show, Kia unveiled its 2017 Niro hybrid CUV, which uses its lithium battery pack to provide power for the headlights, windshield wipers, and other traditional tasks of the 12-V lead-acid battery, which has been eliminated (see http://articles.sae.org/14614/).

“Higher voltages help keeping the peak amperage down,” explained Stefano Zanella, Business Development Manager at Texas Instruments. “Batteries are sensitive to the ratio of their capacity expressed in amp hours and the load current, increasing their lifetime and reducing cable size, making them cheaper and lighter."

A 48V rail will require a DC/DC converter or a second 12V battery, he explained, because there are just too many 12V components in a car that are very cheap and cannot be effectively replaced by 48V components.

Some developers are turning to ultracapacitors to provide quick boosts for stop-start systems. They can provide power to turn the engine over without straining the battery.

“Since ultracapacitors do not rely on a chemical reaction to supply their energy, ultracapacitors can discharge their stored energy very quickly,” GM's Walker said. “The 2016 Cadillac ATS and CTS take advantage of ultracapacitors to provide a quick energy boost during the auto start. This burst of energy allows the engine to start faster and helps in providing a more seamless restart for the driver.”

This design technique can extend the lifetimes of lead acid batteries. The Cadillac system, co-developed with Continental, uses Maxwell Technologies ultracaps to augment battery power. It is not available on the ATS-V and CTS-V performance versions.

“Adding an ultracapacitor lets the battery deliver significantly lower starting currents, which leads to less degradation of the battery chemistry,” said Jens Keiser, Maxwell's senior product marketing manager. “Also, the battery sees fewer high-current peak demands.”

Author: Terry Costlow
Source: SAE Automotive Engineering