Passive Battery Cell Balancing

In the automotive and transportation marketplace, large battery stacks provide high output power without producing harmful emissions (that is, carbon monoxide and hydrocarbons) associated with gasoline-powered combustion engines. Ideally, each individual battery in the stack equally contributes to the system. However, when it comes to batteries, all batteries are not created equally. Even batteries of the same chemistry with the same physical size and shape can have different total capacities, different internal resistances, different self-discharge rates, etc. In addition, they can age differently, adding another variable in the battery life equation.

A battery stack is limited in performance by the lowest capacity cell in the stack; once the weakest cell is depleted, the entire stack is effectively depleted. The health of each individual battery cell in the stack is determined based on its state of charge (SoC) measurement, which measures the ratio of its remaining charge to its cell capacity. SoC uses battery measurements such as voltage, integrated charge and discharge currents, and temperature to determine the charge remaining in the battery. Precision single-chip and multichip battery management systems (BMS) combine battery monitoring (including SoC measurements) with passive or active cell balancing to improve battery stack performance. These measurements result in: 

  • Healthy battery state of charge independent of the cell capacity
  • Minimized cell-to-cell state of charge mismatch
  • Minimized effects of cell aging (aging results in lost capacity

Passive and active cell balancing offer different advantages to the battery stack and Analog Devices offers solutions in our battery management product portfolio for both methods. Let’s first examine passive balancing.

Passive Balancing Allows All Cells to Appear to Have the Same Capacity

Initially, a battery stack may have fairly well matched cells. But over time, the cell matching degrades due to charge/discharge cycles, elevated temperature, and general aging. A weak battery cell will charge and discharge faster than stronger or higher capacity cells and thus it becomes the limiting factor in the run-time of a system. Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current, it drains a small amount of energy from high SoC cells during the charging cycle so that all cells charge to their maximum SoC. This is accomplished by using a switch and bleed resistor in parallel with each battery cell.

Figure 1. Passive cell balancer with bleed resistor.

The high SoC cell is bled off (power is dissipated in the resistor) so that charging can continue until all cells are fully charged. 

Passive balancing allows all batteries to have the same SoC, but it does not improve the run-time of a battery-powered system. It provides a fairly low cost method for balancing the cells, but it wastes energy in the process due to the discharge resistor. Passive balancing can also correct for long-term mismatch in self discharge current from cell to cell.

Multicell Battery Monitors with Passive Balancing

Analog Devices has a family of multicell battery monitors that include passive cell balancing. These devices feature a stackable architecture, allowing hundreds of cells to be monitored. Each device measures up to 12 series of connected battery cells with a total measurement error of less than 1.2 mV. The 0 V to 5 V per cell measurement range makes them suitable for most battery chemistries. The LTC6804 is shown in Figure 2.

Figure 2. LTC6804 application circuit with external passive balancing.

The LTC6804 features internal passive balancing (Figure 3) and can also be configured with external MOSFETs if desired (Figure 4). It also has an optional programmable passive balancing discharge timer that allows the user more system configuration flexibility.

Figure 3. Passive balancing with internal discharge switch.

Figure 4. Passive balancing with external discharge switch.

For customers that wish to maximize system run-time and charge more efficiently, active balancing is the best option. With active cell balancing, energy is not wasted, but rather redistributed to other cells in the stack while both charging and discharging. When discharging, the weaker cells are replenished by the stronger cells, extending the time for a cell to reach its fully depleted state. For more on active balancing, see the technical article “Active Battery Cell Balancing.”

作者

Kevin-Scott-Blue

Kevin Scott

Kevin Scott 是ADI公司电源产品部门的产品营销经理,负责管理升压、升降压和隔离转换器、LED驱动器和线性稳压器。他曾担任高级战略营销工程师,负责制定技术培训内容,培训销售工程师,并撰写了大量关于公司众多产品技术优势的网站文章。他在半导体行业已有 26 年从业经验,历任应用、业务管理和营销职务。 Kevin 于1987年毕业于美国斯坦福大学,获得电气工程学士学位。联系方式:kevin.scott@analog.com。

Sam Nork

Sam Nork

Sam Nork 于 1988 年加入 ADI 公司和凌力尔特公司(现已成为 ADI 的一部分)。作为 ADI 公司电源产品业务部总经理兼凌力尔特设计总监,Sam 领导着一支由 120 多名工程师组成的开发团队,主要负责电池充电器、ASSP、PMIC 和消费电子电源产品工作。Sam 亲自设计并发布了众多便携式电源管理集成电路,而且是 11 项已授权专利的发明人/共同发明人。加入凌力尔特之前,Sam 曾在马萨诸塞州威明顿担任ADI公司的产品/测试开发工程师。他拥有达特茅斯学院文学学士和工程学士学位。