The Lithium-Ion Battery Balancing Act

More and more the world is running on lithium-ion batteries. Consumer electronics are basically built for them-the combination of low-weight to energy and a relatively slow loss of charge making them ideal for portable devices. In this arena, lithium-ion battery technology is fast becoming the standard.

ED Based on the article in Electronic Design, lithium-ion batteries are headed for new frontiers, like electric cars. It's all because a large array of lithium-ion batteries are capable of producing higher-voltages, currents and capacity than their sealed lead-acid (SLA) counterparts. Designers are finding that every breakthrough in technology brings its own peculiar set of problems. The greatest among these is the issue of cell balancing.

Lithium Ion Battery When you look at the use of the lithium-ion battery for electric vehicle application there is obviously a demand for high capacity. This can be reached by creating cells that are purposefully engineered or oversized. These larger battery packs require additional safety measures and better solutions for cell balancing. The chief demand for larger lithium-ion is coming from the electric vehicle market. Not surprisingly, according to Robin Tichy, they are the ones driving the innovation when it comes to practical motorized application of lithium-ion cell technology. Automakers like Tesla and Lotus Elise have been at the forefront. For instance, in order for Tesla Motors to create an electric vehicle capable of doing 0-60 in 4 seconds with a top speed of 125 miles per hour and a range of 220 miles, they've had to design their very own lithium-ion battery as well as a microprocessor to control it. By the way, the battery uses 7000 individual lithium-ion cells and weighs close to 1000 pounds. That's quite a bit of heft to be lugging around.

The Challenges of Lithium-Ion

The decade long rise of handheld devices has allowed lithium-ion to grow at an astonishing rate against the less-practical sealed lead-acid (SLA) batteries that are still currently in wide use. There seems to be little standing in the way of li-ion batteries becoming, more and more the preferred method of power for low-current telecom and transportation industry gadgets. The challenges associated with large battery arrays will not go away quietly.

There are several major obstacles facing the makers of large array lithium-ion cells like problems with heat radiation during periods of charging and discharging is huge. Using heat sinks and active cooling are two of the methods for managing thermal issues.

There is also the fact that most cell manufacturers are discouraging the use of their lithium-ion batteries as part of a multiple-cell arrangement. This is most common in those vendors who manufacture prismatic cells. Often times, you'll find prismatic cells limited to three or four cells in a pack. Much of this is derived from new shipping regulations that categorize large lithium-ion battery packs as Class 9 Hazardous Materials. Thus shipping becomes problematic.

Yet another issue, and, possibly, of greater concern to those who are delving in to large lithium-ion batteries, whether from a manufacturing or an application standpoint, is cell balance.

In order to deliver the high voltage needed to supply, let's say, a car with enough power to operate efficiently, a high-series of cells need to be used. This is fine until you take in to account that batteries have the tendency to become out of balance. Once one battery is out of balance, the other batteries will perform at the same level. Over time, the imbalance will grow through charging and discharging cycles.

The simple solution is to design a battery where cell balance is no longer a problem. Well, to do that, engineers first have to understand the principles behind cell imbalance and that has yet to be done. They have found that heat can play a part in creating a cell imbalance over time. In reality, thermal stress is a fairly common issue and has been linked to faulty design of the batteries' host device.

"What about a microprocessor?" you ask. Well, that can actually add to the problem. According to Robin Tichy, "Because self discharge doubles for each 10°C rise in temperature, even heat from a microprocessor can cause radical differences in self discharge across a multi-cell battery pack. Non-uniform electrical loading of the pack causes the same uneven discharge, and high discharge rates can exacerbate these issues."

Focusing on Cell Imbalance

Engineers have noted several factors that make temperature a major concern in large lithium-ion battery arrays. The gradients are naturally larger. Even the way the cells are constructed within the array can have an impact on temperatures.

Active cooling, while beneficial, may not cool the cells evenly and this is making the science of how a pack is constructed extremely important. The best way to promote even cooling is to build the array so that gradient exposure of the cells is minimized.

Cell balance is indeed a major issue for the practicality of large-array lithium-ion battery usage and there are two essential ways that balance is currently being looked at; bypass redistribution and active redistribution. They are different in how they work, but the goal is the same–to minimize the amount of variation between cells thus extending battery life and performance.

Bypass Redistribution

Bypass redistribution, also known as bleed balancing, takes advantage of a separate path for the current to a cell that is imbalanced. It's already commonplace with many lithium-ion powered low-current devices. But it gets sticky when it comes to larger arrays in that there is a trade off that has to take place between energy conservation and energy delivered. This is due to the fact that, in bypass redistribution, you are, essentially, bleeding off excess energy.

Active Redistribution

Active redistribution compensates for the imbalance by moving charge from higher-charged cells to the adjacent lower-charged cells. In essence, you are equalizing the amount of cell energy across the array. This can happen during every functional period of the batteries' life; charging, at idle and during discharge. There are two ways that active redistribution works: capacitive and inductive topology.

With capacitive topology, a switch capacitor is used to measure the voltage and move energy from cell to cell. The capacitive method however, is limited because it only works when there is peak voltage. Inductive topology, on the other hand, allows for the storage of energy from high-level cells and then delivers the voltage when needed. The two biggest obstacles for inductive topology are cost and a higher part count.

It's safe to say that, as of right now, there are issues present in practical and trouble-free large lithium-ion battery arrays. Where there is a will, there is a way, and over time, the obstacles will be conquered. You can bet your batteries.