Design Considerations for Keeping Your Battery Pack in Top Performance: Balancing and Equalization vs. Pack MonitoringBy Tom Bartley • Mar 28th, 2009 • Category: Battery Technology for Hybrid and Electric Cars
The history of battery packs for electric vehicles is filled with stories of early failures and fires. Optimum and safe pack performance requires balancing and equalization management of the pack state of charge (SOC). A battery pack SOC is the equivalent of a fuel gauge which tells you how much usable charge remains in the pack.
Battery pack balancing and equalization management is the regulation or protection of individual battery cells from hitting the maximum or minimum limits when fully charging or discharging the whole pack.
Here are several methods used by battery pack designers to keep a battery pack in balance in order to get top performance out of the pack.
- Ignore it – For only few cells in series and a limited number of charge/discharge cycles (like a 12-volt starting battery with six cells) that don’t deplete the battery, balancing doesn’t add much to the battery life. For large numbers of individual batteries in a full parallel and series matrix connection the distribution of cells may be good enough to average out variations and provide some fault tolerance without balancing, however, it can create a huge connection and cooling problem. For example, the Tesla Roadster sports car is thought to have a matrix connected type of battery pack with over 8,000 small AA batteries and connections. It almost always turns out that the cells in the middle of the pack are the hottest and hardest to cool. I’ve suggested that a good university student project would be to model such a design and determine sensitivities to manufacturing tolerances, aging, and failures of individual cells.
- Charge each cell or sub pack separately – For example, if a number of 12-volt packs are connected together for a higher voltage pack, each 12-volt battery is charged to a full voltage SOC with a separate 12-volt charger rather than charging the whole pack with a single high voltage charger. If this is done periodically, like once a day, the assumption is that the charge/discharge cycles during the day won’t result in enough unbalance to be a problem. Multiple chargers and connection points can make this a bit cumbersome and time consuming.
- Use a shunt clamp - During charging, as each battery in the pack reaches a predetermined clamp voltage, shunt (switch) the current around the battery to the next battery in line. This process is better known as equalization because every battery in the pack is “equalized” to the same maximum beginning voltage. This requires extra switching circuits that can be tricky for high voltage and high current applications.
- Shuffle the charge - During operation constantly move some charge (current) from batteries with a high SOC to batteries with a low SOC. This requires even more complex circuits and can be limited by how much current can be moved at any time. It works well if the batteries don’t go through a lot of rapid deep cycling because it could take a while to complete all the shuffling. A failing battery in the pack could put a heavy demand on the shuffling system.
- Monitor the pack - Continuously monitor the individual battery voltages in the pack and create a maintenance flag to replace the failing battery. This can cause disruptive and costly down time if not well planned, and typically requires many sensors, extra wiring connections, data acquisition network, some form of processing and reporting (display). This approach is commonly used in addition to balancing and equalization.
- Use a passive network - Connect a resistor in parallel with each cell in the pack. This is a cheap and easy equalizing voltage divider that may work well depending on the resistor value and the length of time available for voltage equalization. However, it can create inefficiencies, generate extra undesired heat, and shorten the battery pack life because of extra deep discharges.
- Others - Dozens of issued patents and published patent applications offer a source reference for finding other schemes to solve this problem.
By managing the battery pack SOC and limiting the operation to stay between a full charge and full charge depletion, battery pack life can be significantly extended.
Why is it bad to fully deplete the pack? Some battery chemistries change the composition of the electrode surfaces at the extreme ends of the SOC (fully charged or full depletion). For example, lead acid batteries at a depleted SOC deposits a sulfate coating on the plates thereby reducing the available electrode surface area. The electrode surface area is where the electric action takes place and the amount available determines the battery performance. Other chemistries such as Li Ion seem to be much less susceptible to electrode damage. However, all chemistries are susceptible to individual cell damage due to imbalances at the extreme ends of the SOC. Most battery pack manufacturers will recommend operating in the middle 30% to 50% SOC for optimum battery pack life.
Take the Toyota Prius as an example. Toyota does extremely well in the battery pack design and operation in the Prius hybrid car. The Prius Nickel Metal Hydride (NiMH) battery pack has 800 to 2000 full charge cycles because of the nature of the battery chemistry. However, by allowing use of only 7% of available battery capacity (usually in the middle of the SOC, about 50-60%), Prius’ battery packs are achieving 8 to 10 years and 100,000 to over 300,000 mile life of the pack. When Toyota says you can drive the Prius on electric alone, this really means the system allows you to drive on electricity alone using 7% of available SOC before the engine comes on for recharging. Because pack life performance has been much better than expected, Toyota has increased the operating SOC range to 15% to increase driving efficiency by capturing more of the braking energy.
The only downside of all of this complex battery pack SOC management is the fact that you are carrying around all the extra battery weight that it appears does not get used. You charge the battery pack only to 50-60% SOC, then use only 7% of that before recharging again. You may be tempted to somehow shave off battery weight to lighten the load the car has to lug around. In reality, it’s actually cheaper to extend the life of the battery pack (through maintaining ideal SOC range, hence heavy battery pack weight) than to save fuel by carrying around less weight.
Similar considerations apply to plug-in hybrids and all electric vehicles that we are going to look at in future posts. You probably don’t need to know all this to drive your hybrid or electric car, but you can give yourself extra battery life by taking advantage of every convenient charging opportunity and not waiting until the battery is fully depleted. We’ll talk more about leakage depletion and how to avoid shelf life damage.
Tom Bartley is an industry veteran with 30 years of experience in general business, marketing, project and product management, and engineering research and development. Mr. Bartley provided executive management support including technical and business oversight to heavy-duty hybrid-electric prototype projects as they evolved into production. He developed cost models for energy storage and fuel savings, and power models for ultracapacitor packs. Mr. Bartley is well known throughout the industry of heavy-duty hybrid-electric buses and trucks, having delivered many papers and presentations since 2003. Mr. Bartley maintains a blog at TomBartleyIdeas.com. Follow twitter.com/TLBartley.
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