The bilevel equalizer is the first hybrid that combines the high performance of an active equalizer with the low cost of the passive equalizer.
“It’s a game changer because we solved the weak cell issue in lithium ion battery storage for packs with hundreds of cells,” said Dr. Ngalula Mubenga, assistant professor of electrical engineering technology at UT. “Whenever we are talking about batteries, we are talking about cells connected in a series. Over time, the battery is not balanced and limited by the weakest cell in the battery.”
Battery makers and automotive manufacturers balance cell voltages in a large battery pack using either a passive circuit, which loses more energy, or an active circuit, which is ten times more expensive. “In spite of their significant losses, passive equalizers are used in most applications because they are relatively simple and low cost,” said Mubenga.
Instead Mubenga developed a technque that groups the cells into sections where each cell within the section is balanced by a passive equalizer, while the entire section is balanced by an active equalizer.
“If there are 120 cells in a battery, divide the cells into 10 groups of 12,” she said. “Then you only need nine active equalizer units and 120 passive equalizer units using the bilevel equalizer. With current active equalizers, manufacturers would have to use 120 active equalizers. For manufacturers that can’t afford to use only active equalizers, the bilevel equalizer is the solution to the problem.”
Experiments have shown that the bilevel equalizer increases the discharge capacity of lithium ion batteries by about 30 percent, and the pack lasts longer because the cells are balanced. “Instead of an electric vehicle’s battery lasting only four years, it would last much longer,” she said.
The research team is licensing the hybrid equalizer and retrofit kit to manufacturers.
After earning her professional engineer license in Ohio, Mubenga went on to found her company called the SMIN Power Group, which develops and installs solar power systems in communities throughout the Democratic Republic of Congo. “My passion is deep,” Mubenga said. “In places like that small town of Kikwit, if you have solar power, you can have electricity and save lives.”
The materials for batteries are also a key factor. “Most of the minerals for today’s electronics are mined in the Democratic Republic of the Congo,” said Mubenga. “The Democratic Republic of the Congo is a leading producer of cobalt, copper, gold, diamond, tantalum and tin in the world. Indeed, the Democratic Republic of the Congo contains about 50 percent of the world’s reserve of cobalt, a mineral used to make lithium ion batteries.”