A charge-control unit was developed as part of a program to validate Li-ion cells packaged together in batteries for aerospace use. The lithium-ion cell charge- control unit will be useful to anyone who performs testing of battery cells for aerospace and non- aerospace uses and to anyone who manufacturers battery test equipment. This technology reduces the quantity of costly power supplies and independent channels that are needed for test programs in which multiple cells are tested. The cost savings that were achieved in a test program are shown in Figure 1. Battery test equipment manufacturers can integrate the technology into their battery test equipment as a method to manage charging of multiple cells in series.
The unit manages a complex scheme that is required for charging Li-ion cells electrically connected in series. The unit makes it possible to evaluate cells together as a pack using a single primary test channel, while also making it possible to charge each cell individually. Hence, inherent cell-to-cell variations in a series string of cells can be addressed, and yet the cost of testing is reduced substantially below the cost of testing each cell as a separate entity.
In the original aerospace application, life-test data on Li-ion cells is critical in order to assess their performances and capabilities relevant to NASA missions and exploration goals. For example, for many NASA missions that involve flight in low orbits around the Earth, batteries are required to endure more than 30,000 charge/discharge cycles, and Li-ion batteries are relatively new to this type of application. The scheme required for charging Li-ion cells is more complex than that typically required for charging the alkaline cells that the Li-ion cells may supplant. This scheme includes the establishment and enforcement of strict cell voltage limits to ensure safe, long operation.
So that this requirement could be addressed in laboratory testing, the present charge-control unit was developed to manage the current through each cell once the voltage of the cell reaches the established limit. By use of this unit, multiple Li-ion cells connected in series within a battery pack can be charged from a single current source, and yet the charging of each cell is controlled independently of the other cells. More specifically, by use of this unit:
- Each cell in the series string is charged at full current until the cell voltage reaches the limit;
- Once the voltage of a given cell reaches the limit, the voltage is held at that limit and the current through that cell is tapered off so that the cell continues to gain some charge without becoming overcharged; and
- Even after some cells have reached voltage limit, other cells that are at lower states of charge continue to be charged at full current until they reach the voltage limit (see Figure 2).
The unit consists of electronic circuits and thermal-management devices housed in a common package. It also includes isolated annunciators to signal when the cells are being actively bypassed. These annunciators can be used by external charge managers or can be connected in series to signal that all cells have reached maximum charge. The charge-control circuitry for each cell amounts to regulator circuitry and is powered by that cell, eliminating the need for an external power source or controller. A 110-VAC source of electricity is required to power the thermal-management portion of the unit. A small direct-current source can be used to supply power for an annunciator signal, if desired.
This work was done by Concha M. Reid, Michelle A. Manzo, and Robert M. Button of Glenn Research Center and Russel Gemeiner of QSS Group, Inc.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17703-1.