The invention relates to the field of electrical battery systems. More particularly, the present invention relates to ion lithium battery cells balanced in an integrated battery.
Lithium-ion battery cells having been connected in a serial string to make a battery. The state of charge of each of the cells can disadvantageously change as the battery is operated or allowed to stand unused over long periods of time. Most batteries maintain the individual cells in a balanced condition through the use of overcharge to bring all the cells up to a highly charged state. The use of overcharging depends on monotonically rising internal losses as the cell state of charge increases. However, lithium-ion cells do not exhibit monotonically increasing losses as the cells are charged to higher states of charge. Lithium ion battery cells do not generally tolerate overcharge without being degraded.
A number of methods have been used in the battery industry to keep individual cells balanced. Voltage regulators are sometimes placed across each cell in the battery to hold the individual cells at a prescribed voltage by shunting any unneeded current around the cell as recharge occurs. Relays have been used to bypass unneeded current around individual cells when a prescribed voltage level is attained. Relays have also been used to switch balancing load resistors onto any cells that rise over a set threshold above the average cell voltage. These relays have been activated either manually as needed, or by differential voltage sensing circuits in the battery. All these methods work well, but require significant active electronic circuitry to always be powered for the purpose of maintaining the battery state of health. A fully passive method involves the placement of appropriately sized resistors in parallel with each battery cell, thus providing linearly rising losses with increasing cell voltage. One disadvantage of the parallel resistor method is that the method can fully discharge the battery, which can damage lithium-ion cells when the battery is left in an unused or open-circuit state. Another disadvantage of the parallel resistor method is the extremely slow rebalancing of the cells. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a battery system having balanced lithium ion battery cells.
Another object of the invention is to provide a battery system having balanced lithium ion battery cells each having a pair of terminals across which is coupled a passive nonlinear device.
Yet another object of the invention is to provide a battery system having balanced lithium ion battery cells each having a pair of terminals across which is coupled a passive nonlinear diode.
Still another object of the invention is to provide a lithium ion battery system having a series of balanced lithium ion battery cells each having a pair of terminals across which is coupled a respective passive nonlinear device.
The invention is directed to a multicell battery system having a nonlinear device coupled across respective battery cells. In the preferred form, the battery cells are lithium ion battery cells and the nonlinear device is a passive diode. The diodes serve to balance charge and discharge currents. The diodes are passive components assuring cell change balance without the use of active components and monitoring systems. The passive device preferably has an exponential current-voltage (IV) curve and is placed in parallel with each battery cell. The IV curve preferably has leakage current below the cell self-discharge rate of ˜C/100,000 in the 2.8 to 3.0 volt range where the cell is fully discharged, and should pass currents on the order of C/1,000 at the maximum expected cell operating voltage that is typically 4.1 to 4.2 volts. Preferably, the balancing devices used for all cells have IV curves that are matched. The use of balancing devices having such nonlinear characteristics will not cause cells to become depleted when the battery is left open-circuited, and can correct for cell imbalances. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.
An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to
The battery has a positive terminal and a negative terminal that is typically connected to a load during operational use of the battery. The load tends to draw energy from the battery. The load could further include a charger, not shown, for charging the battery. Each cell has a matched balancing device placed across positive and negative terminals of the cells. Thereafter, the battery can be charged and discharged with no noticeable changes in performance as a result of the balancing devices, unless the cells in the battery develop a persistent imbalance in voltage. When a persistent imbalance in voltage exists, the higher voltage cells will preferentially leak current through the respective balancing devices, thus being restored to the same state of charge as the other lower voltage cells. Because the balancing devices are designed to pass very low discharge current at voltages below about 3.0 volts, such as when the battery is left in an open circuited condition for a lengthy period of time, for example, over a ten-month period, each cell will discharge in a balanced manner through the balancing devices, down to approximately 3.0 volts. Any further cell discharge is controlled by the internal self-discharge rates of the individual cells.
The cell-balancing device preferably has an IV curve that passes a current less than the cell self-discharge rate ˜C/100,000 in the 2.8 to 3.0 volt range where the cell is fully discharged. The balancing device is also designed to pass an exponentially increasing current as the voltage is increased, such that the device should pass currents on the order of C/1,000 at the maximum expected cell operating voltage of 4.1 to 4.2 volts. The maximum current carrying capability of the device needs to be no more than C/400, which corresponds to 4.0 ma for a 1.5 Ah cell and 125.0 ma for a 50.0 Ah cell. Maximum power dissipation at 4.2 volts, based on a C/1,000 rate, is 5.25 mW for a 1.5 Ah cell, and 210.0 mW for a 50.0 Ah cell. The cell-balancing device is designed to operate over a temperature range of at least −10° C. to 80° C., which covers the maximum operating temperature range of lithium ion cells with a 20° C. margin.
Referring to
Referring to all of the Figures, and more particularly to
The balancing performance expected beyond one month can be obtained by a computer simulation based on the observed capacity and voltage performance of the ideal performance. Also shown in
This balanced battery can be used to maintain cell balance in lithium ion batteries. The nonlinear rebalancing may improve battery life, reliability, and safety. The nonlinear rebalanced battery has no active electronic components, is light and small, dissipates negligible heat, and can maintain state of charge balance between cells having mismatched internal losses more than twenty times more effectively than can alternative resistive balancing systems, and will not discharge cells below safe levels. The nonlinear devices are preferably matched with identical IV performance characteristics. Semiconductor photolithography processes provide nearly identical operating characteristics. As such, matched thin film diodes fabricated by batch semiconductor processes are preferred, that can be applied to both discrete and thin film battery cells. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.
The invention was made with Government support under contract No. F04701-00-C-0009 by the Department of the Air Force. The Government has certain rights in the invention.