Surgery – Miscellaneous – Devices placed entirely within body and means used therewith
Reexamination Certificate
1999-05-14
2001-05-08
Lacyk, John P. (Department: 3736)
Surgery
Miscellaneous
Devices placed entirely within body and means used therewith
Reexamination Certificate
active
06227204
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a charging device for charging of rechargeable NiCd, Ni-metal hydride or lithium batteries of implants (with one or more cells or with one or more secondary elements, as required) by transcutaneous transmission of electric power from an external power transmission part to a power receiving part which forms a part of the implant, the charging device being provided with a charging current detector which in a first charging phase allows a relatively high charging current to flow and which, after the cell voltage of the battery has reached a predetermined limiting charging voltage, in a second charging phase reduces the charging current compared to the charging current which flows at the end of the first charging phase. The invention furthermore relates to a process for charging of implanted batteries of implants by transcutaneous transmission of electric power in which, in the first charging phase, a relatively high charging current flows, and in which, after the cell voltage of the battery has reached a predetermined limiting charging voltage, in a second charging phase, the charging current is reduced compared to the charging current flowing at the end of the first charging phase.
2. Description of Related Art
When a battery is charged, only one part of the supplied electric power is converted into charge. Another part of this power is converted into heat on the internal resistance of the battery and is lost for charging. The power loss can lead to an impermissible temperature rise of the implant housing, and thus, to damage of the surrounding tissue.
Another part of the supplied energy drives secondary electrochemical reactions which, for example, lead to gas evolution within the battery. This applies mainly when the battery has reached a higher charging level, for example, is charged to more than 80% of its nominal capacity. Especially over years of operation of an implanted battery does the capacity ratio of the positive and negative electrodes of the battery cell or cells shift due to electrolyte loss and passivation of the electrode surfaces and/or by corrosion with time, such that, during charging (i.e., re-charging), a greater and greater preponderance of the gas-forming over the gas-consuming reactions occurs, and thus, the internal pressure of the cell rises quickly during charging. As the gas pressure rises the cell housing swells, which under certain circumstances can lead to destruction of the cell or the implant. The increasing corrosion and/or passivation of the electrodes and the decrease of the electrolyte-wetted electrode surface, at the same time, cause an increase of the internal resistance of the battery.
In one such charging device and a charging process of the indicated type, use of a charging protocol is known (U.S. Pat. Nos. 5,690,693; 5,702,431 and 5,411,537) which provides for either the instantaneous charging current or the duty factor of a fixed charging current to be reduced as the charging level of the battery increases. Thus, in U.S. Pat. No. 5,411,537, a multistage charging process for lithium batteries is proposed in which, during the charging process, the no-load voltage of the battery is periodically interrogated to estimate the charging state of the battery, and in which, depending on the no-load voltage determined at the time, one of several predetermined charging current values is selected, the charging current values becoming smaller as the no-load voltage increases. The charging process is ended when the measured no-load voltage exceeds a stipulated boundary value.
U.S. Pat. No. 5,702,431 discloses an alternative of the above described multistage charging process in which the amplitude of the charging current remains constant, but the charging current is pulse width modulated and the duty factor is changed depending on the periodically determined no-load voltage values. Similarly, in U.S. Pat. No. 5,690,693, a pulse width modulation device with a variable duty cycle is used as a controller for the current level applied during the charging process.
SUMMARY OF THE INVENTION
The primary object of the present invention is to devise a device and a process for charging of rechargeable NiCd, Ni-metal hydride or lithium batteries of implants which, on the one hand, for the battery preclude harmful charging conditions with great reliability, and especially limit temperature evolution and gas formation in an implanted battery for the most part but, on the other hand, ensure effective, and thus, comparatively rapid, charging.
This object is achieved in a charging device in accordance with the invention by the charging current detector being made such that, in the course of the second charging phase, it sets the charging current to a value such that the cell voltage during the second charging phase is kept at least roughly to a predetermined constant voltage value.
A process in accordance with the invention is achieved by the charging current in the course of the second charging phase being set to a value such that the cell voltage is kept at least roughly to a predetermined constant voltage value.
In the device and process of the invention, the charging of the battery is regulated depending on the internal resistance of the battery. It is ensured that the cell is charged only with as much energy as the electrochemical state allows, without excess gassing or heating of the cell occurring. Older cells with increasing internal resistance, in this way, acquire less charge than new cells. Charging can be configured according to the individual patient needs without the immediate danger of overcharging, because the charging current is reduced when the limiting charging voltage is reached. An almost full cell reaches this limiting charging voltage very quickly and thus acquires only little additional charge by the end of the charging process due to the low charging current.
The implant can be basically any implantable medical or biological device. Thus, among others, it can be an active electronic hearing implant, a cardiac pacemaker, a defibrillator, a drug dispenser, a nerve or bone growth stimulator, a neurostimulator or retinal stimulator, a pain suppression device or the like.
The term lithium battery, as used here, is intended to encompass any rechargeable lithium systems, especially cells with liquid organic electrolytes, solid electrolytic cells, cells with inorganic electrolyte and lithium ion cells, such as carbon/lithium-cobalt oxide cells, carbon/lithium-nickel oxide cells, and carbon/lithium-manganese oxide cells with liquid or solid polymer electrolytes. Examples of known rechargeable lithium systems can be found in the aforementioned U.S. Pat. Nos. 5,702,431 and 5,411,537 and in Halaczek/Radecke “Batteries and Charging Concepts,” 2nd edition, 1998, page 148, Franzis' Verlag GmbH.
Thus, the charging current detector is preferably made such that the predetermined value at which the cell voltage is kept in the course of the second charging phase is at least roughly equal to the value of the limiting charging voltage which is reached at the end of the first charging phase. In this way, after reaching the limiting charging voltage, in the further course of charging harmful effects on the battery are precluded while, at the same time, the charging rate remains high.
It goes without saying that the voltage value at which the cell voltage is kept in the course of the second charging phase need not be exactly constant. Practical experience has shown that this voltage value will deviate from the limiting charging voltage reached at the end of the first charging phase generally by not more than ±30%, preferably by not more than ±20%, and especially preferably by not more than ±10%.
According to one development of the invention, an arrangement is provided which acquires the time change of the charging current and ends the charging process when the change of the charging current per unit of time falls below a predetermined minimum value in the course
Baumann Joachim
Leysieffer Hans
Volz Andreas
Implex Aktiengesellschaft Hearing Technology
Lacyk John P.
Nixon & Peabody LLP
Safran David S.
Szmal Brian
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