External ambulatory infusion devices for the infusion of a liquid drug over an extended time period may be utilized for a number of therapies. In particular, such devices form the basis for a therapy of Diabetes Mellitus by CSII (Continuous Subcutaneous Insulin Infusion. Besides diabetes therapy, those devices may be used for a number of additional therapies, such as cancer treatment or pain therapy, without requiring substantial modification.
Such devices are typically powered by one or more electrical energy storages. In the following description, the energy storage is assumed to be a rechargeable or non-rechargeable battery. It may, however, also be another kind of electrical energy storage, such as a high-energy capacitor or a fuel cell.
As described herein, a simplified battery model is used to aid: the energy storage is considered as a non-ideal battery, which includes an ideal battery. More specifically, the non-ideal battery may include an ideal DC voltage supply, having an off-circuit-voltage, in series with an internal resistor having an internal resistance. The off-circuit voltage is the voltage that can be measured between the battery terminals when no current is drawn from the battery. In contrast, the terminal voltage is the voltage that can be measured at the battery terminals under normal operational conditions. When current is drawn from the battery, it is generally smaller as compared to the off-circuit voltage because of a voltage drop over the internal resistor. The voltage drop is defined according to Ohm's law by the internal resistance and the current that is drawn. The terminal voltage equals the off-circuit voltage if no current is drawn. This is the case, for example, if the voltage is measured with a voltage measurement unit of substantially infinite input resistance.
When an energy storage component such as a battery is being used, neither its off-circuit voltage nor its internal resistance is constant over time. The off-circuit voltage decreases and the internal resistance increases. Both effects reduce the terminal voltage if current is drawn. These effects are illustrated in FIG. 9, showing an exemplary off-circuit voltage U0 curve 600 and the corresponding internal resistance Ri curve 605 as measured over the usage time of an exemplary battery.
In many existing devices, a battery may be connected to a test load, such that the battery is stressed with a defined testing stress and the corresponding terminal voltage is measured in order to ensure that the device does not abruptly terminate operation due to a depleted battery. The measured terminal voltage is compared to at least one alerting voltage threshold and the user of the device is alerted if the voltage is below the threshold. Testing is typically carried out repeatedly with a time interval of some minutes. If the infusion device is, for example, an insulin pump that is designed for pulsed insulin administration of a small amount of insulin every few minutes, the testing may be carried out prior to, during and/or after each administration.
Testing is accompanied by the drawback that each test is associated with some power consumption by the test load, thus reducing the remaining usage time of the energy storage. Furthermore, it has been found that some batteries show a defect or anomaly which results in the terminal voltage dropping from a high level at which no alert would be generated to a very low level that is not sufficient for operating the device, with a steep drop of the terminal voltage within a short time. The device may accordingly terminate operation without alerting the user. Since the user is, in case of this event, not aware of the terminated infusion of the drug or substance, severe adverse effects may result.