Medical device with battery testing

A battery powered extracorporeal medical device configured to be attached to the body or carried close to the body of a user, the medical device comprising:

• battery contacts configured to contact the battery;
• a capacitor in parallel electrical arrangement with the battery contacts;
• and
• a controller configured to control operation of the medical device, the controller including a battery tester configured to:
• determine a first capacitor voltage U1 at a first point in time t1;
• determine a second capacitor voltage U2 at a second point in time t2 subsequent to time t1;
• draw between time t1 and time t2 a test current, wherein time t1 is determined by the beginning of drawing the test current and time t2 is determined such that the capacitor voltage at time t2 is in a steady state and substantially constant while the test current is being drawn;
• and
• determine a charging state of the battery from the difference between the voltage U1 and the voltage U2 and/or from a time difference between time t1 and time t2.