One of the current problems in the current project is the battery pack. According to the electronics man, it's a small problem, but he explained as follows: the battery pack consists of a bunch of lithium-ion non-rechargeable batteries, custom made by an American company. The batteries come with a specification that they deliver up to a certain voltage for a certain temperature. The spec sheet shows a curve; the lower the temperature, the lower the voltage. At room temperature the batteries give around 3.8 V but the Telis electronics have to operate on a balloon. The balloon its trajectory will take it in the atmosphere where temperatures can occur between -19 and -70 degrees Celsius.
This is a much broader range than what's usual for electronics on a satellite. Once these are packed in isolation, the temperature range is quite small.
The problem is now that tests show that the batteries don't deliver up to spec for temperatures around -40 degrees Celsius and possibly lower. The electronics man thought up one solution, which would involves a separate battery pack. Together with a temperature sensor in a control loop, the second pack makes certain that the main battery pack (feeding the electronics) is kept at the right temperature. It must be checked now that there is enough room above the electronics casing in the frame that's carried by the balloon.