I remember in somewhere I read that they should not be stored in the eCharger and this must be only used to charge the batteries.
possibly overcharging the aids or not being able to fully dry out moisture? I keep my non-rechargeable aids in a drying/UV kit each night and take the batteries out.
Copied from user manual. https://usa.bestsoundtechnology.com/media/2015/04/echarger-user-manual.pdf
Charging and drying The hearing instruments are first charged and then dried.
Press the front button to open the eCharger and unlock the lid.
Place your hearing instruments into the charging slots
Hearing instruments in eCharger Drying process is finished.
Remove hearing instruments from eCharger when you want to use them.