There is a bug in the Big Shield Capacitor 2 :
My shields are lully charged on 384 (basic shield + shield booster + military enhancement), capacitors are charged on it's maximum capacity - 512 (shield capacitor + big shield capacitor + big shield capacitor 2). After some hits shields drop to zero, but capacitors are not discharged to the shield. When I sell Big Shield Capacitor 2, maximum capacitor capacity is 256 and all works normal - after some hits shields drop below 128 (384 - 256) and capacitors are discharged to the shields.
