Complex sentence: As a result, each generator can only operate for 105 seconds before needing to cool off for another 120 seconds; alternating between the two I-field generators still leaves the Gundam defenseless for about 15 seconds.
Split into simpler sentences: As a result, each generator could only operate for 105 seconds before needing to cool off for another 120 seconds. Even when alternating between the two I-field generators, the Gundam still would be defenseless for about 15 seconds.