So my players had some fun with this and I added some aspects to it that I felt add to the tension that the players really enjoyed.
The party decided to stealth into the area where they needed the charges. You had stated that the charges had a 3 minute delay... so without them knowing the strength of the charges, once the first one was hidden and pressed, I started initiative. Every minute mattered and the charges were going down as they headed down to fight the final boss.
It made them handle the encounter more tactically and I think they got more into it. Overall... my players had a great time and for me, that is considered a win.
[4 of 5 Stars!]