Need extended run time on UPS

[DaveGoTech]

We need our server to stay up for at least several hours instead of 10 minutes when power fails. The UPS is APC Smart UPS 700. The load is currently about 43%. It has 2 internal 12V (24V total), 7.2AH batteries.
I was thinking on getting 2 large 77AH batteries and connecting them externally.
Has anyone done this?
Do I disconnect the internal batteries or leave them connected? The UPS does have an outlet on the back for an external factory battery pack, but their pricing is outrageous.
Are there any other concerns or precautions? We do intend to build an enclosure for them if it works.

 

[cajuntank]

I'm am no electrical engineer by any means... so that being said, what is you voltage on youe 77AH batteries?

 

[DaveGoTech]

I have already completed this project.