Sorry Stephane wasn't meaning to hi-jack the link. I was interested in knowing the why nots though...
Although it seems that changing the battery type out will fix a lot of the stated problems, what you can replace it with will depend on the battery bay inside the unit...
But I did like the idea of having com's ports and several power type outlets available, that and I have good access to ex-government sales in Canberra.
More of a glorified battery box at the end of the day.
Ps. Just checked youtube and someone has been there and done that already. Nice sup'd up ups...
$80-100 for a 2nd hand UPS about the same for parts and batteries at a guess. I would stack a fan between my batteries packs before adding another cell layer, but, I am a bit bias against li-ion.
https://www.youtube.com/watch?v=fXXGhgvwCtQ (part two)
Might be something to think about with the newer Al-ion coming online soon(2024?) it would make the "awesome" setup... a lot safer(you can swallow a Al-ion battery), no operating temps as per Li-ion, three times the power and incredible recharging times.