Batteries for Back Up Power Supply

I have been using back up power supplies on my Omniturn controls for 25 years. They keep the machine from crashing in the event of a power plip or failure. The other benefit I get is I can unplug the control without powering down and wait till a hot electrical storm goes by.
The original batteries for these APC power supplies used to last about 10 years. now they are not lasting much longer than the warrantee period.
A friend suggested using deep cycle trolling batteries for this instead of the APC batteries. These heavier UPS unites use a pair of 12 volts to get 24 volts. Would there be any issues with the larger batteries hurting the charging circuit? Any other issues? I am thinking of drilling a hole in the unit and running the wires to the pair of batteries on the floor.

Yes there may be an issue. The heatsink on the battery charger is only large enough to charge the original small batteries. They are known to catch fire.

If your setup is such that you power down the system as soon as possible, then it will work.

But if the batteries are fully discharged then you should disconnect them and charge them separately. You can ensure the ups doesn't turn on, by feeding it 120vac mains through a motor starter style contactor with pushbutton start.

A 12v fan running off the ups output (wall wart) installed inside the ups may be enough to prevent burnup. (Which can also result from the increased run time of thr larger external batteries)

Alternatively just buy two flooded 10 amp hour motorcycle batteries, they should last a long time.

Johansen, I was thinking of the larger batteries for longevity as I am getting tired of replacing them every couple of years. These things are just for power blinks and occasional heavy electrical storms. During a storm they maybe on for 30 minutes but no machining is going on, so the power drain is minimal. I will look for premium smaller batteries.
Thanks.

It depends on the type of charger in the UPS.

If it is a "dumb" charger, as many older UPS had, which is essentially just diodes off the existing transformer, then yes it may be an issue. If it is a "real" controlled current charger, then it should just take longer the larger the battery is, and not increase the risk.

Those UPS are cost sensitive consumer products. The batteries in them are often worked very hard, at currents up to 10 to 20 times higher than the design was intended to handle, when supplying power.

Naturally the batteries do not last long. They are not intended to. The manufacturer likely does not expect that the UPS should have to work more than a total of perhaps 2 hours in it's "lifetime".

The UPS we had actually never had to operate during a power failure, even though we have rather a significant number of them. It only ever had to supply power once, when I tested it.

While they CAN supply high currents, that will decrease their life. For the one linked, the max discharge is 105A (for 5 seconds), but the RATINGS are based on a discharge at the "20 hour rate", which would be 0.35A.

There is a table of voltages corresponding to various discharge rates. You can see how the time varies with current and final voltage. The table is almost certainly "typical" values and not a guarantee.

You can also see the charge current rated at 2.1A.