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I see that manassehkatz has already given you a good answer and plausible explanation for what you observed, so I'll fill in a few other points.

Your battery is 5 years old, so is getting to the point where it could start to fail.  I wouldn't <i>expect</i> a 5 year old battery to be bad, but it's to the point where it's plausible.  If the battery were 10 years old, I'd just replace it immediately, then see what the symptoms are.

Don't compare UPS batteries with car batteries, even though both are probably lead-acid (in a consumer grade UPS).  Cars are very harsh environments for batteries.  Car batteries have a lot of robustness built in.  This results in a much heavier and larger battery for the same amount of energy storage, but you get reliability in return.  They are also over-specified so that they can still do their job with some degradation.

Your UPS battery is probably a sealed lead-acid type, probably 12 V, but I've seen 24 V types used too.  These are designed for storage efficiency, deep-cycle ability, and no chemical leakage.  They are not designed to handle much vibration or wide ambient temperature swings.  As a result, they are physically more delicate.

Individual lead-acid cells produce about 2.1 V when fully charged.  A "12 V" battery actually has six cells in series.  These cells are packaged together into this unit we call a "battery", but are really independent electrically.  A plausible failure mode is that one cell went short inside the battery.  This can happen simply over time due to stuff migrating, dendrites growing, or a physical failure.  Again, car batteries are physically beefier, so you don't usually see this kind of failure before other effects make the battery no good anymore.

If one cell out of six is shorted, the battery voltage will only be 10.5 V instead of 12.6 like the UPS expects.  That's enough of a difference that the UPS would either consider the battery "bad" or too discharged to use, especially when the UPS knows the battery should be fully charged.

Your UPS manufacturer will be more than happy to sell you a replacement battery, at a hefty markup for the convenience and for people that don't feel comfortable doing this themselves.  I just looked, and Mouser lists 836 different varieties of sealed lead-acid batteries (https://www.mouser.com/c/power/batteries/sealed-lead-acid-battery).  Of course, there it's up to you to know what you need.  You need to know the voltage, roughly the capacity in amp-hours, maximum size, and physical connection type.

If you're not comfortable with all that (or can't find someone to help you who is), just get the replacement from the UPS manufacturer, and consider it the cost of having a UPS every 5 years.

If you're more adventuresome, consider that the new battery doesn't have to go in the same physical place the old one did.  You can extend the cables to the battery outside the UPS box and have the new battery sitting next to the UPS instead of being nicely tucked inside.  That's fine electrically, as long as the cables are thick enough and not too long.  I'm envisioning a 1-2 foot extension, just enough to place the battery next to the UPS.

Now you only need to know the voltage and capacity.  If you're lucky, the old battery has its spec printed on it, or there is a plaque on the UPS or you can find it in the UPS documentation.  The voltage needs to match exactly.  Fortunately, it's probably either "12 V" or "24 V".  There is some leeway in capacity.  I'd say &plusmn;50% is OK.  Capacity directly relates to run time.

You can estimate capacity from other specs.  Let's say you found the battery needs to be 12 V.  You have already said the UPS is rated for 450 W, but you also need to know the run time at that 450 W.  Let's say for sake of example that the run time is rated for 15 minutes at 450 W.

Ultimately, the battery provides a fixed amount of <i>energy</i>, which the inverter in the UPS converts from 12 VDC to 115 VAC.  Energy is power times time.  If the inverter is putting out 450 W for 15 minutes, then the total energy delivered is 113 Wh (Watt-hours).  There will be some loss in the inverter.  For a consumer-grade UPS, let's figure the inverter is 85% efficient.  That means the battery has to cough up 132 Wh.

The battery has to be able to hold 132 Wh in this example.  However, that's not how batteries are sold.  You need to convert this to capacity, which will be listed in Ah (Amp-hours).  You do this by dividing the Wh by the battery voltage.  (132 Wh)/(12 V) = 11 Ah.  So in this example, you're looking for a 12 V sealed lead-acid battery that has 11 Ah capacity.  Anything in the 10-15 Ah range should be OK with your UPS.  10 Ah will reduce the run time a bit.  Higher capacity will give you more run time, but if you go too high the UPS may think something is wrong when it can't charge the battery as quickly as expected.