The runtime of a UPS battery backup system is determined by the size of the battery bank. The larger the bank of batteries, the longer the system will run. Think of the size of the battery, or batteries, as something comparable to the size of a water tank. The larger the tank, the water it can hold. The photo to the right is of 4 batteries that came out of 120v AC, UPS battery backup systems. Let's see how much power each has (watt hours) and how many toys we can run for one hour (amps @ 110v).
Battery 1; 12v, 4.5ah = 54 watt hours or .45amps at 110v for 50 minutes.
Battery 2; 12v, 6.0ah = 72 watt hours or ..60 amps at 110v for 50 minutes.
Battery 3; 12v, 80ah = 960 watt hours or 8.73 amps at 110v for 50 minutes.
Battery 4; 12v, 215ah = 2580 watt hours or 21.5 amps at 110v for 50 minutes.
Equations: V*Ah = Wh Wh/110v = amps @ 110v for 50 minutes
Battery #4 can run the kitchen (a 20 amp 110v circuit in most homes) for 1 hour basically folks. That is the kind of power to run the microwave, the fridge, and the toaster at the same time, for an hour. So if you want that kind of reserve time for your servers, or you need to run a monitor, computer, and printer while the power is out, the size of the battery is very important. Without the power in the tank, the electronics go OFF!
Just like a water tank, the important thing to remember during installation, hook up the wires or pipes right. The wires in our situation are just like the pipes for the water tank. Our concern is to keep the voltage (pressure in a water system) inside the levels needed so we don't break anything. Long story short, you install a 12 volt battery in a 12 volt system. When installing batteries, you add voltage (series connections) or amperage (parallel connections), depending on how you wire it up. JUST MAKE SURE YOU WIRE 12 VOLTS to 12 VOLT SYSTEMS. As long as you follow the rules above, and match the battery bank voltage, you can upsize the battery amp hours how you please.
What happens when you oversize the voltage in a UPS battery bank?
Should you try to hook up a 24 volt battery bank to a 12 volt system, it will never recharge. The charger in the UPS will not have adequate power to recharge the system. In addition, you may have a fire or burn out the electronics in the UPS when you first plug it in.
What happens when you undersize the voltage in a UPS battery bank?
Hooking up a 6 volt battery, in a 12 volt system isn't just a good idea, it can be explosive. THIS IS A VERY BAD IDEA! Simply put, a battery under an overcharge condition outgases hydrogen and oxygen. The first is explosive, the second will accelerate the fire. When a 12 volt system sees the 6 volt battery, it will try to charge it, with 14 volts or so. This is the overcharge condition that creates the problem. In addition, the 6 volts are not enough to drive the inverter, so your electronics won't run anyway.