A Detailed Look (Cont’d)
Access to the replaceable battery is provided via a panel cover on the bottom of the Ultra 850VA power protection unit. You first have to remove a small screw (requiring a tool is safety feature) then push in the release latch and slide the cover off to the side to expose the battery.
Ultra uses a standard 12V, sealed lead-acid battery to provide backup power when the lights go out (CSB 1272 F2). It has a capacity of 7.2Ah with a maximum discharge current of 100A (130A/5 sec). This is a general purpose battery commonly used in home security systems. It is designed to operate 3-5 years in standby service and deliver up to 260 cycles at 100% discharge. (Typical replacement cost is approximately $25.00 USD.)
The amount of energy stored in this type of battery is directly proportional to its physical size. The size of the battery (and the number of them used) will determine how long the UPS can provide power to the load. The 850VA/425W rating states the maximum power the Ultra 850VA power protection unit circuits can deliver but says nothing about how long that power can be sustained. In simple terms, backup time is a function of battery size, which ultimately is affected by the finished product size and cost. Multiple, large batteries can provide very long backup times (hours or days) at the expense of large size and high cost. A single, small battery may only provide a few minutes of backup time (long enough to permit a safe shutdown) but results in a compact size and low cost. The capacity of the Ultra 850VA UPS battery is 7.2 Amp hours and is designed to provide up to 16-40 minutes of backup time, depending on the connected load.
Fact: The greater the connected load, the shorter the backup time!
Here are a few more pictures of the inner workings showing the main inverter and power management circuits. (Don’t try this at home – dangerous voltages inside!)
(click to enlarge)
(click to enlarge)
Backup Time Testing
As we said before, the backup time of a UPS is determined by the battery capacity (Amp-hour rating) and the magnitude of the connected load. Since the battery size is fixed, the main variable of interest to us is the connected load. Ultra gives two different examples of backup times for their 850VA Power Protection Unit:
- 16-20 min for a PC and 15” CRT monitor
- 36-40 min for a PC and 15” LCD monitor
Unfortunately, this really doesn’t tell us much. No specifications are given for the PCs and when was the last time you saw anyone using a 15” monitor? I have a hard time believing that one little battery could power a modern PC for 20-30 minutes.
To find out, I tested the Ultra 850VA Power Protection Unit with three different loads. The unit was allowed to run in standby mode for 24 hrs before each test to insure the battery was fully charged. A Seasonic Power Angel AC wattmeter was used to measure the connected load of multiple incandescent lights (PF=1). Each test was performed starting with a fully charged battery and terminated when the UPS shutdown due to low battery.
Wow, even worse than I thought. For a computer system pulling over 300 AC watts, you will have less than 2 minutes to conduct an orderly shutdown. But if the power just blips out for a few seconds and then comes back on, you should be fine. The good news is that many modern PCs don’t use this much power (in spite of what some power supply manufacturers would have you believe). And don’t forget, Ultra offers a full line of UPSs, with capacities up to 2,000VA if the entry level 850VA unit isn’t enough to safely meet your needs.
For perspective, one of my older, high-end test rigs (P4 955 EE dual core with two 7800 GTX video cards and a 19” Dell flat panel monitor) only pulls 403W when fully loaded. This is pushing the upper limit of the 850VA Power Protection Unit and might not give enough time for an orchestrated shutdown. I would select a larger UPS (larger capacity) for use with this system.
Again, it’s important to remember that a compact, inexpensive UPS for home use is intended to supply backup power for relatively short periods of time. This allows the PC to keep operating during brief power outages or to be safely shutdown for longer outages. But it’s very important to properly size the UPS to your load.