Today, I decided it was time to test my UPSs out to make sure they were still functioning properly. Like any well designed product UPSs just work. They fade into the background which makes it easy to forget that they may need maintenance too.

Computers can be very sensitive to power conditions. The hardware expects the power to be within a certain tolerance. Peaks or dips in the power can cause unexpected behavior. I don’t know how often bad power conditions result in crashes but it can’t help the stability of your computer. There may also be problems with cutting the power to hard drives. During a clean shutdown a hard drive will spin down and park the head. This cannot happen if the power is suddenly cut off. A good UPS not only provides power during a brownout or blackout it will also do some amount of filtering to ensure a clean power source. On the software side of things modern operating systems use RAM to cache file system operations. This means that the file you just told you word processor to save may not actually be written to the disk immediately. If the power were to drop at just the right moment the file system can be left in a inconsistent state resulting in lost data. For these reasons I view having a UPS on a computer as an absolute requirement.

The description of the file system caching above suggests a problem with testing the run time of an UPS. If a complete power drop can result in a corrupted file system then running the UPS to the point when it shuts down has the potential to be a bad thing. The solution to this problem on a Linux system is to mount the file system as read-only before running the test. If the file system is read only the OS cannot be caching any writes (because they are not allowed) so power loss should be OK (there may still be hardware problems). This can be accomplished by switching to a console (CTRL-ALT-F1 if you are in X) and then running the following commands:

init 1
df (to see the mounted file systems)
umount -r FILESYSTEM (for each file system).

It should now be safe to run test your UPSs until they cut the power. I do not know how to accomplish something similar to this on a Windows system but I expect there is a way. If anyone does know how please comment below.

It turns out my suspicions were warranted as my UPSs clearly need battery replacements. The APC Office 280 that powers my gateway computer, DSL modem and Ethernet hub lasted only a couple of seconds after the power plug was pulled. I have my primary monitor (19″) attached to an APC Back-UPS 300 which lasted only 4 minutes 22 seconds. The only good news is that the APC Back-UPS Pro 280 that powers my work station (not the monitor) lasted 12 minutes 24 seconds. Not stellar but at least it would stay online during a short blackout.

Now, the question becomes what should I replace, the batteries or the whole UPSs? It looks like batteries are going to cost about $45 for each unit. I can get a new Back-UPS CS 350 for about $100. I would hope that the power noise filtering in a more modern UPS would be better but I’m not sure that is worth double the cost. It looks like battery replacement is the way to go.

There are a couple of lessons in this adventure. First, if your UPS is more than a couple of years old take the time to test it. It’s quite likely it is not functioning as well as you think it is. Second, battery technology still sucks.