That UPS you bought for your home server may not be as useful as you think

Some years ago I decided to install a server at home for use as a NAS (network-attached storage) in my home network, and for an Internet-facing server. I live in a place where blackouts are very infrequent (perhaps a couple per year), but occasionally the mains drops out for only a second or two. I suspect these very short dropouts occur when substation switchgear operates, but have no way of being sure. Anyway, with a server running 24/7 I obviously wanted protection against any loss of the mains supply.

I ended up buying a 700VA APC Back-UPS ES-BE700G-UK, which has four mains sockets that are battery-backed and also have surge protection, plus another four mains sockets that have surge protection but are not battery-backed. Additionally, it has two RJ45 sockets to provide pass-through filtering for an Ethernet connection. It also has a USB port for connection to the server so that it can transmit unsolicited status information to the server (including requesting the server to shutdown) and can also be interrogated by the server using the apcaccess command. The APC UPS daemon works with this model of UPS, and was relatively easy to set up. APC, formerly American Power Conversion Corporation, is a subsidiary of European company Schneider Electric. My UPS was manufactured in The Philippines.

I have three devices plugged into the battery-backed sockets on the APC UPS: the server, an external 6TB USB HDD connected to the server for automated daily backups by the server, and a 5-port Ethernet switch. The battery in the UPS would provide between 15 and 20 minutes of power when the mains fails, although I have configured the UPS to trigger the server to shutdown when 30 per cent of the battery power remains, as battery life is reduced considerably if its power is allowed to drain completely. In case you’re wondering why my router is not also plugged into the UPS, due to the position of the broadband provider’s socket the router is in a different room and I have therefore had to connect it to a different UPS, an iLEPO multi-functional DC UPS (the ECO PLUS 412P, which is tiny but can keep the router powered for several hours). Obviously the router needs to be connected to a UPS, otherwise the server would not be able to send me e-mails when there is a mains power cut. Being able to receive UPS status e-mails from the server is important to me when I am away from home on work trips.

So I thought I had covered all bases, and, indeed, the UPS proved useful on several occasions. I would quite often be on a work trip and receive an e-mail from the server informing me that mains power to the UPS had been lost, then another e-mail soon after informing me that mains power to the UPS had returned. Only once did the power cut last longer than the battery capacity, and the server was shutdown automatically.

Now, the life of the 12-volt lead-acid battery in the APC UPS is supposed to last approximately three to five years. The life will depend on how many times the battery is discharged and ambient temperature.

While I was away from home on a long work trip, suddenly I could no longer connect to my server and I had not received an e-mail from the server informing me of any problem. Luckily it was near the end of my trip so I was not too inconvenienced. When I arrived home I found that the UPS was sounding an alarm and was not supplying power to the server even though there was mains supply to the UPS. It transpired that the UPS battery had suddenly died without warning and could no longer hold a charge, and this had happened while there was mains supply to the UPS, i.e. there had not been a power cut while I was away. Fortunately there was no loss of data on the server; I was able to run fsck during boot-up.

This failure was annoying on two counts. Firstly, the battery was only about thirteen months old (the manufacturing date stamped on the UPS box was only two or three months before the date I purchased the UPS). Secondly, I certainly did not expect the UPS to stop supplying power to the server while there was mains supply to the UPS. The APC white paper on UPS topologies, ‘The Different Types of UPS Systems‘, does not make this behaviour clear.

It turns out that the type of UPS topology (‘Standby’ — see the model’s Technical Specifications) in this model of UPS does not continue to provide power to connected equipment when the UPS battery either fails or is disconnected for whatever reason when there is mains power supply to the UPS. A quick search of the Web showed me that I was not alone in discovering this ‘feature’: an APC Back-UPS 350 owner posted ‘UPS Battery Backup — useless when the battery dies?‘.

From what I have read, the so-called ‘Line Interactive’ UPS topology does not suffer this shortcoming, so, the next time I have to buy a UPS for a piece of equipment that requires power 24/7, I will buy a line-interactive UPS rather than a standby UPS. But, before purchasing, I will be sure to ask the manufacturer what the precise model would do if its battery fails or is disconnected while there is mains supply to the UPS. The APC line-interactive UPSs are more expensive than the APC Back-UPS models; now I know why. It’s a pity I was not aware of the shortcoming of the Back-UPS models, as I would have spent more and bought a UPS that continues to work when the battery dies while there is mains supply. I would also hope the UPS would issue an alarm if the battery has failed or is disconnected while there is mains supply. Be sure to ask the manufacturer all these questions if you cannot tolerate a sudden loss of power to your equipment if the battery dies while there is mains supply.

Anyway, after checking that the dead battery was indeed useless I replaced it with a new Yuasa battery that has lasted nearly three years now. I will be replacing it shortly as a precaution, even though it has not had to be used much at all since I installed it. I have not replaced the APC Back-UPS model but I will be replacing the battery at least every three years just to be cautious, and of course taking the old batteries to my local waste disposal centre to be recycled properly. By the way, it is possible to purchase a 12-volt battery manufactured by one of the reputable battery manufacturers such as Yuasa with the same specification as the APC battery, for a significantly lower price than APC charges for replacement batteries (which I suspect are badged by APC in any case).