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I'm a HostGator customer, and they frequently post notices that such-and-such server will be momentarily offline for a "battery swap".

What exactly are they swapping? I assume it can't be the CMOS battery... I've only ever had to change one of those once in ~20 years of computer experience.

...and if it was the UPS, why would the server need to be powered down?

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    Have you tried asking them? – womble Sep 24 '11 at 4:04
  • UPS Battery. CMOS Battery. RAID battery. Flashlight battery. Pacemaker battery. Assault and battery... well OK maybe only the first 3 are likely candidates (and the first two not even so much as you pointed out). Like womble said - Ask your hosting provider but my bet is on RAID battery :) – voretaq7 Mar 3 '12 at 7:07
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I'm going to assume that hostgator isn't using crappy UPSes that require all connected systems to be powered down for a battery swap.

With that assumption out of the way, the "battery swap" they're talking about is likely replacing the batteries that power their battery-backed RAID cache cards. These batteries typically get put on a regular replacement schedule in order to make sure they're able to perform their duties when called upon.

  • How long do those batteries last? And I'm curious why you removed my just curious line :-) – Drew Sep 24 '11 at 3:00
  • In my experience, vendors typically want them to be swapped out every three years proactively. The RAID cards also will do periodic tests of the batteries, and if the tests fail, they'll get replaced as well. I removed the "just curious" line because it was noise and didn't add anything to the question. – EEAA Sep 24 '11 at 3:02
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It's the UPS. Servers are plugged directly into the UPS for power. The servers run on the batteries and in the event that a battery is faulty, then the server would have to powered down to replace it.

If the server has two power supplies inside, for redundancy. One would be plugged into one UPS and the other PS in another UPS. Then in the case of a bad battery, they would just have to take one power supply offline and the server could continue to run.

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    The kinds of big fancy UPSes you have in data centers will have a "bypass switch". This will shift the power from the battery circuit to a straight-through circuit, thus letting them do battery work without interrupting power. So you really only have to power off the downstream devices if the UPS itself is being worked on and prevents the use of a bypass. – Scott Pack Sep 24 '11 at 4:05

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