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Comment by riobard | Hacker News Reader

Speaking of RTC battery, I've recently come to the realization that I have to make sure that BIOS battery is not absolutely dead in always-on PC boxes.

Background: I use an x86 box as home router. I've changed the configuration in the BIOS that it should automatically boot up on power. However if the BIOS battery is dead, the config will be lost and it will revert to default settings, which is not to boot on power.

But there is no way to know how much juice is left in the BIOS battery, thus there is no way to issue warnings that the battery should be replaced soon. If there is power loss AND the battery is dead, when the power comes back up again, the router will just stay off indefinitely until it's manually turned on.

That's when I realized why most consumer routers do not feature RTC nor do they require batteries.

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> But there is no way to know how much juice is left in the BIOS battery,

True, but its voltage can be used as an estimate. Most PC MB still around monitor the supply voltages, some also of the battery feeding the RTC. If you have the (Debian) package 'lm-sensors' installed, look for

    sensors[<pid>]: Vbat:           +2.91 V  (min =  +2.70 V, max =  +3.63 V)  ALARM

in /var/log/syslog.

Ran `sensors` and it only shows CPU core temps. I guess my MB does not report RTC battery voltage?

One thing are the sensors, 'nother thing is the software. Have you configured the lm-sensors package (by running `sensors-detect`) yet?

https://hwmon.wiki.kernel.org/lm_sensors also has a FAQ.

Yes I did run `sensors-detect` previously and just to be sure ran it again. No voltage sensors on this box unfortunately :(

Some unsolicited solutions, maybe not for you but somebody in a similar situation: maybe replace it with a supercapacitor (needs some wiring changes, otherwise it won't ever be charged, should last long enough for most power outages), or use a stack of coin cells in parallel (difficult due to physical dimensions, and they'll still go dead at some point). You can also short two wires on the ATX supply to automatically turn on the computer when power is supplied (which may not be good enough if you have to press a key during boot to dismiss the low battery warning). Or hack the BIOS :)

Somehow I feel reliability might take a hit… :|

If using supercapacitors, use some known not for leaking else you're back to square one (do a search for capacitor recap).

That issue doesn't apply to supercapacitors (nor to any capacitor manufactured in the last 15-20 years, really). It affected some aluminum electrolytic capacitors manufactured with a defective electrolyte in the early 2000s.

Most newer systems persist EFI options to NVRAM now days as well. Pulling the battery isn't generally enough to reset them anymore.

My desktop remembers all settings, but after power loss still goes into a warning where I have to press F1 to enter settings, then exit them without making changes. I couldn't find an option to not do that.

Yes, this is very annoying.

Voltage check on battery is part of regular PC maintenance. Like cleaning dust, checking all fans are spinning, capacitors are not getting bigger, checking for weird sounds in PSU, overnight memtest...

You should do it every year or two...

Cannot do that if it acts as the always-on router…

Companies have regularly scheduled (weekly in some cases I know) maintenance windows affecting thousands of people. A home environment with an "always-on" router can surely find a 10 min window (say, 3 AM on a Sunday morning) where nobody cares.

Funny enough, instead of doing that every year, my policy has been "replace the machine if it breaks" and in general, I've gotten a good five-year cycle out of all my PCs without having to do disassemble-maintenance with an air can and a static strip. This has been good enough because I do enough high-graphics-demand gaming that five years is about the cycle on which some new-shiny has come out that renders my machine too old to play modern games.

Maybe I'm just lucky.

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