[lug] apm/UPS

D. Stimits stimits at attbi.com
Tue Aug 20 13:25:35 MDT 2002

David Morris wrote:
> On Fri, Aug 02, 2002 at 11:53:07AM -0600, D. Stimits wrote:
>>I'm looking at an UPS for the Linux bridge and cable modem. Obviously, 
>>cable modems do not have any "auto power down" feature (nor would the 
>>modem need it, but not totally depleting a battery's power is good for 
>>the life of the battery). In the case of the bridge machine, it is 
>>older, and does not support having the o/s power itself down, it has a 
>>good old-fashioned switch. However, in the case of the Linux machine, I 
>>can probably rewrite some of the UPS scripts for apmd, so that it starts 
>>out by changing the ext3 journal speed from 5 second intervals to 1 
>>second intervals; and then if time goes far enough, either remount the 
>>drives read-only, except for /var/, or else run init 0 to halt when 
>>things get really close.
> I cannot speak for the batter depleation question, but just
> as an FYI on the above, a system that does not power-off
> upon shutdown is actually preferable if you are looking to
> maximize uptime.  So long as you have a UPS that talks to
> the OS (and that is a function of the OS and the UPS, not
> the system), you can get Linux to shut itself down.  Then,
> after the system has shut itself down, the power is still
> on.  This way, when power is restored, your system will
> power itself back on automatically, and it will be up and
> running without your intervention.

The machine involved is quite old, the power supply itself has nothing 
but an ordinary switch on it, so it can init to a halt level, but it can 
never turn itself off. By coincidence, this limitation becomes a feature.

> NOTE:  As I said at the top, the below is far beyond what I
> can say with any certainty.  Don't flame me if I am don't
> have a single fact straight, this is simply how I understand
> the situation....an understanding which could be completely
> wrong!
> Yes, this will depleat the battery....a UPS is, I believe,
> designed to account for this, and it does not significantly
> affect the battery.  Now, I could easily be assuming far too
> much intelligence on the part of UPS designers, but a
> properly designed recharchable battery / battery recharger
> system does not care if the batter gets fully depleted or
> not (in fact, it is a *good* thing to fully depleat the
> batter periodically to both verify runtime and protect
> battery performance....or at least, that is the way I
> understand it.).

I certainly hope the UPS has the intelligence to shut down before it 
goes into deep-discharge, but I have no way of knowing (which is why I 
worry about it). If the UPS is smart enough to not go into 
deep-discharge, then all of this is ideal.

The following may be boring and irrelevant trivia, read at your own risk:

As far as battery life goes, the discharge-for-cycle-life only applies 
to batteries where the chemical path for charge and discharge are not 
reverse of each other. If you take a Ni-Cad type battery, the chemical 
reaction during charge is quite different from the reaction in 
discharge...the characteristic of cycle memory is applicable due to this 
rather complicated reaction. On batteries with symmetric mirrors of 
charge and discharge reactions, the cycling is not beneficial (lead-acid 
batteries are this type of battery). I believe the gel-cell, chloride 
cell, and many other "sealed" batteries, aside from Ni-Cad, are 
symmetric in charge/discharge path. I have not read up on the Li-Ion, or 
many of the laptop type batteries, I have no idea whether they have any 
cycle memory. I am fairly certain though that the UPS battery is 
symmetric, and does not have a cycle memory.

In the case of symmetric discharge/recharge reaction paths, any 
intermediate chemical can be used to go in either direction; with a 
non-symmetric reaction, the chemicals of an intermediate reaction may 
not work to recharge, and fully discharging to obtain a new chemical 
capable of charging might be necessary (and vice-versa). For reversible 
chemical reactions, they often oxidize or reduce a metal plate or 
physical structure, in essense turning it into a form of rust. That 
rust, if it is too complete, chips off parts of the plates and then the 
plate never reforms (deep discharge can do this in a single discharge). 
Even if the plates do not chip away, it is not as easy to reach the core 
of the plate as it is to reach the surface, and the core parts may be 
harder to "repair by recharge" than the surface portions. If you ever 
looked at electro-plating of silver or other metals, these reversible 
chemical path batteries are essentially doing chemical plating during 
charge, and rusting during discharge.

D. Stimits, stimits AT attbi.com

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