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RE: Debian Compatible UPS?

> From: Craig M. Houck [mailto:chouck@binghamton.edu]
> Sent: Tuesday, September 20, 2005 12:33 PM
> The more I think about this the more I think using car batteries for this
> is a tad dangerous, sealed, old-school with caps, no spill batteries, etc
> no matter the type/style. Chemically created energy in a thin walled box
> is not to be played with, particularly when it sits ideal a lot.

Actually, these batteries don't mind sitting idle, as long as you keep them
fully charged.  They have no "memory effect", like the NiCad batteries in
older cordless phones.  Just don't overcharge them, that dries them out.

> When in a car
> they are 'exercised' regularly. Sitting in the situation described here
> they will be doing nothing. Much like dry-cells, I have seen car batteries
> that experience a chemical breakdown when they just sit.

Lead-acid batteries that sit in a discharged condition gradually develop
sulphation on the plates.  This permanently reduces the capacity of the
battery.  If it goes on long enough, the battery will be useless.  It is no
longer chemically able to accept a charge at the normal charging voltage,
and when connected to a charger, its voltage will rise to the float voltage
immediately.  The charger thinks the battery is fully charged and tells you
everything is OK.  In fact, it is fully charged, its just that its capacity
is close to zero.  When you put a load on this battery, its voltage will
instantly drop to near zero.  Since all lead-acid batteries lose capacity as
they age, even if they sit in a float charge condition, it is a good idea to
give them a load test every few months just to make sure they still have
enough capacity left for your purposes.

> BUT if you are going to ....
> I'd build an outside and well vented box on posts (though given size and
> extra gear you might need some of 'shed' size), made of wood on a solid
> base that would have in it one (or more) of those Rubbermaid(TM) style
> non-metallic storage containers for the battery(ies) to sit in. I would
> keep a dedicated trickle charger handy in the box/shed to be hooked up to
> each/the battery after it is brought into use and had a drain put on.  As
> soon as power was restored it would easily be hooked up to get a recharge.
> Don't keep a dedicated charge in the battery area and you won't find one
> when you need to use one. That would be due to Mr. Murphy BTW.

Elaborate, but it will do the job.

> I don't think this would work, but the ideal situation would have the
> trickle charge hooked up to the battery all the time. I am not sure how
> that would react with the 12v dc-dc atx adaptors. IF the charger
> >> battery >> atx adapter could be in a chain, you'd have a nearly set
> and forget setup. EXCEPT FOR ... given my first paragraph.

If you are using the ATX adapters that Alvin mentioned in the other thread,
you have to supply +12V to the adapters for normal operation.  If you just
connect it to the battery and trickle charger, the trickle charger output
will be overwhelmed and the battery will supply the power until it is
discharged, just like during a power failure.  To prevent this, you need a
line operated +12V power supply and some means of isolating the battery when
line power is present.  The switchover design can be a little tricky, though
it doesn't cost much once it's designed.  Once you add that, your idea is
perfectly reasonable and should work.  You've just built an AC-DC UPS!

As long as the trickle charger is sized appropriately for the battery, so it
doesn't cook off all the electrolyte, and the charger is designed so it
doesn't draw battery power when line power is off (easy to check), this is a
good setup.  The only rub is that if you actually use up a significant part
of the battery charge during an outage, a trickle charger will take a long
time to restore the battery to full charge.  That just means that you will
have less than normal backup capacity while the battery is re-charging.
Since even a small car battery is at least 50 Amp-hours, that won't happen
too easily.  This is one case where the get-a-bigger-hammer school of
engineering makes the job easier.

If you get frequent, long power failures or you connect a lot of ancillary
equipment to the battery (aquarium, TV set, stereo, espresso machine), you
may wish to consider a two-step charger.  This charges a battery at a higher
rate until it is fully charged, then switches over to float mode to maintain
that charge.


Seth Goodman

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