hand powered linux box

[Duane Attaway]

On Wed, 15 Dec 2004, djgoku wrote:

> I have spent $70 on a 400VA 700W apc UPS, I would like to seek out 
> another alternative that maybe better. I wouldn't mine making something 
> that would that would cost a little more then what I paid already that 
> would say better quality and will last longer. How long do UPS batteries 
> last do they need to be changed every couple of years? I would like for 
> all my computers to last 2-3 days if needed... How much are marine 
> batteries and how do you charge them? How you know how much juice it 
> currently has? Do you charge them periodically? And is all this in an 
> enclosure that could pass safety regulations?

All APC UPS batteries will dehydrate their gell cell batteries in about 
two years.  The more expensive APC UPS units will dehydrate their more 
expensive bigger batteries in the same two years.  APC is evil and sets 
their float voltages too high.  Its a great business model.

Marine batteries are about $60 for the 125 Ah sizes at Walmart.  Pretty 
big bang for the buck.  That's about 20 times more power than the original 
UPS battery.

You can attach marine batteries to the UPS, but they will need to be 
watered every 3 months due to the high float voltage.

If you do decide to use a marine battery, invest in safety.  Pay attention 
to wire size, strain reliefs, and high current connectors.  Also, its not 
a bad idea to have your battery in a sturdy battery box, since not only 
are they HOT with respect to the mains, they can leak and corrode other 
things.  If your home engineering skillz make it look good and attractive, 
an inspector might even be pleased and admire it.  They just want things 
to be safe and not blow up.

Your UPS will charge this huge battery in a few days and keeps it charged. 
The charge on lead acid batteries at rest can be approximated by voltage:

12.0 volts: 0% dead
12.5 volts: 50%
13.0 volts: 100%

If you see more than 13.5 volts on the UPS unit, you can assume its 
charged and ready for the Great Ice Storm.  Lead acid batteries are 
voltage sources.  If a dead lead acid battery sees 14 volts, it will 
consume infinite current until it is nearly charged, then taper off the 
current.  This is the reason why you don't charge a motorcycle battery 
with a car alternator.  It will get too hot.  The UPS only has about an 
amp to charge with, so its slow and safe.  Your UPS is friendly to lead 
acid batteries, except for the high float voltage.

-=Duane
http://dattaway.org