I’ve been doing mad research on this topic lately, so please forgive 
the following wall of text.  (Or feel free to correct it if I make 
any mistakes!)

Most desktop computers these days will suck any but a very RV or 
marine battery down pretty quickly.  The average gel cell UPS is 
designed to give your computer ample warning to shut down, and little 
more.  Bigger units last a little longer, but still have computer 
usage times measured in minutes, even if you’re talking 60 of them  
You aren’t getting much more unless you either sell your first-born, 
or MacGuyver yourself together a serious Franken-UPS!  Which you can 
do, but others have suggested already that laptops would be far more 
efficient for the purpose—and you can already get 12v chargers for 
them!

There are a few battery types to consider, and just all of them are 
at least based on the basic lead acid battery.  The exact mechanics 
of each one vary, but what they have in common is that they are best 
maintained by a two-stage charging cycle wherein you provide a 
trickle charge to fill the battery most of the way, and then now and 
then you feed them a higher voltage for a short time to top off the 
battery.  Failure to do this two-stage charging will kill the battery 
before its time.

The basic lead acid battery is called either a wet cell or a flooded 
battery.  You fill it up with battery acid (careful with splashes!)  
You need to make sure that these things get some ventilation, and you 
probably don’t want them closed up inside your house.  They can and 
will give off a little bit of explosive gas now and then.  You gotta 
be careful with that unless you wish to become a baked ham!

The advantage of the lead acid wet cell is that it’s going to be the 
cheapest for its size and power rating.  But the thing to remember is 
that most of these lead acid batteries are made for what usually get 
called "cold cranking amps"—a lot of power really really fast to get 
a big vehicle engine to start!  Most are relatively lousy at lower 
power over a sustained period of time.  Those that are better at it 
are called "deep cycle" batteries.

To avoid the problem associated with wet cells, there exist gel cell 
batteries.  These are more expensive and smaller.  They are a type of 
sealed lead acid batteries.  They tend not to be a whole lot better 
than the average wet cell at power over a duration, but the smaller 
sized ones are plentiful and cheap.  Most un-interruptible power 
supply systems use them, so obviously they will get the job done!  
They’ll power a mobile radio a lot longer than they’ll power any 
computer, though, so keep that in mind.  Like lead acid wet cells, 
they need a two-stage charge cycle to maximize their life—but at a 
slightly different pair of voltages for best results.

Fortunately, two-stage chargers meant to keep these batteries topped 
off and ready to go are everywhere, because these things often do get 
used for precisely what you want!  You just often have to build banks 
of batteries to get the same kind of capacity you could from the deep 
cycle RV or marine batteries.

The newer sealed lead acid battery technology (consequently said to 
be "more expensive", though if you look around I’d say the difference 
can be quite negligible nowadays) is called AGM, or Absorbed Glass 
Mat.  It’s still lead acid, and it’s sealed like the gel cell, but 
instead the acid has been soaked into a fiberglass type material.  It 
too wants dual-stage charging for longest life, but they behave 
better as deep cycle batteries.  Bonus, you can often find drop in 
replacement AGM batteries for most gel cells.  They want charging and 
topping voltages close enough that you can put them on a gel cell 
charger without any noticeable loss in life.

That leads to power ratings and the like.  You are looking for deep 
cycle amps, typically measured in amp hours over a given time period 
(usually 20 hours).  So, looking at a small AGM battery of 7.5 Ah, 
you could pull 375 mA for 20 hours to drain a fully charged battery.  
But!  If you pull say 750 mA, the battery will probably not last 10 
hours.  You see, it’s not quite linear.  Could be as little as 6.5 Ah 
at 10 hours, depends on the battery.

The best choices for batteries are the AGMs if you didn’t get the 
point, and the other good choices are the batteries made for solar 
power systems.  Powerwerx has a nice system that will keep gel and 
AGM cells charged without allowing the charging system to drain the 
battery when it’s not connected.  Other systems exist like that for 
solar and other systems, but the Powerwerx system is one that’s kind 
of made for ham radio emergency power, and it seems reasonably enough 
priced.  Don’t have one yet, so I can’t tell you how well it works 
for its intended purposes.

What the Powerwerx device does have that I’m rapidly becoming a very 
big fan of is Anderson powerpoles.  Figure out the red and the black 
however you’ve got to do it, and then remember Red Right, Tongue Top 
when you put 'em together.  Once you do, you’ll never screw up a 
power connection at the Powerpole end of the cable.

At the other end, well, you’re on your own.  I suggest a handy-dandy 
talking DMM like the one sold by the NFB.  There were others, but the 
people who made the things took them off the market rather than allow 
blind people to be able to buy the things.  Because you know we’re 
all blind and helpless and everything.  If you give us tools, we 
might get hurt!  *snort*

(If you don’t know the story, YES, they were serious, and they did 
actually take the meters off the market rather than be forced to sell 
them to blind people as required by the ADA.)

Joseph - KF7QZC


On Sun, Aug 14, 2011 at 05:14:06PM -0400, Kevin Minor wrote:
>Hi.
>
>
>
>I'm looking for a way to run ham gear in my apartment when the power goes
>out.  I figure a battery backup is the way to go.  I need to run a desktop
>computer on it as well.  I can use an inverter to run the PC, and I'll hook
>the radio directly to the backup.  I was hoping that UPS units would do the
>job, but the ones I got only lasted about 10 to 15 minutes.  I know a
>generator would be ideal, but I don't think the complex would let me put one
>outside, plus the fumes wouldn't be the healthiest thing if I ran it inside
>the apartment. <grin>
>
>
>
>Thanks for any ideas.
>
>
>
>Kevin Minor, Lexington, KY
>
>[log in to unmask]