[Chaos-l] Where to buy 12V batteries?

[Robert Nielsen]

Michael,

Congratulations for stepping on one of the more "opinionated" subjects 
in amateur astronomy!

I'm actually interested in how this "battery" stuff goes, because next 
weekend I'm going to try to go to Medoc Mountain State Park (more on 
this in a different e-mail message) and run my Tak off of batteries.   I 
had done a little research leading up to this, and found a few 
interesting things:

1) Everyone seems to think that AGM (absorptive gel mat) batteries are 
the best, because they are the safest (something about not leaking 
hydrogen gas!) and because they seem to have the best discharge/recharge 
characteristics of all the battery types over time.  In other words, if 
you get an AGM battery, the story is that for perhaps 5 to 7 or perhaps 
even 10 years, you will get consistent discharge and recharge cycles.   
With other types of batteries, you may only get a couple of years of 
decent recharge cycles.

2) Everyone says to take all your equipment, figure out the current used 
by each, and add it all together for the amount of time you want to 
observe.   That will give you how much amps (current) you will need ... 
and you need to have batteries rated for TWICE that much to make it 
actually work.   The "twice as much" bit has to do with the chemistry of 
the battery ... if you discharge it too much, it will never recover.   
So, for example, if you dew zapper takes 2 amps per hour, and your mount 
600 mA per hour, and you want to observe to 6 hours, then you will need 
(2 + 0.6) * 6 or 15.6 amp-hours of juice, and you will need a battery 
rated at 2 * 15.6 or at least 31.2 amp-hours to make it work.

3) For some reason, running certain equipment off a 12V to 115V power 
inverter that is plugged into the batter is more efficient!   I can't 
really figure this out, but there are multiple articles on the web that 
claim this is true, and I've found similar results.   Perhaps it is the 
efficiency of transmitting the power ... or perhaps (as stated in one of 
the articles that Jim Pressley referenced) it is because the voltage is 
better regulated through the inverted.   But this is something to think 
about.

I did a little experiment with a battery I have (that has a voltmeter 
built-in) and my EM-500 mount, which claims to use the 600 mA in 
tracking mode (more for slewing).   I let it run for 8 hours, THROUGH 
THE 12V -115V INVERTER and checked the meter every 15 minutes ... sure 
enough, the drain on this particular battery (a Kendrick Astro power 
pack, which is essentially a marine-sealed AGM unit) plotted *exactly* 
as expected, and so I think it will do what it says.   It also 
vindicated the money my folks and I paid for my physics degree :-)

Of course, next weekend at Medoc Mountain will probably be COLD ... and 
I'm wondering if the battery's chemistry will work as efficiently at 
that temperature.  I guess we'll find out!

What I don't know is how a "marine deep cycle battery" will work ... or 
what it even IS!  I think the "marine" part means that the containment 
vessel is sealed better, to reduce the amount of hydrogen gas that leaks 
... and I'm sure the "deep cycle" part means that it will work for 
longer periods of time.   I'm just not sure about (1) how quickly the 
battery will recharge (sounds like you have a good charger) and (2) how 
many cycles you will get over time.  Some of the articles on the web 
claimed that because you get a smaller number of recharge cycles, the 
AGM batteries were cheaper in the long run.

And as far as running a laptop is concerned ... I don't have much 
experience at all.   I have an extra-life battery in my Thinkpad, which 
normally will run the thing six hours or so as long as I don't do 
something compute-intensive or something that spins the drive (like 
watch a DVD).  Again, that will be part of the experiment next weekend.

As I said before, I'm really interested in how all this turns out, and 
everyone's empirical experience!

Robert

Michael Hrivnak wrote:
> I need to pick up a 12V deep cycle battery this weekend. It will power 
> my dewbuster and sometimes the drives on my equatorial mount.  Where 
> is the best place to buy?
>
> I think I need about 30Ah to guarantee 8 hours of observing. I already 
> have a nice 3-stage charger.
>
> Thanks,
> Michael
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