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

Sat Jan 17 14:26:28 EST 2009

Remember that a marine battery is not necessarily a deep cycle battery
(meant for recharging).  Make sure you get the deep cycle variety.

Yep, those jump start thingies don't have many amp hours.  They do okay for
powering scopes, or some heaters, but won't last too long if you're running
an inverter for a laptop.

I bought my deep cycle batteries at Wal-Mart and Advance.  Big, and heavy,
but work great.  I do have a smaller, 32 amp hour gel cell that I can run
the scope and heaters on if I want.

Jeff

-----Original Message-----
From: chaos-l-bounces at rtpnet.org [mailto:chaos-l-bounces at rtpnet.org] On
Behalf Of Mark Lerch
Sent: Saturday, January 17, 2009 1:58 PM
To: 'robertnielsen at nc.rr.com'; Chapel HillAstronomical Observation Society
Subject: Re: [Chaos-l] Where to buy 12V batteries?

> I'm really interested in how all this turns out, and everyone's 
> empirical experience

I own three "el cheapo" jump start batteries I bought at Target for about
$40 each.  I think they're in the 20 amp hour range.  One for my heaters,
the other for my scope and a 3rd to go on standby.  I've run these down to
zero more times than I can count.  I've left them out in the rain in my
pickup truck bed causing them to give off this ear piercing whine until they
drain down.  One time at a campground I was frustrated with being unable to
shut one off and I was afraid of waking my neighbors since it was early
morning, so I bashed it repeatedly against the ground until it shut up,
trying to forcibly destroy it.  I failed.  It still charges fully and
operates like the day I bought it 5 years ago.  These things have been
ridiculously stubborn performers.

My WalMart marine (called such because they're used by trolling motors)
battery is much newer and has seen much less field time.  It recharges from
60% via a trickle charger in 8-10 hours.  It's a heavy beast but was
inexpensive.  I can run a laptop off it 6 hours and it takes it down to
maybe 75%, which is just silly.

I've had no luck using inverters.

I'm definitely in for Medoc next weekend if the weather is good.

Returning to work somewhat reluctantly, Mark

-----Original Message-----
From: chaos-l-bounces at rtpnet.org [mailto:chaos-l-bounces at rtpnet.org] On
Behalf Of Robert Nielsen
Sent: Saturday, January 17, 2009 1:37 PM
To: Chapel Hill Astronomical Observation Society
Subject: Re: [Chaos-l] Where to buy 12V batteries?

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
> _______________________________________________
> Chaos-l mailing list
> Chaos-l at rtpnet.org
> http://rtpnet.org/mailman/listinfo/chaos-l
>
>

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