1 A at 120V is ten times the current of 1A at 12V.

-----Original Message-----
John Mahony
Sent: Sunday, February 29, 2004 1:31 AM
Subject: RE: [M]: Help With Deep Cycle Battery


You've got that backwards.
If you were talking about applying a voltage to a given resistance, then the
current is higher when the voltage is higher. But no one is talking about
applying 120V directly to the scope (I sure hope not, anyway ;)
An inverter is the electrical analog of a lever. If an inverter with 12V
input and 120V output is putting out 1A, then the input is 10A (actually a
bit higher due to unavoidable ineffeciencies). The law of conservation of
energy in action.
-John


>
>I think you are missing the fact that the amphour rating of the battery is
>for amps at 12V. If you invert it to 120V, then the amperage at 120V is ten
>times that at 12V.
>
>Steve
>
>-----Original Message-----
>Roger Hamlett
>Sent: Saturday, February 28, 2004 4:36 AM
>Subject: Re: [M]: Help With Deep Cycle Battery
>
>
>
>----- Original Message -----
>Sent: Saturday, February 28, 2004 8:07 AM
>Subject: RE: [M]: Help With Deep Cycle Battery
>
>
> > >
> > >I went to Walmart and got a 75 Amp Hour Deep Cycle battery and a 10 amp
> > >charger with trickle >charging capabilities. I am thinking something
>is
> > >wrong because, 1. As I mentioned a while ago, >my lx200 hooked up to
>this
> > >battery with the 1812 sounds like it's going to stall when it slews at
> > > >high speed (this could be the 1812 too) 2. We had an outage the other
> > >day
> > >that lasted 3 days >because of a wind storm and we used this deep cycle
> > >battery to watch TV. It only lasted 2 >hours with TV (27 inch) VCR,
>and
> > >TiVo hooked up to it through a Coleman 800W inverter.
> >
> > Sounds about right. A full size TV takes a few hundred watts, say at
> > least
> > 300W with the extra equipment. Inverters are not that efficient. I
>don't
> > really know the numbers, but say it takes 450W from the battery to
>produce
> > 300W output. Your battery has a total capacity of 12V X 75 Ah = 900 Wh
>=
> > 450 W X 2h.
> >
> >
> > >SO, I am hoping you guys can give me some theory so I can troubleshoot.
> > >These are the >questions I have:
> > >
> > >1 - I am weak in electronics. To make the equation easier, if I have a
> > >7.5
> > >amp total charge pulling >from this 75 amp hour battery, would it last
>me
> > >10 hours from when it's fully charged?
> >
> > Roughly speaking, but the 75Ah total capacity rating actually depends on
> > the
> > rate of discharge. I believe they get less efficient with a heavier
>draw,
> > so it might support 1A draw for 75 hours, but not 25A for 3 hours.
> > Also, running a lead-acid battery all the way down is hard on the
>battery
> > and will reduce the lifetime of the battery.
>The 'quoted capacity', is normally also at a defined temperature (usually
>something like 20C). Battery capacities decline in cold conditions, and
>with
>age. Hence I normally take half the quoted capacity as my 'working' value,
>when calculating battery operating times.
>
>
> > >2 - The LX200 pulls less than 1 amp correct?
> >
> > That depends a lot on whether you're slewing and at what speed, and on
> > scope balance and accessory weight.
>I think this could well be part of the problem. If the scope is not well
>balanced, and is being asked to slew quickly, the drives themselves do have
>problems even if the supply is 'perfect'. This is why a lot of people
>(most...), turn down the maximum speeds they use on the drives. It makes
>the
>batteries last better, and increases the life of just about every part in
>the mechanism as well. You really don't want the LX200 to 'slew at high
>speed'....
>
> > >4 - Do these inverters waste power? Would I be better off trying to
>run
> > >straight from the battery >for everything instead of using the
>inverter?
>Inverters waste a _little_ (reckon on 10-15%). But in the case of the
>LX200,
>the motors run more efficiently at the higher voltage, so this is largely
>made back. Remember though, that if the scope requires 1A at 18V, this will
>need to draw perhaps 1.7A from the battery at 12v (18/12 * 1 * 1.15).
>
> > Use the 12V straight for what you can, except maybe the scope. The
>LX200
> > classic has a dangerous (to equipment) wiring scheme where the metal on
> > the
> > scope is not at the same voltage as the ground of the input supply, so
>the
> > scope's power source should be electrically isolated from other
>equipment
> > running off the same supply. Otherwise, for example, if you attach a
>CCD
> > camera to the scope, and the CCD runs off the same battery, the outer
>cases
> > of the camera and scope will be at different voltages. The 18V ac
>adapter
> > will isolate it, but I don't know if the 1812 adapter will.
>It doesn't.
>A seperate battery is a good idea here. It avoids this problem, and also
>isolates the scope from 'noise' sources like the dew heaters...
>
> > >5 - I bought 4 cigarette lighter outlets and mounted them in this 22"
> > >toolbox with the battery >inside, in parallel. Meaning that the
>positive
> > >terminal from the battery feeds all the positives and >the negative is
> > >connected to all the negatives, with a 30 amp switch that turns power
>to
> > >the >outlets on and off. Is this correct?
> >
> > Yes.
> >
> > >Tomorrow I am going to turn everything on in the morning and see how
>long
> > >it lasts. Every half >hour or so I'll slep the scope a couple times.
> > >We'll see what this test yields, but I wanted to see if >anyone had
>some
> > >advice.
> >
> > It should last a long time if you're not slewing much. My 8" LX10,
>which
> > has no high-speed slew, not even "find' or "center" speed, will run 4
>full
> > nights on 4 AA batteries. Of course, it has minimal other circuitry,
>too.
>
>Best Wishes
>
>
>

_________________________________________________________________
Stay informed on Election 2004 and the race to Super Tuesday.
http://special.msn.com/msn/election2004.armx

View index by [date] [author] [subject]
Previous message: RE: [M]: Help With Deep Cycle Battery, John Mahony
Next message: Re: [M]: Help With Deep Cycle Battery, Glen Sharp II
Next message in thread: Re: [M]: Help With Deep Cycle Battery, Glen Sharp II
Previous message in thread: RE: [M]: Help With Deep Cycle Battery, John Mahony