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Same battery for mount and dew system?

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#1 rflinn68

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Posted 11 November 2012 - 12:54 PM

I bought the ScopeStuff dual outlet cigarette lighter adapter with digital voltage gauge to use to power my CG5 or CGEM mount with my DewBuster controller. I have since been told NOT to do this because it could damage my mount. Is this true? Is anyone else doing it this way? Thanks

#2 wolfman_4_ever

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Posted 11 November 2012 - 03:25 PM

how could it damage your mount if you are using the proper fuses inside the cig adapter? Won't pull enough current?

#3 rflinn68

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Posted 11 November 2012 - 09:31 PM

how could it damage your mount if you are using the proper fuses inside the cig adapter? Won't pull enough current?


I dont know, that is my question. I believe he said voltage spikes could damage the electronics in the mount.

#4 Arizona-Ken

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Posted 12 November 2012 - 12:10 AM

Absolute wackery.

I have used dew control on my CG5 and CPC mount for years with no problem.

Arizona Ken

#5 wolfman_4_ever

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Posted 12 November 2012 - 12:22 AM

The current will fluctuate when it's pulsing but it's in the tenths range.. nothing you will notice..

#6 DwainM

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Posted 12 November 2012 - 08:33 AM

+1 to Dave's comment.
I've been using two small lawnmower style batteries with volt and amp meters connected and all you see is a pulsing current when the controller is duty-cycling the heaters.
I bought into the same line of thought but I've seen no evidence of anything that would cause a problem with the mount.
Dwain

#7 rflinn68

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Posted 12 November 2012 - 09:16 AM

Anyone else heard of this? OK, I will name drop, I was told this by Dr Clay Sherrod. I'm sure he knows what he is talking about. He told me "Do NOT use the same battery for your mount and dew control...." It sure would be more convienent for me to use just 1 battery. That is why I bought the dual outlet cig plug.

#8 DwainM

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Posted 12 November 2012 - 09:39 AM

Wow. My first post and I kicked the shin of a legend....
I'll be popular.
:foreheadslap:

#9 BigC

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Posted 12 November 2012 - 11:54 AM

The only problem I see is the dew heaters could drain the battery so that the voltage becomes too low for reliable mount operation.

I would be surprised to see voltage spikes measured across the battery terminals .

If powering a goto mount,dew heaters,and active cooled cameras a large capacity battery pack is a very good idea.Some of the booster packs have only a 7AH battery inside ,better is one with a 15AH or 17AH .It will cost more and weigh more but will be more reliable.

#10 rflinn68

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Posted 12 November 2012 - 03:25 PM

The only problem I see is the dew heaters could drain the battery so that the voltage becomes too low for reliable mount operation.

I would be surprised to see voltage spikes measured across the battery terminals .

If powering a goto mount,dew heaters,and active cooled cameras a large capacity battery pack is a very good idea.Some of the booster packs have only a 7AH battery inside ,better is one with a 15AH or 17AH .It will cost more and weigh more but will be more reliable.


I use a full size car battery

#11 rflinn68

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Posted 12 November 2012 - 03:29 PM

Wow. My first post and I kicked the shin of a legend....
I'll be popular.
:foreheadslap:


HAHA! Its ok, I kinda set you up for that...Sorry. I'm still not convinced of this though which is why I asked the question here. Since I use a full size car battery maybe it will be ok? I just dont want to take any chances with my CGEM DX and CG5 mounts. I should try to get back in touch with him again and get some clarification on this though it seems many havent had a problem.

By the way...Welcome to Cloudynights!! I see you are right up the road from me :D

#12 rflinn68

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Posted 12 November 2012 - 03:48 PM

Ok, I just sent him another email so hopefully I'll get some explaination on it soon. Actually I was hoping to hear what I have so far (that it is fine to use 1 battery). It sure would make things easier on me. I'm also hoping Uncle Rod will chime in with his thoughts.

#13 jrbarnett

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Posted 12 November 2012 - 04:03 PM

I always use a single battery to run my mount (CG5, Atlas, CGE, Nexstar SE) and dew controller. Never an issue in decades. I use a splitter as you describe. I also use quality batteries (Optima deep cycle blue tops).

- Jim

#14 rflinn68

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Posted 12 November 2012 - 04:32 PM

I always use a single battery to run my mount (CG5, Atlas, CGE, Nexstar SE) and dew controller. Never an issue in decades. I use a splitter as you describe. I also use quality batteries (Optima deep cycle blue tops).

- Jim


Good to hear Jim! I'm hoping I've been worrying about it for nothing and can get back to using 1 battery.

#15 DwainM

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Posted 12 November 2012 - 05:10 PM

Thanks! It’s good to be here.
Actually my battery box is rather convenient with the two lawnmower battery set up.
I’m not doing AP (yet) so my power needs are pretty low.
Here is a picture of how both batteries fit neatly inside of one Group27 battery box.

Dwain

Attached Files



#16 rflinn68

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Posted 12 November 2012 - 06:46 PM

Just heard back from Dr Clay...I believe I'd rather be safe than sorry. He said I WILL eventually end up with a voltage spike of 40 volts or more if running more than one thing off a single battery. Some of you may be lucky but lucky has NEVER been associated with me so I think I will take his advice.

#17 Lord Beowulf

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Posted 12 November 2012 - 08:28 PM

The funny thing here is that the most likely thing to cause a voltage spike like that are the motors in the drive itself. The heaters draw a good amount of current, but shouldn't be highly inductive. However, every time a motor cuts off there's a nice big reverse EMF spike. Diodes on the drive circuits are usually used to damp that down, and similar tricks could be used on the heater coils. There's no doubt that the more things that are connected, the higher the chance that something could go wrong, but I doubt those chances are all that high. Likewise a lot of people have the same fear about AC supplies and although I've never had a problem with any of mine, that doesn't mean it's not possible. Still, I suspect that most electrical damage to control circuitry in mounts has been due to ESD from the user in the nice dry cold air. And of course the ESD can damage a power supply and then cause havoc on what it's powering too!

Beo

#18 mgwhittle

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Posted 12 November 2012 - 08:30 PM

I have no knowledge of battery operation so I am curious how you get 40 volts out of a 12 volt battery?

#19 rmollise

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Posted 12 November 2012 - 08:40 PM

Ok, I just sent him another email so hopefully I'll get some explaination on it soon. Actually I was hoping to hear what I have so far (that it is fine to use 1 battery). It sure would make things easier on me. I'm also hoping Uncle Rod will chime in with his thoughts.


It IS possible to introduce some noise from the dew heater, which COULD bother a computer, I reckon. But the only problem of this sort I have ever had with a dew controller is creating interference on the Stellacam or Mallincam video cameras when I've run them and the heater off the same battery.

Still, I run the scope off a different battery to avoid _potential_ problems and to keep the battery up enough to run the go-to scope reliably. Many go-to mount problems caused by low voltage there are... :jedi:

#20 Lord Beowulf

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Posted 12 November 2012 - 08:48 PM

Still, I run the scope off a different battery to avoid _potential_ problems and to keep the battery up enough to run the go-to scope reliably. Many go-to mount problems caused by low voltage there are... :jedi:


Yeah, following up on my last post, both switching power supplies and motors have the general behavior that the lower the voltage of the power supply, the more current they draw. The higher the current, the higher the back EMF when things switch. So actually a low battery can cause more problems than a fully charged one, and not just due to the dipping supply.

Beo

#21 Eddgie

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Posted 12 November 2012 - 09:04 PM

Well, I don't know what Dr. Clay is a doctor of, but my guess is that it is not Electronic Engineering.

I have a double E.

There is no way to get a 40 volt spike out of a 12 volt battery.

I am not a Dr., but have 20 years of experience with electronic (Aviation Electronics - Fire Control Radar and Navigation/Communication electrons, and 10 years as a field engineer with IBM).

You are not ever ever ever going to get a 40 volt spike out of a 12 volt battery.

Now if it were an unregulated power supply, I would say "Maybe" (though it would be a very rare occurance even in an unregulated power supply) but a battery is not an unregulated power supply.

A battery can only produce its rated voltage. Never more, but often less.

There is this though. A big dew strip (like for a C14) draws about 4 amps.

The most common dew controllers use a timer that cycles the power strip on and off. Some people beleive that the knob regulates the heat by regulating the current to the strip directly, but it does not.

The knob controls a timer circuit that cycles the strip on and off. If you turn the knob to a higher position, the on/off cycle becomes longer.

This means that it is either drawing full current or no current.

Let's say that your strip draws thee amps, but you have your controller set to 1/3rd level. At this setting, your controller might cycle the strip on for 3 seconds, then off for 6 seconds. For the three seconds it is on, it is drawing the full 3 amps. For the 6 seconds that it is off, it is not drawing any current.

This means that at this setting, it will draw 3 amps for 20 minutes out of the hour, or one amp per hour.

A Go-To telescope will usually draw 2 amps when slewing and about .8 amps when tracking.

But you always have to plan your power for your peak draw and for your amperage load.

This means that your wiring needs to be capable of handling the full amperage draw of the dew strip and of the telescope when slewing.

If you run your scope from a battery, you will need to ensure that the wiring that goes to the socket can handle the current (and my guess is that for a medium size Go-To scope and dew strip this will be about 4 amps) and that you have a fuse in the line that is rated for the peak current. Do not wire direclty to the battery. You really want to fuse the circuit on the hot side with a fuse that is about 1.3x the expected load. So, if you expect a peak draw of 4 amps, you would want to use a 5 amp fuse.

Next, you need to calculate the amount of time you think the dew strip will be on and calculate the amp hours of the battery. Again, a Go-to scope will draw about .8 amps while tracking, but 2 to 2.5 amps while slewing (depending on the scope), but over an hour, figure you will draw about 1.5 amps out of the battery to run the scope.

For the dew strip, again, the strip will never draw over it's rated current (which again depends on how long the strip is) but if you have to turn it up high, this means that you will draw that 3 amps for a bigger and bigger percentage of the hour.

If your conditions are bad, this means that you may be running the strip with fairly high amount of on-time.

If the on cycle of a 3 amp strip is 66%, then you will draw 2 amps from your battery for an hour, plus perhaps another 1.5 amps for the scope.

This means that you will need to plan on a battery that will provide about 3.5 amps per hour of operation.

And here is the bad part. While it is very unlikley that you will damage the scope, here is what will happen to a Celestron controller if the voltage of the battery falls off (which causes the current to fall).

When slewing, the scope will start to audibly bog down. As the dew strip kicks on and off, you will hear the motor speed up and slow down.

This is the exact opposite of what Dr Clay is saying. You don't get volatage spikes, but rather you get voltage dips.

I get them all the time, and it has never damaged a controller.

My bet (and it is only a bet) is that the LEDs for the emitters are not getting sufficient voltage and the emmitters loose position. The sensor does not see any pulses so the motor keeps turning.

Agian, this does not appear to hurt anything (I have had it happen many times) but it does mean that you loose your alignment and have to re-align (assuming that you have another power source).

Bottom line.. If you think your current draw is about 4 amps per hour and you want to run your scope for 3 hours, I would recommend that you provide at least 50% reserve capacity. This means that if you anticipate a draw of 12 amps for 3 hours, you should bring a 24 amp battery to the party (Lead-acid). This is because as the charge on lead-acid batteries gets lower, the volate goes down.

Once the voltage drops below about 10 volts, when the dew strip controller shoots the current to the strip, you will get a brief volatage drop. If the scope is slewing, you will know when you are there, because the slew will start to slow down and then speed up as the strip cycles off again.

When this happens, it is time to quit.

Again, I have had this happen many many times, but it has never hurt anything.

My advice though is to run on the mains whenever you can. Even if you have to set uncoil a 75 foot extension cord, it is actually almost easier than messing with a battery.

Of course if you are going remoted, you have little choice, but I usually just run off of the car battery, making sure that the accessorys are off (in particular the blower fan for the A/C or heater) and being sure to start the car every half hour or so to keep the battery from going dead (and yes, it happend once.. Left the fan running for 2 hours... click click click.. Bummer).

So dude, if Dr Clay scared you, I am sorry to hear that, but my experiecne and my electronic background both say that this myth is busted.

You can run both your scope and your dew strip from a bettery and you are not going to get a 40 volt spike.

#22 wiruna

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Posted 13 November 2012 - 03:09 AM

FWIW Astro-Physics are pretty emphatic in their manuals that the mount should have its own dedicated power source.

#23 Paul G

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Posted 13 November 2012 - 06:10 AM

I run my mount and dew heater off the same battery, no problem.

#24 wiruna

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Posted 13 November 2012 - 07:46 AM

Actually, I raised this question on the ccd new-astro yahoo group and there was quite an informative discussion. See this thread http://tech.groups.y...o/message/71590
Geoff

#25 Lord Beowulf

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Posted 13 November 2012 - 08:47 PM

There is no way to get a 40 volt spike out of a 12 volt battery.

Not directly, no, but remember, the OP wasn't going to hook the two different loads directly to a battery. He was going to use a dual lighter adapter connected to the battery (or whatever power source) and plug his devices into that. This is where the potential trouble starts. While the voltage across the poles of the battery may not deviate (much), there's resistance and inductance in the leads to the splitter. That WILL allow the voltages at the feeds of the two devices to deviate from the battery voltage. How much depends on the loss (no big deal, just a reduction in power) and the combination of the inductance and the current being switched on and off. It's that switching off of a high current that's the killer. Once you get a current flowing, it wants to continue flowing. This is what causes arcing when you flip off a switch and why we have fancy things like zero crossing breakers and other tricks to blow out the arc on a breaker when it trips due to too much current. The higher the inductance and the more current flowing, the higher the voltage spike when you suddenly try to turn it off. That can get high enough to break down the dielectric of air and cause arcing. That generally doesn't happen when you turn something on, but only when you turn it off. Arcing you notice when plugging something in is actually due to temporarily making then breaking the connection. It takes about 3 kV/mm to break down the air dielectric, so your 12V supply or even 120V AC won't do that except over microscopic distances. However, yank a plug out of the wall while you're running a high current load and you'll see plenty of sparks, assuming you catch the A/C wave near full current.

At any rate, if you want to be safe(safest), make completely separate connections at the battery.

Beo






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