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Homemade Battery Pack for CGEM

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#1 jacobmarchio

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Posted 09 March 2013 - 08:52 PM

Hi all,
I wanted a way to power my CGEM in the field when there was no AC power, so I decided to build a battery pack. Looking at the Celestron website, it said that the CGEM only needed about 1.5 amps (12v) max. So, my mother had this handheld vac that she didn't want. We (my dad and I) decided to build the battery pack off that. (pic 1). Looking online, it appeared that the vac battery was 2amps (ok, 25 air watts, which is something like 25.04 watts, which is something like 2.09amps [12v] :grin: ). So, we took a spare plug that fit the mount off of something different out of my dad's electrical junk. After some cutting and soldering, (pics 2, 3) we had a small power pack. (pic 4). We did some testing against running off of normal house current (with converter): I would slew the mount to a point on the RA axis, time it, and do the same with the power pack, and timed it. The slew speeds were the same (ok, within .01 sec. I'll call that the same). We then did another test: we measured voltage of the battery, then did almost 5 minutes of continuous, heavy slewing, and measured again. Dropped a quarter of a volt.

So, what do ya'll think? How long do you think (on an average night of use) it'll last? The only thing it's powering is the mount.

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#2 jacobmarchio

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Posted 09 March 2013 - 08:52 PM

pic 2

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#3 jacobmarchio

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Posted 09 March 2013 - 08:53 PM

pic 3

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#4 jacobmarchio

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Posted 09 March 2013 - 08:54 PM

pic 4 (the power pack is the little thing on the floor)

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#5 OneGear

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Posted 09 March 2013 - 10:39 PM

It's not really a homemade battery pack, more like repurposed technology. So I like it :)

Aren't rechargeable batteries rated in amp-hours?

I'm not up on the math, but my understanding is for certain applications you buy a rechargeable battery based on exactly your concern - how many hours will it run the given application?

My understanding is you can determine the draw of your mount and the total energy content of your battery to estimate the length of charge. And that is the limit of my understanding :)

IOW, there is a way to guesstimate without relying on WAG. Temperature and usage will influence the actual runtime, but you can compare the capacities of various battery packs in this manner.

#6 Chuck Hards

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Posted 10 March 2013 - 12:20 AM

Is using the car battery not an option? I have a CGEM and if only running the mount (not imaging equipment and laptop too), a car battery will run the mount for a week before needing a charge.
Balance your setup as best you can. The better the balance, the lower the current draw. Also the more you slew, the faster the discharge rate. Take that into account when calculating your amp-hr requirement.

#7 jacobmarchio

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Posted 10 March 2013 - 05:58 PM

OneGear- I don't know the amp-hour for this battery. It's a pretty small battery, so I don't think it's very high.

Chuck- If you are talking about just a car battery by itself (not in a car), than not really. Didn't have one lying around.

Please note that I DIDN'T balance the mount before doing the tests- I guess I should have- so, it was probably off and put more strain on the mount.

#8 Chuck Hards

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Posted 11 March 2013 - 07:49 AM

Chuck- If you are talking about just a car battery by itself (not in a car), than not really. Didn't have one lying around.

Please note that I DIDN'T balance the mount before doing the tests- I guess I should have- so, it was probably off and put more strain on the mount.


I was talking about plugging it into the lighter socket. It's what I do when in the field and power isn't available.

Keep the scope balanced as well as you can in all attitudes, to minimize current draw when tracking and slewing. It also helps minimize wear on gear trains and motors.

#9 jacobmarchio

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Posted 11 March 2013 - 04:59 PM

Chuck Hards- unfortunately, on our cars the car has to be on to have power in the sockets. Besides, I would rather not depend on the car.

#10 Chuck Hards

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Posted 12 March 2013 - 09:30 AM

Chuck Hards- unfortunately, on our cars the car has to be on to have power in the sockets. Besides, I would rather not depend on the car.


I understand. I also have a cable with alligator clips on one end, and a female lighter plug on the other, for times like you mentioned. I have been known to use a motorcycle battery (left over from my RC airplane days) and it lasts all night in summer, when nights are short.

After you get the scope all balanced, why don't you set it up indoors the morning, aim it at the eastern horizon, and let it run on your homemade battery pack until it dies. You will have to do a meridian swap after six hours, and reset it again after twelve, if it lasts that long, but it will give you a good idea of the capacity of your battery pack before you take it out in the field. Do a few random slews or slo-mo corrections every once in a while if desired.

#11 Baxstar

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Posted 12 March 2013 - 11:22 AM

I have this for my CGEM925. 65Ah AGM.

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#12 Baxstar

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Posted 12 March 2013 - 11:23 AM

2nd photo:

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#13 jacobmarchio

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Posted 12 March 2013 - 05:31 PM

Right. So, these things (Chuck Hard's rc battery, and Baxstar's battery) can be pretty big and heavy. How do these compare to the $140 Orion 12v 17amp-hour battery pack in terms of size, weight, how long charge lasts, etc.? Obviously the Orion has more bells and whistles (different ports, LED lights, and so on).

#14 Chuck Hards

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Posted 12 March 2013 - 07:33 PM

A motorcycle battery is about half the size of a Triscuit Box. The Orion pack probably has a similarly-sized and capacity battery in it. Not sure about the type, though. If it's lead-acid, it's probably sealed, but it may be a type of dry cell such as a NiCd. I'm sure someone on this forum knows for sure, if it's not listed on the Specs on the Orion page.

#15 avarakin

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Posted 12 March 2013 - 11:09 PM

Based on what I see, it looks like the battery is very small.
Also if it is unwanted item, it is very likely that it spent a lot of time sitting idle, which is a certain death for some types of batteries.
Walmart sells U1 sized tractor batteries for $25, they also have battery boxes for $10, 12v replicators for $10 and fuzes. Gaven all the materials above, one can build a nice battery for around $50 which is a perfect companion for CGEM: not too heavy, ~30Ah should last for very long time.

Alex

#16 ed_turco

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Posted 15 March 2013 - 02:15 PM

On Fleabay, you can buy sealed, rechargeable lead-acid batteries for motorcycle use, no maintenance too! They are about 5x4x3" and are rated at 7 amp-hours. I think the price is $18 and a charger goes for $8, both are shipped free. They are from two different sources.

Just a thought, but not bad!






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