1. If you look up the datasheets for the batteries you plan to use, you can look to see if they'll be sufficient. For example, if you look at the common 1.5V alkaline batteries you might use in a small scope, you can see why you might want an external pack.

http://data.energizer.com/PDFs/E91.pdf

Things to note are the internal resistance, how the capacity changes with discharge rate, and how the battery performs at temperature.
Internal Resistance - The actual battery voltage you get at the terminals is the nominal voltage (1.5V) minus IR*current. In this case, a 300milliohm internal resistance would mean that if you pull 50-100mA from the battery it won't drop the too much, but if you try and pull 500mA or 1A while slewing both motors, you could get a large voltage drop. If your batteries aren't fresh, internal resistance goes up and nominal voltage goes down, so with heavy draw you could drop the voltage below what your the scope needs to work pretty quickly.

If you look at the top right graph, you see how the drain affects maximum life. If you set 1.0V as your minimum voltage before the scope goes wonky (8x1.0V=8V, probably a little too low already), then at room temperature you're already looking at only 2.5 hours of life if you're drawing 500mA. High draw drains the battery faster, but it's not linear. Twice as much current drains the battery more than twice as fast.

Lastly, normal alkalines have about half their normal capacity when the temperature drops from 21C to 0C as you can see in the second chart down on the right.

Compare that to a pretty standard sealed lead acid battery, like this 7Ah 12V battery.
(Sorry for the crappy quality)
Compared to the above, the capacity is only about 3 times larger (7Ah compare to 2.8Ah), but performance is way more than than. At the lowest draw curve (0.36A) the terminal voltage will reach 10.8V after 20 hours. At 0C, you have to derate that to 85%, so you might get 17 hours to 10.8V at 0C. Compare that to 8 2800mAh alkaline batteries, where you would reach 9.6V after about an hour and a half at 21C and 360mA draw, or under an hour at 0C. Granted the SLA bettery weighs 15 times more than 8 AAs, but you get that much better performance out of it. You can get way better performance out of better AA batteries as well, like Energizer's Ultimate Lithium, but after replacing 8 of those every few nights you might as well just buy a power pack.

2. ## The Following User Says Thank You to DaveW For This Useful Post:

prc (07-08-2012)

3. 12 volt power backup is not proper for telescope mount. first things if we go for outing and use at night then it could be fail and shut off so all ways use original backup.

[U][B]
Battery Maintenance and Repair--Ok been doing a lot of searching and reading ever since my powerpack "died"... seems after I moved I should have charged it every week or so (or at least after you use it!). Seems low charges cause the sulfur to precipitate on the lead electrodes, causing the battery to fail to accept charging and decrease in its capacity to keep charge (due to the loss of electrolyte).
There are several methods to combat this...
I had the same problem you've had, and finally -- due to the need to power a laptop and a dew heater -- I gave up on the powertanks/car starters and GOT BIG IRON...big marine batteries.

These are not needed with many scopes but "you'll know it when you need it" as it were. And that's when the powertank or car starter won't run your system for more than a few hours; when it cannot recharge properly; or if you should have the misfortune EVER to plug a dew heater into one for even a very few minutes!

In order to be "colorful" and maintain interest when I wrote the article I'm going to cite, below, I used a "tone" that was overtly cynical and sarcastic. It did cause some affront and I wondered if I *dare* refer to it here...because I do agree with the nature of your approach, and understand the DIFFERENT needs of many of the respondents in the thread.

With this caveat, please consider the article I wrote to be a kind of 'worst case situation' by someone who wants to power a lot of stuff for perhaps two days in the field, or at the very least, during a 10-hour winter observing session (which is likely, with my gear to be something you can't do with just any old powertank/car starter battery.)

OK. We're all still friends, right? The sarcasms in the article are not directed to any of us, here, right? The 'tone' is supposed to sort of keep you reading and maybe give you a chuckle or two, right? OK?? OK!

Just sort of kidding, but it might strike you as kind of an odd, exaggerated article...though it DOES chronicle my long quest for battery power!

THE RECHARGEABLE BATTERY MYTH

Will you still respect me in the morning?

Steve W/8HHaggis

5. ## The Following 2 Users Say Thank You to 8HHaggis For This Useful Post:

seigell (04-12-2012),t1mm0 (09-04-2014)

6. ## Re: Powering Your Telescope and Mount

Here is my solution:

I used a DeWalt case I got with some tool and mounted a 12V analog meter and a 12V outlet.

Inside you will notice an 18V DeWalt NiCd battery sitting in a failed recharging cradle that I repurposed. Included in those changes was the addition of a DC to DC converter that accepts a range of inputs and converts them to 12V. I got the DC-DC converer on eBAy. I also added a fuse.

I added two compartments to the case for accessories.

Here it is in operation.

The batteries can be hot swapped when using my Celstron.
They are NiCd so they don't mind being depleted below 50%.
I have quite a few of these for my own power tools so having charged batteries around is not an issue.

7. ## The Following User Says Thank You to ColdMotive For This Useful Post:

rferrante (06-09-2017)

8. ## Re: Powering Your Telescope and Mount

This is wonderful information. I will be looking for an external supply for my Celestron NexStar 6SE. I will be learning on this system for quite a while and will need the added power for use. Thanks for all of your experience.

9. ## Re: Powering Your Telescope and Mount

I plan to use what most people use with their Lawn Tractor. Not very big, high Amp Hour yet only about 5 pounds.
Lawn Tractor battery.jpg

10. ## Re: Powering Your Telescope and Mount

Got me a very practical batterybox at a local boat shop. It has a carrying handle, charge tester and two 12v ports.And a perfect fit for my two 26Ah deepcycle batteries. My plans are to add some more 12v ports so i can use the batteries apart from each other.This little power box will be used to power a mount, dew heater, DSLR camera,and a laptop.
powerbox1.JPGpowerbox3.JPG

11. ## Re: Powering Your Telescope and Mount

I need a little help understanding how a setup like t1mm0's above works. I'm good with AC circuits, but have less experience with DC. The Celestron AVX mount I am considering specifies 12V-3.5A positive pin. The marine battery I use to power the backup motor on my crab skiff puts out 16A when fully charged. How do I step that down, so that smaller leads and the mount itself can handle it? I know that some devices are self protected against higher amperages than needed and are more at risk from lower than specified amperages. Still, 16A would fry the leads on a standard 3.5A positive pin connector. My guess is that the cigarette lighter/power port adapters handle that. Do they come in different ratings, and are there other types of connectors that also work, since some people have reported having issues with them popping out? How would one add USB charging port other than sticking an adapter into a cigarette lighter port?

12. ## Re: Powering Your Telescope and Mount

the idea is that you use a 12v adapter on each sigarette lighter socket so that each part of your setup gets the right amps it needs.

My heaters have there own battery now plus socket.

13. ## The Following User Says Thank You to t1mm0 For This Useful Post:

Heyref (06-16-2015)

14. ## Re: Powering Your Telescope and Mount

I don't know if any one has had this problem I have the skywatcher 17ah power tank at first it charged brilliant then I noticed it started to take longer to charge and just before going to Norfolk the charge light started to flash so I checked the battery it seamed fine as it was newish I then checked the mains charger and it was only giving out 12v instead of 15 then I put a circuit tester on it and the light was flashing instead of giving a permanent light the charger was at fault so now I charge it off a smart charger off the jump start terminals hope this helps anyone having the same problem

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