Telescope mounts, cameras and anti-dewers all require power to run them. Most mounts and anti-dewers require 12 volts DC. DSLR cameras usually require less voltage, usually around 7.4 volts, but take proprietary batteries. If you work from your driveway or backyard, you can buy an adapter to change the 120 volts AC current from the wall outlet to 12-volts DC. At a remote location where you don't have access to 120 volts, the best way to power your mount and anti-dewers is with a 12-volt deep-cycle battery.
The best kind of deep-cycle battery to get is either a gel cell or AGM type. These batteries are sealed and the acid they contain will not leak out if the battery is tipped over. You will also need to get a good charger for the battery. The charger may be more expensive than the battery. Get a charger that has a maintenance setting. After the battery is charged, you leave the battery connected with the charger running on "maintenance", and it prevents the battery from sulfating during periods of storage. When charging, a 12-volt deep-cycle battery may read 13.5 volts. This is normal. When it's fully charged, and just sitting there with nothing hooked up to it, it should read about 12.6 volts. As you use the battery to power your equipment, you drain energy out of it, and the voltage goes down as it is discharged. If you only discharge the battery to about 50 percent each time, you can double it's lifetime. 50 percent charge is when the battery reads about 12 volts under load. When a deep-cycle battery goes down to 10.5 volts, it is fully discharged. You should stop using it at this point, or you will run the risk of shortening its life considerably. The amount of energy stored in a deep-cycle battery is given by its "amp-hour" rating. This is the amount of amps that it can deliver over different periods of time. For example, a 33 amp-hour battery can deliver 33 amps for 1 hour, or 1 amp for 33 hours. Power Requirements in the Field To determine what size battery you need, you will have to figure out how much power your equipment uses and how long you plan on using it over the course of a night, or several nights. Somewhere in the equipment's documentation should be information on how much power it requires to run. For example, an anti-dewer may require 0.5 amps. A computerized Go To mount may require an average of about 0.7 amps. Take the total number of amps that the equipment requires and multiply it by the total number of hours you need. In our example, we have 0.5 + 0.7 = 1.2 amps. I typically stay out all night, so I might need 8 hours. Multiply 1.2 amps x 8 hours and you get 9.6 amp hours. You really want more power than this to be on the safe side though. At least twice as much because cold will reduce a battery's power, and you don't want to drain a deep-cycle battery down below about 50 percent of it's capacity if you want it to extend its working lifetime. So for our 1.2 amp setup, we would want to get a 12-volt deep-cycle battery with a capacity of about 20 amp hours. If you are going to use it for two nights at a remote location, you will need an even bigger battery. I use a 12-volt deep-cycle battery with 33-amp hours capacity. It weighs about 24 pounds. Car Batteries A car battery really should not be used to power your telescope equipment. Car batteries are made to supply a high current for a very short period of time to get a car started, and then an alternator charges the battery continuously while the car is driven. They are not made to be drained slowly over a long period of time to a deep discharge level. If you try to use your car battery for your equipment, you will eventually ruin the battery. Then one night after you pack up to go home, your car may not start. Trust me, this has happened to me more than once, I am embarrassed to admit. I once got stuck deep in the woods with a dead car battery after using it to power my scope equipment for a long winter night of photography of Comet Hyakutake. If you plan on using your car battery, you should also plan on having a cell phone with you, and you should hope to be in an area that has cell service. You definitely will also need to have a really good friend that you can wake up early the next morning to drive out into the woods, hopefully find you, and give your car's battery a jump start. Camera Batteries Cameras have built-in batteries. Some of the newer cameras are very power efficient and you may be able to power your camera all night long with just one battery. The Canon 1000D (Digital Rebel XS) is very good in this regard. Older cameras are usually not that efficient though, and you may need several batteries to get through a night. This is why it's a good idea to turn off the LCD display on the camera which uses a lot of power. If the in-camera battery dies in the middle of writing a file you may lose that image and it may corrupt the images already stored on the card. Keep a close watch on your battery level. If a battery dies on you in the middle of using it, it is best to pull the memory card out of the camera and store it until you get home. On the chance that it is corrupted, you may be able to save most of the files on the card with a recovery program. This is why it's a good idea to have an extra battery and extra memory card with you. You can pick up an additional inexpensive generic battery from someplace like SuperMart on Pricegrabber.com or get the official LP-E5 Canon battery for the 1000D Camera. Another option is to get an adapter that will run the camera off of 120 volts AC or 12 volts DC. With these adapters, you take the regular battery out of the camera and replace it with a dummy battery that connects to the power adapter that runs off of a 12-volt deep-cycle battery. This can easily power your camera all night long.
A battery's power will be reduced in the cold. This should be taken into account when calculating your power requirements. For example, my Canon 20Da's lithium-ion battery will power the camera for long exposures for only about 3 hours at 50F, and a lot less if it is much colder. My newer Canon 1000D (Digital Rebel XS) is more efficient, and will last almost 6 hours in temperatures in the 30s. Turn off the LCD display on the back of the camera. It uses a lot of power, and the battery will last longer if you only use the display when you need it, instead of having it display the image after every picture.
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