If you’re reading this then you’ve got at least a passing fancy for astronomy. If you know who I am, you know I have an obsession with it. I’m here today to talk about how telescope field batteries and astronomy allow us to enjoy dark skies and high resolution views away from stable utility provided power. I’m here today to introduce you to my monster.
In February of 2014, my trusty (but inadequate) portable emergency car jump-start battery died. This little thing was purchased from an auto parts store for a modest $30. It came with a 12 volt DC outlet rated for 7 amps of draw (84 watts of power) so I could run my telescope. It carried enough reserve capacity for at least 4 hours of observation. I kept it on the charger all the time, as you are supposed to do for these kinds of batteries. However, after a year of faithful service at star parties and in my astronomy classes at the local community college, it eventually gave me its last night out under the stars in January 2014.
I needed a new telescope field battery, but I wanted one that would last all night (6 hours). I wanted one that would last several years. I wanted one I could afford. I needed to build my own. But how?
I did a ton of research and read lots of interesting blog posts of how people made their own batteries for their telescopes, or other electric needs. It became clear that the actual construction would be very simple to do. I’ve wired up several car stereos in my youth, and this looked to be less complicated of a project. My biggest challenge would be to determine how much power I would need for two full nights of imaging.
The basics of electrical theory
My old battery had 12 volts at 7 amps, which was equal to 84 watts of power. Whatever system I built had to have much more than this since whatever system I ended up with would possibly power my DSLR, Laptop, CGEM Mount, and charge my cell phone (hey, why not?).
I’m going to cover some basic math here. You only need to be able to compute algebra to determine the proper battery size for any project.
First, we need to define a few concepts about electricity. Mainly the terms Volt, Amp(ere) Watt and Amp Hours
My old battery was a 12 volt battery with 7 amps. What does this mean? Well, Voltage is the difference of electrical potential between the positive and negative terminals in the battery. To use an analogy: Voltage is to electricity as pressure is to water. An Amp is the measure of electrical capacity for the battery. Amps are to electricity as current is to water.
Watts are the actual work that can be, or is being done. We figure this out by multiplying Amps and volts. For example: if I had one amp of current flow through the electrical potential difference of one volt, I will have produced a Watt of power.
Lets start with my telescope mount – since it’s the primary reason for needing a battery. Looking at my AC adapter, I can see that the output is 12 volts at 2500mA which is equal to 2.5 amps. For every hour my mount is powered on, it can draw up to 30 Watts of power. If I were to image for 6 hours per night the I would need to figure out the power requirements for my laptop, DSLR and mount for 12 hours each.
12v * 2.5a = 30w
30w * 12h = 360wh
19v * 4.75a = 90.25w
90.25w * 12h = 1083wh
7.2v * 1.2a = 8.64w
8.64w * 12h = 103.68wh
Grand Total: 1546.7 Watt Hours
That’s a lot of power! But is it correct? Remember, I calculated that my AC adapter outputs 30 watts of electricity every hour. If my mount was truly drawing 30 watts, then my original 84 watt telescope field battery that I started out with would have died in less than 3 hours. I know for a fact that I’ve run my mount for more than 4 hours at star parties, and at least 3 hours in class for our weekly observations.
I dug a little deeper, and came to the conclusion that these outputs are the maximum draw that is available, but the devices can actually use much less power. I imagine that my mount draws very little power when it’s simply tracking at 1.5x sidereal rate. High speed slews obviously require more power. I don’t do high speed slews for a full 6 to 8 hours, so the 360 watt hours requirement is overkill.
Likewise, my laptop came with a HDD which was replaced by an energy efficient SDD. Also, I run power saving mode when I’m in the field (dim display, no power boost in the CPU, etc) so my laptop is not going to draw the full 90 watts (which incidentally is the thermal design package of my mobile CPU). Not doing any intense CPU work and keeping the laptop lid closed will drastically reduce my laptops power requirement as well. Besides, who wants to be staring at a bright LCD monitor when you can be looking at the stars?
The DSLR, however, will probably be working at 100% the entire time. It’s better to assume that figure is correct.
So, what kind of a battery would I need to buy that would satisfy 1550 watt hours of power? We know it needs to be a 12 volt battery because my electronics require more than 6 volts. Because watt hours are equal to Volts * Amps * Hours, we can divide 1550wh by 12v, and find a 130 amp hour battery would actually provide me with all of my power needs, and them some if I correctly assume that I wont be pulling maximum current from the battery all of the time.
Building the telescope field battery
It just so happens that CostCo sells high capacity, deep cycle batteries at a reasonable cost. For a measly $60 – I walked out with a 115 Amp Hour battery that is designed to be deeply discharged and recharged all the time. This battery will be the core of all my power needs in the field. There’s only one problem. My CGEM and everything else uses a 12v DC plug and the battery only has positive and negative terminal posts.
I needed to do some fabricating.
At a local electronics superstore, I found everything I needed to make my power system and feed my electronics. I had purchased two Lenmar Power Port 4 power splitters. These splitters will be used to allow me up to 8 total connections. In theory, I could run two mounts from my battery with this configuration should someone find out their own telescope field battery is flat.
My process for converting this to run off of a battery was simple. First, I had to open up the 4-way power plug to figure out which of the wires coming out of the splitter was the red (+ / positive charge) wire.I found a red cable coming from the LED light that is on when the splitter is energized. I traced that red cable to the plug, and only cut that lead away from the power plug. I then used a butt connector to attach an in-line fuse holder, and another butt connector to attach about a foot of red wire.
The next step in the process was to connect a ring connector to the end of the red wire so I could fit it over the terminal of the soon-to-be telescope field battery. This process was tricky, since the ring connectors that I purchased are ready to be shrunk with a heat gun. I cut away the pre-shrunken part of the rubber and fed the copper filaments through to the ring. I flattened them out a little bit, then using a pair of pliers, I crimped the hollow pass-through down around the copper.
I must warn you, I am horrible at using a soldering iron. But, I soldered the copper as best as I could to the ring terminal connector. In fact, if you’re looking at my soldering job here, and you think to yourself “he’s really good!” DO NOT SOLDER ANYTHING! As soon as you have secured the ring terminal connector with the red wire, you’ve completed the process of wiring the positive (+) electronics. The negative wiring is much simpler. Cut about two feet of black wire of the same thickness as the red wire. Remove the original 12v DC plug from the still attached electrical cable and use a butt connector to secure the two feet of black wire to the 12v DC splitter’s negative lead.
At this point, you should have all of the wiring from the 12v DC power plug complete. There should be a RED and BLACK wire that you have attached ring terminal connectors to. You may (or may not) have soldered them to the ring connectors. That step is totally optional. I wanted to practice, so I did it.
The photo to the right demonstrates how a final rewired 12v DC power plug should look. I put a fuse holder on the positive cable, which will hold a 10 amp fuse. This is important, because my battery holds a ton of energy. I want to ensure that if I plug several devices into a socket, I can’t pull too much current. If I draw too much current, I run the risk of an electrical fire, or could potentially damage other devices plug into the array of sockets. If the total draw of all my devices exceeds 10 amps, the fuse literally burns up and the circuit breaks. Yeah, I’ll lose power to all of the devices, but is sure as hell beats having my devices burn up because I pushed too much current to them.
The next order of business is to affix my power outlets to the top of my battery box lid. My Lenmar Power Port 4’s came with mounting plates. I screwed them to the lid as you see here. I took a Dremel and cut two holes. One is on top, and allows my power cables to go into the battery box. The other is for the future option of placing an on/off switch so I can actually turn a bank of 12v DC outlets on or off. I tried to get this to work when I first built the battery, and I wired it incorrectly. My switch actually caught on fire when I turned it on. I could also hear the battery boiling inside from the lead – acid reactions caused by the sudden discharge of so much current.
I have not revisited this idea since…
Having screwed the mounting plates to the top of my battery box lid, I passed the cables through the hole I cut and locked the 12v DC outlet. The hole I cut is just big enough for the fuse holder to go through first, followed by the black (-) cable. I can’t pass both through at the same time.
My next order of business was to be able to monitor the power in the battery. I decided that I needed a voltmeter that would plug into a 12v DC plug and tell me how much voltage was still in my battery. I quick drive to my local auto parts store turned up the this little gem. Designed to plug into a cigarette lighter socket, it will tell you how your car battery is doing. That’s perfect for what I need, since I am quite literally powering everything through a cigarette lighter socket hard wired to a car battery!
When you go to connect the positive ring terminal connectors to the terminal post, be sure that the fuses are not in place. This is important! You cannot short circuit the battery by connecting the other wires if the fuses are missing. People often worry about getting shocked when connecting the leads to the battery terminals. If the fuses are in place, you can see sparks fly when you touch the ring terminal to the post. However, when the fuses are not in place, then there is no complete circuit through the wiring. That means there won’t be any sparks, and therefore reduce the chance of getting shocked. Remember, you are playing with electricity here. **Never assume there is no risk!**.
The Finished Project
With everything connected and the lid placed back on the battery box, I can insert my battery voltage monitor.
I don’t know why, but a 12v battery is said to be flat when the potential difference is 10 volts or less. When charged to capacity, my telescope field battery will measure 13.2 (though I’ve seen mine read 13.4). I can connect my telescope mount to this battery, and it will run it all night. Also, I keep a phone charger in my OTA case, so I can keep my phone totally charged while I’m out. I never know when I need to call my wife for something.
As long as there is more than 10v in the telescope field battery, I’ll be good to go.