Let’s build a power box

A permanent observation site has lot of advantages. One of these is the availability of power for your mount, CCDs, and other astrophoto devices.

But this situation changes when you work with a mobile setup, While giving electric power to a mount for hours is not a tricky thing (using a commercial “power tank”), it turns out a serious issue when you add a cooled CCD, and a laptop.

That is my case. Now that I own a mobile astrophoto equipment, ready to get into my car and driven to the backcountry, the supply of electricity for this setup for hours has turned out to be a major problem.

First of all, let me tell you this: I am not skillful at all, and I have only basic knowledge of electricity. This is to encourage you. I had a lot of fun while building the device, and then a lot of satisfaction seeing how it works in the field.

My first thought when I faced the problem was to check prices for commercial power tanks. To my surprise, I found them to be very (very!) expensive. But even worse, they wouldn’t be able to satisfy my needs. Let’s see why.

Ok, let’s begin taking a look at the power needs. You can find the relevant information in the documentation that comes with your equipment, or in internet.

You have a mount. The power requirements for a mount are not high. Much of the time the mount is tracking. Only when it’s slewing the power consumption gets a bit higher. On average, let’s say that we need around 1 ampere (A). Then you have the main CCD. Most cheap, not cooled CCDs get the power they need from the USB port of your laptop. But a cooled CCD, as the one I use, needs its own power supply. In your CCD specs you’ll find the consumption figures you should expect. For example, you can have something like 1A minimum, and 2A maximum when cooling, moving filters, and taking a picture. Let’s stay on the conservative side, and get the maximum number for our maths.

You have to check the power needs of all of your equipment, including the laptop of course. Laptops  are really hungry devices regarding power consumption. Get the right information from your documentation. You may end up with a number like 4 or 5A.

You may need to take into account anti-dew devices if you use them, but let’s suppose that this is all for now. So we have a total of: 1A (mount) + 2A (CCD) + 4A (laptop) = 7A.

Back to the commercial power tanks. The one I own, a Celestron Power Tank, has 7A-hour of nominal capacity. What does it mean? That means that the maximum it can deliver is 7A in an hour. If I need 7A, the power tank will be dried out in just an hour!

Ok, some considerations here: we have adopted the maximum figures. So, the CCD won’t be all the time cooling, and the mount won’t be all the time slewing. But, on the other hand, as far as I know after reading a lot of information, a battery (a power tank is nothing else that a battery) cannot deliver the nominal power, and they are dried well before. When their charge goes below some point, they are not able to maintain the expected power supply. Depending on the battery type, this figure can be awful: below 50% of charge, many batteries are not reliable!

Anyway, what it’s clear is that my poor power tank would’t do. As I said before, power tanks are expensive, and Celestron (and other brands) has more powerful models, up to 17A-hour, for over 200€! That’s a lot of money, and again it is not enough: 17A-hour can provide, in our example, maybe up to 2-3 working hours only!

Enter DIY. When I searched internet for solutions, I found some people showing how they had built their own power boxes, and I thought it was a nice approach: having a powerful power tank, for less the money I’d need to buy a commercial one.

And it worked.

So here I share my project, hoping it will be interesting for somebody facing the same issues.

My first decision was about the battery. I needed a battery, and after some search I learnt that I needed a “deep cycle” one. Deep cycle batteries are designed to allow for a great discharge. A normal battery cannot be discharged under let’s say 50% without compromising its lifespan. Deep cycle batteries are long distance runners. They can be greatly discharged, which is what I need.

In internet you can find hundreds of deep cycle batteries. As I wanted something powerful, no very heavy (the more powerful a battery, the heavier it is), and with a comfortable size, I selected an AGM battery of 40A-hour, weighting 16Kg. I bought it through internet to a Spanish batteries provider. I also bought a battery charger, one of the  so-called intelligent models. These models detect when the battery is fully charged, and adapt while charging to protect the life of the battery.


Then, the box. Most of the DIY projects in internet used cheap plastic boxes. But I wanted something smarter, and I found a wooden box, the size of which seemed just fine for my project (30 X 23 X40 cm). I also bought resin to protect the box from moisture, weathering, cold, etc. However I noticed how thin the box was at the base, and I doubted it would be hard enough to hold the weight of the battery. So I decided to nail a solid base there.


caixa 1

caixa 2

The connectors: of course, I’d use cigarette lighter plugs, as many of my equipment comes with that kind of plug. I found these plugs in Amazon. I thought it’d be nice to have them in pairs, but I liked even more that they can be screwed through the holes of their plastic holder.


I needed switches. I wanted to have 3 of them: one would be the main switch, and the other 2 governing the plugs.

As I wanted to protect my equipment (first priority!), I decided to install fuses. To my surprise, the plugs that I have bought in Amazon already came with fuses. Nice! They are easily removable and changeable. They originally come with 10A fuses, but I changed to 6A ones to be on the safe side.

Ok, and that was more or less all I needed.

First step was to paint the wooden box with the resin.


Then I drilled the holes for the plugs and switches wiring.


And I installed the plugs and switches in place. As I explained before, the first switch is the main one, and each pair of plugs has its own switch.


I completed the box with the installation of a fancy item: a voltmeter and ammeter. The voltmeter is useful to check the battery status. When fully charged, a battery should read over 12V. When dropping towards 12V, the battery is signaling it’s been dried up. Regarding the ammeter: ok, I thought it would be a nice add-on, to read intensities. But I soon discovered that it’s far from being precise. The reading is not reliable, only the trend is. Maybe I haven’t installed it the right way. But probably the cheap model I found in Amazon is the culprit.

I took advantage of a tiny box I found to be used as a receptacle for the meter, this way avoiding to drill a precise rectangular hole for the device. I just nailed the little box near the plugs, as you can see in the picture, drilled a hole for the wiring, and placed the meter inside it.


Here you can see the wiring diagram of the box.


The shunt is a resistance that comes with the ammeter and must be installed in series (and the ammeter in parallel with its shunt). The ammeter and shunt are set always at the end of the path, just before ending in the battery.

And here you can see the box performing well in the field.

in the field