I have a Celestron NexStar SLT 127 telescope. For those who don’t know, it’s a battery-powered telescope which has a small computer to navigate around the skies and steer the telescope. It’s quite hungry for power – the telescope takes 8 AA batteries. Depending on the type of batteries you use and the temperature, these can last as little as 2 hours, which isn’t long enough for a decent evening of observation and is certainly an expensive habit if you are a regular observer.
Fortunately, the telescope has a traditional DC-in jack so an external power supply can be used. I made up a 20-metre lead with a 12V, 1A mains transformer so I can use the telescope in my garden without fear of the batteries running flat. But it’s rare that I want to use my telescope in the garden of my house in suburban Bristol – there’s so much light pollution that the sky is the colour of Fanta.
I also made up a cable to run the telescope from my car’s 12V cigarette lighter socket. This works well, but you still have to use the telescope relatively near the car and there’s always the slight risk that you might over-discharge your car battery and strand yourself in the countryside. Best avoided.
I decided that a portable power supply would be the best option. There are commercial offerings out there, but they seem expensive for what they are, sometimes have low-capacity batteries, and almost always come with crap you don’t want. So I decided to build myself a battery box which would be useful not just for the telescope, but any device you’d expect to be able to run in your car – e.g. a satnav or a phone charger.
You will need
- A lead-acid battery
- A small toolbox
- A 12V cigarette lighter socket, surface-mount variety
- One or two switches
- Panel-mount voltmeter (optional)
- Charging terminals. I made mine out of some spare bolts I had lying around
- Wire. While your telescope may only draw 1A, charging the battery may push significantly more current, so bear this in mind. I used some old mains cable, rated at 13A.
- Spade connectors and/or a soldering iron, depending on how permanent you want it to be.
- Stick-on rubber feet
Choosing a battery
You should preferably use a deep-cycle leisure battery rather than a car battery, since car batteries don’t like being over-discharged. It will need to be 12V but you can get various ratings of Amp-hours (Ah) which tells you how long the battery will last. My Celestron telescope has a rating of 1A maximum, which means a 6Ah battery will last for 6 hours at a draw of 1A. In reality, 1A is the worst-case power draw, so 6 hours is also the worst-case lifespan. You’ll probably get twice that. I decided to buy a 12Ah battery so I wouldn’t have to recharge it too often.
You should also make sure it is “maintenance free”, otherwise you will have to keep topping it up with water.
Fixtures and fittings
First things first, decide which components you’d like in your battery box. Measure them, and make the correct holes to mount them. A plastic toolbox is dead easy to drill, saw and file. I chose this particular Stanley toolbox because of the small compartments on the top, which I turned into covers for the sockets and switches.
Analogue voltmeters are more expensive than their digital counterparts, but are so much cooler. The voltmeter springs into life when I flick the TEST switch.
My charging terminals are actually 8mm bolts with a nut on both sides to keep them in place. The charging terminals can, of course, also be used to run equipment that has a spring clamps instead of a 12V plug.
The schematic is dead simple. It’s a switch to close the circuit that feeds the 12V socket and the charging terminals, and a second optional switch to close the circuit for the voltmeter, so you can keep an eye on the battery.
Don’t forget to add a fuse on the positive terminal of the battery, in case you short something later on. Choose a suitable fuse rating that is high enough that you can charge the battery without blowing the fuse. Check your battery and charger for their specs.
When working, be careful to keep your positive and negative in order, and double check everything with a continuity meter before connecting the battery. Lead acid batteries can deliver a heck of a lot of current, and you don’t want to fry a component, or let the battery boil and shoot acid in your face.
Here’s how I wired the internals of my battery box. I decided to use spade connectors for the battery as it is a consumable component, and I soldered everything else.
The battery must be fixed in place securely, or it will move around and ruin your soldered joints. I had originally intended to secure it with some hefty Velcro straps, but in the end I was lazy and I glued it to the bottom of the toolbox. The flimsy plastic sagged with the weight of the battery, so I attached self-adhesive rubber feet in the four corners of the toolbox, and under the four corners of the battery so its weight would be supported when on a flat surface. I glued some of the longer runs of cable into place, to prevent them from being trapped in the hinge.
And finally, the finished product, complete with carry handle and weather-resistant flaps!