Making a battery box for my Celestron telescope

Background

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.

Top panel
Top panel

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.

Voltmeter and switches
Voltmeter and switches

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.

12V socket and charging terminals
12V socket and charging terminals

Electrics

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.

Battery box schematic
Battery box schematic

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.

Inside the battery box
Inside the battery box

And finally, the finished product, complete with carry handle and weather-resistant flaps!

Toolbox
Toolbox

8 Comments

  1. Simon
    January 7, 2013
    Reply

    Pretty cool Jofo!

  2. Drew James
    January 7, 2013
    Reply

    That’s ‘smart’ thinking !

  3. john
    February 4, 2013
    Reply

    First of all well done and points for creativity. I wish to also make my own custom battery supply. I wanted to know the cable you are plugin in from the telescope to the DC socket is that a straight through connection? Does it matter if the amps are higher that what the device specifies?

    • February 4, 2013
      Reply

      Hi John,

      Yes, the cable between the battery and the telescope is a straightforward cable with no cleverness. One end has a 12v car-style plug and the other end has a small DC plug to match the telescope’s socket. They are wired straight through with two-core cable, but you have to double-check that you’ve got the polarity the right way round so you don’t zap the ‘scope.

      The regular mains power supply that comes with the telescope supplies up to 1 amp, so the telescope will always draw less than this. A lead-acid battery can supply tens of amps. You always need a power supply more powerful than the device you want to run, and the device will only draw as much current as it needs. You just have to match the voltage. It’s always a good idea to use a fuse – I included one in my battery box but also many of the 12v car cigarette lighter plugs have fuses inside them.

      Regarding view time, well so far I’ve done several astronomy sessions and never had to charge the battery. I don’t know exactly how much current the telescope draws as I’ve never measured it, but its absolute maximum worst case is 1 amp, in which case the 12Ah battery will last for 12 hours. In reality the telescope draws much less current than that, and you’ll probably be able to get at least 24 hours out of the battery. Be sure not to run the battery too flat though, or you’ll have problems charging it. This is why I also added a voltmeter.

      Happy observing!
      Jonathan

  4. john
    February 4, 2013
    Reply

    Oh forgot to ask how much view time to you get out of 12 AH battery?

  5. DAVSPURS
    April 23, 2014
    Reply

    CAN YOU USE A 12V GOLF BATTERY OR CAR STARTER BOOSTER BATTERY

    • April 23, 2014
      Reply

      Yes, you can use pretty much any 12V battery. A booster battery will be fine, Celestron actually sell their own rebranded booster battery but any will do.

  6. Lionel
    January 12, 2015
    Reply

    Hi Jonathan,

    I have a Nexstar 130. Typically a 12 volt battery measures about 13.8 volts on my voltmeter. Does the Celeston car adapter step it down to exactly 12 volts or will it end up putting 13.8 through the scope. Will 13.8 volts fry the electronics or is it ok?

    I would prefer just making my own lead to the scope straigh from the battery, instead of purchasing the Celeston adpater.

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