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How to Construct a Portable Power Supply

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More Power Scotty!
The Construction of a Portable Power Supply By Robert Kerner

August 2005

Amateur astronomers who utilize electronic equipment in the field need a reliable power supply. Whether it is for a GOTO mount, laptop or sophisticated CCD imaging system, portable power comes in many forms. Quick start batteries of the type used to jump start a dead vehicle are available from several commercial sources; however, they often have limited capacities. Many astronomers use deep cycle marine batteries, to which they connect alligator clamps or cigarette lighter outlets. This method is inexpensive and, if you select a big enough battery, can provide power for longer periods than possible with a jump start unit. This can result in a tangle of cables and clamps and, most importantly, there is always the risk of reversing the polarity of the cables when setting up or breaking down in the dark. Polarity reversal can be dangerous and damage the equipment.

Commercial power packages specifically designed for astronomy are available in the retail market. Kendrick makes several models of sealed 12 volt batteries complete with 12 volt cigarette lighter jacks. Unfortunately, theses units are not inexpensive (one dealer sells a 33 amp hour 4 outlet model for $305), particularly when you consider that a marine battery can be purchased for as little as $50, depending on the size. By shear luck, I met a gentleman at NEAF who had a home-built battery pack much like the Kendrick models but his cost a fraction of the price of a commercially built battery pack. The gentleman, Paul Hyndman, has his own web site www.astro-nut.com complete with pictures of his power pack. Paul was gracious enough to share his wiring plan and this article is a description of my effort to build Paul’'s design. The entire project cost me $135 without a charger, substantially cheaper than a commercial battery pack and my battery has a generous 55 amp hour rating.

The project took me one afternoon to build and about 3 hours to purchase the parts. I obtained the battery from Interstate Battery, a distributor in Syosset, for $80. I suspect that I could have obtained a less expensive battery if I had spent more time shopping. I’'d suggest not purchasing a marine battery at West Marine, as they wanted $100 for a 30 amp hour battery. The remaining electronics parts came from Radio Shack and the battery case came from West Marine.

The beauty of this project is that you will have a deep cycle marine battery of whatever size you want in an enclosed case with 4 power outlets, an on/off switch and a power meter. You will also have fuses for the outlets and the peace of mind knowing that you need not worry about polarity issues again, unless you disconnect the wiring assembly from the battery posts. And did I mention it is cheaper than a commercially made product?!

And now the legal disclaimer. Batteries can be dangerous instruments, particularly if handled incorrectly. Reversing the polarity (positive for negative and negative for positive) can have dire consequences, particularly for your equipment. So be careful and take your time. The wiring plan came from Paul and I trusted it because I saw his power pack working at NEAF. If you have specialized training as an electrician or electrical engineer and feel the plan is incorrect or unsafe please let me know. That said, the project is pretty easy. I’'m writing as someone with very minimal skills and confidence when it comes to wiring. In fact, it took me longer to work up the courage to try this than it did to actually build the unit.

Here is the parts list:

  • One battery. You choose the size but I’'d recommend a marine deep cycle. Make sure it will fit inside the battery box.

  • A battery box. West Marine has a fair selection of these plastic cases that have carrying straps. About $13.

  • 4 12v power outlets/cigarette lighter adapters. Radio Shack part #2701556, $6.99 each complete with an in-line fuse.

  • Rocker switch. Radio Shack part # 2750730 $2.99.

  • “Gold plated” connector for 5/16 inch stud: to connect wires to the posts of the battery.

  • Auto Volt tester. Radio Shack part # 2200112 $5.99

  • Assorted connectors: the female half of the bladed connectors, since you will discard the ones that come with the power outlets.

  • 2 nuts and bolts to run through the post of the battery to secure the wires.

  • 5 drops of Crazy Glue to secure volt meter to top of box.

  • Drill with bit suitable to drill out the holes for the outlets and/or Dremel tool

  • Wire cutter/crimpers

Begin with the box. I wanted 4 outlets and chose to mount them on the ends of the box. The Radio Shack packaging instructs you to drill a certain sized hole for the outlets. I did not have the requisite drill bit and chose instead to use a 1 inch diameter bit and trim out the excess material with a Dremel tool. This worked out very well because you want the hole a little tight on the outlets; they have to snap into place. I next tackled the battery meter. Paul’'s website shows that he cleverly concealed the wires leading from the bottom of the meter into the box. I wasn’'t that clever and could not determine how to do that without dissembling the meter, which I did not want to do. So I cut a tiny hole on the top of the box with the Dremel tool and threaded the wires down into the box. Lastly, I drilled a ? inch hole and Dremeled out the excess for the rocker switch. I chose to conceal it in a nook rather than on the top of the box so that it cannot be inadvertently switched off while working. The entire surgical procedure took about 90 minutes, going slowly because once you cut out plastic you cannot replace it.

After drilling a pilot hole, I trimmed out the excess using a Dremel tool.

A small hole was made on the top of the case to accommodate the wires from the meter. I later secured the meter with Crazy Glue.

I tucked the switch into this location rather than have it on top

of the box, where it could be accidentally shut off during use.

The wiring took the lion’'s share of the time due to my inexperience. If you are comfortable stripping wires and attaching connectors, you’'ll likely complete this part with little worry. I worry, so I laid out all the wires on a table before I cut or attached anything so I could visualize how it would look assembled. Paul Hyndman was kind enough to describe the routing and I can do no better than to pass along his directions:

Put five black wires on the battery's ground lug and run four of them directly to the negative (outer shell) lug of the 12VDC adapters (AKA cigar lighter sockets) and one to the negative side of the radio Shack power monitor.

Run one red wire from the positive lug to the master switch and three wires out of it, one to the Radio Shack voltage monitor plus side and one to each of the two fuses. Run two red wires out of each fuse, one each going to the positive lug of the 12VDC adapters.

All of the wires you will need are included with the outlets and the meter. You will need to cut off the crimp connectors that are supplied with the outlets as well as the probe and alligator clip supplied with the meter. I shortened the wires so they would all fit comfortably in the box. Paul’'s instructions call for modification to the fuse wires. I did not understand how to modify the fuse lines and could not see the purpose in using only two fuses when each outlet came with its own. Therefore, I connected one red fused line to each of the positive terminals of the outlets, so that each outlet retained its original fuse. The system works fine like this. This part of the construction calls for splicing ends of wires to join them into crimp connectors, but it is not an overwhelming task. You could also solder theses connections.

For the hot and cold terminal connectors, I used the “gold plated” connectors mentioned above. They are basically round eye connectors, through which I passed a nut and connected them to the posts of the battery. My battery has straight posts with holes in the top, so it was convenient to run a bolt through the hole, into the round eye connector and secure it all with a nut. This way I never have to worry about fouling up the polarity in the dark; the wires are permanently bolted to the battery posts.

With the wiring complete, the only thing left to do is tidy up the wires with some zip ties, tuck it all into the box and secure the meter to the top of the box using some Crazy Glue. To fire up the unit, just throw the rocker switch to the “on” position and plug in your devices. The meter comes on with the power.

This is a straightforward project that does not require any special skills. Upon completion, you will have a power supply hard wired to four fused outlets, an on/off switch and the convenience of a power meter, all in an easy to carry case. My battery pack weighs in at 42.2 pounds for a 55 amp hour battery.

Many thanks are extended to Paul Hyndman for sharing his wonderful creation and wiring instructions.

The finished box with two outlets and the meter visible. Two additional outlets are located on the other end of the box.


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