Mark I) with Voltmeter
A simple and compact power source for backpacking. Easy to build, but it will take
some time for the metal work.
The Back Packing Battery Pack (BP-BP Mark I) with Voltmeter
I have a 10 cell 'AA' NiMH battery pack (12V @ 1.6AH) that I use for backpacking. It needed a home! I also wanted a voltmeter built-in so I could keep track of it's condition and charging (I live in the woods, you have to keep chasing the sun with your solar panel..). Radio Shack didn't have any cases / boxes that suited me so I got thinking about that Super-Sized Giant Altoids tin I had and thought it might work. Bingo! The battery pack, LED voltmeter (http://www.rainbowkits.com/kits/vm-1p.html), switches and connectors all just fit!
Jumbo Altoids tin with LED voltmeter (early)
AmQRP Homebrewer, Issue #5 2 Copyright 2005, AmQRP, All rights reserved
Inside view of tin ribs and voltmeter
Actually, I'm telling a little white lie - don't go off to the market to find that jumbo-sized Altoids tin, I made it by cutting the bottoms out of 3 regular tins and soldering them all together with a 4th.. A lot of folks use the Altoids tins for QRPp rigs, so I started to think of them not as just a project box, but as a project form-factor. A four unit high box fit my needs perfectly.
Start by removing the lid of the bottom tin with pliers and flatten out the hinge tabs back into the space they were stamped out of. They should be below the outside surface when done, we will get back to them later. Warning - the following steps produce LOTS of tiny flying, SHARP and HOT pieces of metal, USE EYE PROTECTION!!! The thin metal edges created are also sharp, be carefull! It is also a VERY NOISY process, USE EARPLUGS!
Now drill three 1/4" (6mm) holes next to each other in the bottom corners of the tin you are stacking on top, following the corner arc and leaving about 1/8" (3mm) between the tin wall and the holes. Small pilot holes are good in this thin material, and a reamer works well to enlarge the holes to the 1/4" size. Grind or mill out the metal between the holes, I use a conical milling bit in a Dremel Moto Tool®. You should now have four arc-shaped holes, one in each corner. Now cut in a straight line between the holes with mini tinsnips / heavy sissors (sheet metal nibblers tend to jam on this thin metal), also leaving about 1/8" (3mm) of lip.
Voila, the bottom of the tin is gone!
Lay the tin down on some wood and tap flat any bends made by the cutting (I use the butt end of my Exacto® knife). Sand / grind all the paint off of the bottom you just finished and the top edge of the tin below. Stack the tins and hold them in alignment with big rubberbands.
Tack solder the corners quickly and then remove the rubberbands. Flow solder into the gap all the way around the tins. The corners will wick in more solder as they cool, so you will need to reflow more solder there.. Now file and sand everything down smooth. Tiny flying, SHARP and HOT pieces of metal warning again, just in case you missed the first one - USE EYE PROTECTION and EARPLUGS!!! Use the Dremel tool with a milling bit to cut away the extra
metal inside down to the factory rolled lip of the lower tin. Scrape and sand off any left-over burs. Repeat as many times as needed to get the height of box you want. The resultant ribs inside create quite a sturdy box for being made out of thin tin.
Now scrape away all the paint from around and on the former hinge tabs and flow solder into the depression to make it flush with the rest of the box. Sand / file away the excess solder. Last tiny flying, SHARP and HOT pieces of metal warning - USE EYE PROTECTION and EARPLUGS!!! Drill out ALL (I learned the hard way and had to repaint.) the holes needed for the project, deburring with the Dremel milling bit. Now you can mask off the bottom and lid and paint the box white again.
Checking battery pack fit.
AmQRP Homebrewer, Issue #5 4 Copyright 2005, AmQRP, All rights reserved
Very simple.
My battery pack has 3 bananna jack outlets. The middle one is switched for my camp light (uses 6 white LEDs!). The voltmeter presents a load to the battery (3-7 LEDs on at a time in typical operation), so I used a momentary normally-open switch for the battery check. In fact, I might replace the voltmeter with one of my own design using a bargraph IC that can be setup to display the voltage with a single LED lighting up at a time for lower current consumption. This one is fine for now, much better than dragging a DVM along.
Finished unit and voltmeter demo
Yes, the voltmeter is backwards, I wanted to be able to calibrate it (the black trim pot on the right side of the voltmeter board seen on the inside photographs above) without
disassembly or drilling a hole in the bottom of the case. I also wanted the primary wiring at the top of the box in case field repairs are ever needed. I have added lettering to make all the functions clear.. I wrapped the battery pack in pipe wrapping closed-cell foam to act as a shock absorber and insulator from the sun's heat. With two small desiccant packs inside the battery pack weighs 12 Oz. total.
Battery pack wrapped in closed-cell foam
AmQRP Homebrewer, Issue #5 6 Copyright 2005, AmQRP, All rights reserved
If you choose your solar panel capacity carefully (Solar panel amperage = (battery capacity in amphours / number of good sun hours per day) X 1.1) and monitor your battery, you
should not need a charge controller. Just remember that a drained battery will take all the current you can give it (it won't like it, but it'll do it), so do not try using a high capacity (in respect to the battery) solar panel. Solar panels are not constant-current sources, so if you have a high capacity solar panel you should use a DC-DC current-limited power converter with a timer circuit. A little more weight to pack in but it will save those expensive NiMH batteries.
Out catching some rays with my Z-fold solar panel (an old project)
All material in HOMEBREWER is copyright 2005 and may not be reprinted in any form without express written permission from the American QRP Club and the individual author. Articles have not been tested and no guarantee of success is implied. Safe constructions practices should always be followed and the builder assumes all risks. HOMEBREWER Magazine is a quarterly journal of the American QRP Club, published on CD-ROM. Each issue typically contains over 200 pages of QRP-related homebrewing construction and technical articles intended for builders, experimenters, ham radio operators and low power enthusiasts all around the world. HOMEBREWER features include construction projects for beginners all the way up to the advanced digital and RF experimenters. Annual subscriptions are $15 (for US & Canada) and $20 (for foreign addresses). For information, contact editor/publisher George Heron, N2APB at[email protected]or visit HOMEBREWER Magazine home page atwww.amqrp.org/homebrewer.
The journal of the American QRP Club ... for builders, experimenters, ham radio operators and low power enthusiasts everywhere.
Wade Lake, KR7K