What's All This Battery-Powered Stuff, Anyhow?

This article is in Bob Pease on Analog Volume 2 in the Analog section of the Electronic Design Library. 

Members can download the PDF ebook.

When I went hiking in Nepal in 1989, obviously I should have taken along a camcorder. But I didn't, and none of the other guys on the trek did either. I did bring a good 35mm camera and a small tape recorder, because I was well warned that many things in Nepal are very photogenic. I shot over 1000 slides. But, if I go on a trek like that again, a camcorder would be a very good thing to bring along. (I would still bring a small 35-mm camera in my pocket, for landscapes and pictures with higher resolution than any TV).

But when I began to ponder the actualities, I realized that it would be very hard to get enough battery power to do even an hour a day of recording. So, when I bought a camcorder in 1991, I began planning how to bring the camera on a trek.

I bought the Sony TR-51 because it's quite compact and light at 1-3/4 lbs. (including strap but no case). It weighs less than my 35mm single-lens-reflex camera (Nikomat FTN, 2-1/2 lbs., including strap but no case), and it isn't much more bulky. If one had to carry a bulky old 1989 camcorder that weighed about 5 lbs.—well, I refuse to think about that.

I will presume that a small 8mm or similar compact camera is the right way to go. You can carry it easily and fire off a quick shot when you want. The TR-51 draws about 1 ampere from its 7-volt battery, about 7 watts.

Batteries... Sony's model NP-55 (about $40) is good for 1 hour of steady recording, about 1 ampere-hour. But if you turn the camera on and off a lot, taking little snips of action and scenery, you're lucky if it lasts 40 minutes. Obviously, one of these batteries, fully charged, will barely last you about one day in a photogenic place like Nepal. The larger NP-77 (about $60, 10 oz.) is good for almost double that amount, 1.8 ampere-hours. I have one NP-55 and one NP-77. So after the first 3 days in the wilderness, away from ac power, I will have to invent some kind of battery charger....

It's possible to put a set of five AA cells in a tiny little $50 battery box sold by Sony. But they're not effective energy-wise in terms of weight or cost.

You could rig up five alkaline D cells in an extension-cord battery pack. That would be four times more effective than AA cells from the standpoint of weight and cost vs. energy. But after several days, you would need more D cells. Not cheap nor light. I bought a set of five rechargeable, sealed lead-acid cells. These are heavy and cost-effective, and would keep you in business about five days—but then they would run out.

So I needed some kind of battery-charging scheme. Solar cells would be much too bulky, and maybe too fragile? They're also unreliable on a rainy or cloudy day, and not easy to use if you're hiking through a forest. I mean, it's not easy to mount them on top of your backpack. Note, most solar-cell panels will inherently put out ZERO current if any portion of the panel is in the shade! But I do plan to buy one and check it out.¹

A gasoline-powered generator would be gross overkill. Remember, an NP-55 (about 1 ampere-hour × 7 V) is just 7 watt-hours, which is only about 1% of a horsepower-hour of energy. So a 1-horsepower gasoline generator would be absurd.

Frank Goodenough suggested, could you rig up a little model airplane engine to a tiny generator? That's about the right amount of horsepower, perhaps 1/10 horsepower, and the weight would be reasonable. But they're not very efficient; they would guzzle lots of fuel if you ran one half-an-hour a day. And the noise would not be acceptable, even after you added a muffler. So I do not see a solution down that alley.

One other possibility is if there are any decent, fairly compact camcorders that draw a lot less than 6 watts. I haven't looked into that, but I don't foresee much chance here for a really significant energy saving.

I asked my friends at work, but nobody had any good ideas about battery-charging. I have seen a portable radio with Nicads and a hand-crank generator.² You crank it up for a couple minutes and then you can listen for a half-hour. But this radio obviously runs on a lot less than 6 watts—probably down near 0.06 watts. Even if you rigged it up for additional output to an external battery, it might not be at all suitable.

I spotted a little hand-crank generator, in an Edmund Scientific catalog, that could put out "10 V, 100 mA." The price of $48 was absurd, but I decided to try it. When it arrived, it was a cheap little motor with flimsy plastic gears in a light but flimsy plastic framework. If you cranked it fast, the gear ratio was barely adequate, so you could just barely get 7 volts. And if you wanted to get 100mA charged into a 7-volt battery, you had to crank almost at a frenzy. Bad investment. Not suitable for a battery charger.

Then a friend at work, Fran Hoffart, said he had a "gear motor." It had a 12-volt motor with an integral 65:1 gear box that he had bought for a couple bucks at the Electronics Flea Market.³ I put a long crank arm on its "output" shaft and tried cranking. At about 30 rpm, a leisurely 2 seconds per revolution, I was able to get 200 to 400 mA into a 7-volt battery pack. Not bad! The gear motor weighs 1.2 lbs. (including crank and wires), which is not a trivial amount to add on top of your 45-lb. pack frame. But the alternative is to carry a 1.7-lb. camcorder that you can't use because its batteries are dead.

I strengthened the crank handle, hooked up some connectors and a little strap-on socket for the battery, and paid Fran $5 for the gear motor. Then off we went on an 8-day backpack trip in the Sierras. To be honest, it took a lot of cranking to get much charge. I started the trip with both batteries fully charged, and after 4 days, both were pretty low. Still, with an hour of cranking, I could get an honest 1/3 ampere-hour into a battery, and that would be good for 15 minutes of intermittent recording.

And we always had a quiet hour in the evening when we would be sitting around sipping juices and nibbling snacks. So I could easily put in my hour a day of cranking, and that was enough to get some coverage of a day's events. I also was able to con my friends into helping out with the cranking. After all, how could I record their antics on tape if they didn't help bring up the charge on the batteries? So they helped, and grinned.

Eventually, I installed a good knob on the crank so I could easily crank at a good torque for a long time (just turning over the crank easy would put 100 mA into the battery, but if you want to get a good 300-mA output you have to crank with quite a bit of force). I rigged a spare piece of cord so the torque from cranking could be carried by a long lever arm and the cord itself to my foot (as I sat cranking), or to my neck (as I ambled along a trail). This cut out a great deal of the effort needed to hang onto the motor as I cranked. I found I could crank as I walked along easy trails. But if I had to do any serious hiking, it would be pretty hard to crank at the same time.

Still, if I went on a trek, I could crank an hour or two per day and keep my batteries charged up pretty well so that I could record a few dozen minutes every day. If I went to Nepal, I would pay one of the porters a few rupees to crank for an hour every evening. If I go backpacking with friends, I can get my friends to chip in a few minutes, just for a lark. And they did enjoy helping on my recent trip, just like Tom Sawyer got his friends to help him white-wash the fence. SO, I think I have a handle (literally) on how to drag along enough batteries for a big trek. I would need 3 or 4 batteries, and a couple of chargers, and I'd have to put in some of my spare time. I might even have to do some cranking while I hiked on easy trails. But it WOULD work.

Where would you buy one of those gear-motor sets? Well, the one I got is made by The Pittman Co.,⁴ and is rated at 12-volt dc with a 65:1 gear ratio. I peeked inside, and the gears do seem well-lubricated and well-designed for industrial service. I got a catalog, and this model is still available. The specs make sense. But the amount of torque I was putting in was excessive for the output gears.

So I ordered a similar gear motor with 38:1 gear ratio, GM9413-38:1. This is about as small and light as you can get for a gear motor that will put out 300 mA at 7 volts—anything smaller than that could not handle the torque (175 inch-ounces at 50 rpm) or the amperes.

What about cassettes? The VHS-compact tapes only run 20 minutes, so you would use up a pretty big box of them on a 20-day trek. The 8mm cassettes are compact and can record for 120 or 150 minutes, so 10 of them would last for 20 days (at 1 hour per day) and that's the same size as 10 audio cassettes—not bad.

Rain-proofing.... If you're out hiking and rain begins, a plastic bag or two can keep your machine pretty dry. If you wanted to take some shots on a rainy day, and it's not too windy, you can carry an umbrella. I tried that, and I was delighted to find it worked quite well. Or, for about $150, you can buy a camcorder raincoat that lets you shoot through an optically flat glass while the camera stays perfectly protected from the rain. You just put your hand up into that bag from below. For about $350, you can get a completely sealed version of a big heavy plastic bag with excellent waterproofing and an optically flat glass front panel, so you can take your camcorder diving with you down to 30 feet deep.

I'll list the outfit that sells these⁵, because many camera stores won't know where you can buy them. Still, $150 is a lot to pay for a plastic bag. But it's better than getting your camera soaked or doused with salt spray. I just brought some plain plastic bags and a zip-lock plastic bag, and I put on an extra UV lens to protect the camera's front end.

This camera is small enough (7-1/4 in. × 4 in. × 3-1/4 in.) to fit into a small water-repellent "fanny pack," so I can easily strap it around my tummy and bring it just about anywhere. The zipper is a minor annoyance, but it's not quite bad enough to force me to redesign it with a velcro cover (that would also have drawbacks). When it rained, my rain parka covered over the fanny pack, keeping everything perfectly dry but still accessible.

When we were returning home from Nepal, the Security Guards at Kathmandu Airport were very polite, but they confiscated a dozen AA-cell alkaline batteries from one of us. We later asked at several places, "Where does it say you should not bring flashlight batteries on planes?" (SMWISICDI)⁶ We never did get a good answer. But recently a friend who travels a lot told me that in Korea and several other Asian airports, the Security people do confiscate flashlight batteries in your hand-baggage for fear of saboteurs using them to ignite explosives. So be sure to keep your flashlight batteries in your checked baggage—that is OK.

Of course, lead-acid batteries can be pretty dangerous, and they're normally forbidden on planes. But I found one company that sells lead-acid batteries for electric wheelchairs that are approved for airline travel.⁷ So if you need larger batteries than ordinary Nicads or small sealed lead-acid batteries, that may be a good resource.

One other thing about your camcorder or portable computer—when you go to the airport, the security people like you to be able to turn it on and demonstrate that it really does work (not just a dummy packed full of dynamite). So make sure your batteries are charged up.

This is especially important if you are travelling outside the United States, because security in Europe and Asia is a lot more rigorous. Of course, this is contradictory to the premise that the airport security doesn't want you to carry any batteries. Ask your travel agent what to do!

Now, I have been talking mostly about charging problems for a camcorder. But in reality, if you have a little portable computer, or radio, or transceiver, and you travel out of the United States or far away from line power, you will probably have similar problems. Maybe these ideas will be useful. Stay tuned for more!

All for now./Comments invited! RAP/Robert A. Pease/Engineer

Address: Mail Stop C2500A, National Semiconductor, P.O. Box 58090, Santa Clara, CA 95052-8090

1. John Christensen suggests trying a solar panel, about $60 for 14 V at 200 mA. I am ordering one from Solar Electric Inc., 1450 Harbor Island Dr., Suite 204A, San Diego, CA 92101; (800) 842-5678.
2. Crank-Dynamo AM-FM radio, about $34. REI CO-OP, 20640 Homestead Rd. at Sunnyvale-Saratoga Rd., Cupertino CA 95025; (408) 446-1991.
3. The Electronics Flea Market is held on the second Saturday of each month from April to October, 6 AM to Noon, at Foothill College, Los Altos, CA at the Moody Rd. exit off I-280. Free entry for shoppers.
4. The Pittman Co., Harleysville, Pa. 19438-0003. Model "GM9413-3", 12 V, 175 in-oz., 65.5:1 gear ratio. About $28 in quantities of 1-9. The same GM9413 gear motor with 38:1 gear ratio is preferable, because it can put out twice as much power without exceeding the gear box's maximum torque rating. Call (215) 256-6601.
5. Raincoat for rain and spray protection, Models E190, E191, E192 (depending on the size of your camera), about $150; Ewa-Marine c/o Pioneer Marketing & Research Inc., 216 Haddon Ave., Suite 522, Westmont, NJ 08108; (800) 257-7742. Also, waterproof plastic-bag housing for down to a 30-ft. depth, Model E176 or similar, $350. (These are actually manufactured by Goedecke & Co., Kirchheim, Germany.)
6. "Show Me Where It Says I Can't Do It," Pease Porridge, Electronic Design, May 9,1991.
7. Mobilectrics, 4311 Woodgate Lane, Louisville, KY 40220; (800) 876-6846. Deep-cycle, sealed-lead-acid batteries, $69.95 plus UPS, "Airline Approved."