Ni-Cad Battery Charger

Kiss wasteful disposable batteries goodbye by building this $6 ni-cad battery charger battery charger, including proper care a feeding, the design, circuit board diagram, cost and materials list.

| November/December 1985

You can kiss wasteful disposable batteries goodbye by building your own less-than-$6.00 ni-cad battery charger. (See the ni-cad battery charger diagrams in the image gallery.)

The common battery is a prime example of a product that usage transforms all too quickly from a valuable device into a piece of hazardous waste. And much of this accumulation of dangerous junk is needless, since rechargeable nickel-cadmium cells can easily be substituted for conventional batteries in many applications. These sturdy little power sources may cost twice as much as even high quality alkaline cells, but they'll make up the price difference many times over as they go through hundreds of charge-discharge cycles.

From a practical standpoint, though, you shouldn't replace every battery in your house with a ni-cad. The rechargeables are most valuable in appliances that are used regularly and extensively—for example, a desk calculator that's operated daily, or a toy that a toddler just can't remember to turn off. In a year's time you could run through dozens of conventional cells in a heavily used appliance, while the same use wouldn't make a significant dent in a nicad's life expectancy. On the other hand, the flashlight that sits idly in the glove compartment month after month will do better with alkaline cells. Because nicads self-discharge at a rate of 1% to 3% of their capacity per day, they'd go flat in an unused flashlight within a month or two; an alkaline cell, however, might stay on call for a year or more.

Proper Card and Feeding of Nicad Batteries

To get the most from your investment in nicad cells, you need to treat them right. Don't use ni-cads in applications that drain their power too quickly: A case that's hot to the touch is a sure sign of strain. In general, nickel-cadmium batteries shouldn't be drawn down from full charge to flat in less than an hour, though short spells of high discharge won't harm them. Also, it's best to avoid repeated cycles of equivalent partial discharge and charge—as with a calculator you use a bit each day and charge every night. A ni-cad will develop a sort of electrochemical memory of the partial discharge, and you'll lose access to its full capacity. (However, such a cell can be restored by draining it all the way flat and then charging it fully.)

Most important, though, a nickel-cadmium battery must be charged at the correct amperage. A good slow charger applies a current that's equal to about one-tenth of the battery's amp-hour capacity. Thus a 1.1-amphour ni-cad (a C or D cell) should be charged at about 0.110 amperes. A completely discharged battery will be restored in 14 to 16 hours at this charging rate, though it will be usable in 6 to 7 hours. (Ni-cads can be charged at rates high enough to restore their capacity in an hour, but the charger must then be shut off to prevent battery damage.)

The Ni-Cad Battery Charger Design

When contributor James E. Bialy sent us a slick design for a AA ni-cad battery charger that works off an automobile's electrical system, we were impressed . . . but we just weren't able to leave well enough alone. Our tinkerers figured that for a minor increase in cost, Bialy's brainstorm could be made to charge a wide range of ni-cad cells. In our adaptation, amperage is controlled to 0.135 amperes by a power resistor (R1), and three switched resistors (R2, R3, and R4) make the final drops to provide 0.007 amps for 9-volt batteries, 0.045 amps for AAs, and 0.110 amps for C and D cells.

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