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

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.

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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.

We also added a transistor (TR1) to Bialy's schematic. The semiconductor, when biased by resistor R5, limits charging current to the value selected with the switch. Without it, the charge rate would change as the internal resistance of the ni-cad dropped during charging. This addition is an improvement over most commercial ni-cad chargers. (Note: Any 2-watt or heat-sinked 1-watt NPN power transistor could be substituted for the 40-watt Radio Shack item. It's simply the lowest-rated suitable device available from the company.)

You can use the printed circuit design we've provided, or a 1-13/16 inch by 2-1/2 inch piece of perforated circuit board, to secure the capacitors, resistors, diodes, and transistor. Make sure that the negative sign on the large capacitor is running toward the negative power connection solder pad (down, as you look at the printed-circuit side of the board) and that the diode Dl's band is toward the powering end of the board. Diode D2, a 15-volt zener, must have its black band on the end nearest the positive power-connection solder pad (up). The capacitors and diode D2 absorb (or bypass) transient power surges from the car's alternator, and diode D1 prevents the ni-cad's power from back-flowing when the charger is connected to a switched circuit in the car's wiring system. Also, be sure to connect the transistor so that B, C, and E on the circuit board match the key on its package. The resistors can be wired either way, but be sure that resistor R1 has at least a 2-watt power rating. (Again, Radio Shack's minimum suitable item is rated at 10 watts.)

Because the resistors and transistor dissipate power to limit the charging rate, the circuit board generates heat. Consequently, you should drill a half-dozen 1/4 inch holes in the bottom of the Radio Shack project box to allow air to circulate. The circuit board-with the parts pointing down-sits on the ridges in the plastic box and should be glued in place. The rotary switch is secured in a 3/8 inch hole in the box's cover. (Note: If you can locate a 4-position switch, such as the Digi-Key item noted in the Bill of Materials, it can be substituted for the 12position Radio Shack switch.) To wire the switch, look for the contact brush underneath the connectors to determine which post connects to which circuit. Then double-check your wiring with a continuity tester-this is an easy place to make a mistake. And when you mount the switch on the case cover, mark the positions for 9-volt, AA, and C and D clearly, to match 82 ohms, 15 ohms, and 5 ohms respectively. Two pairs of wires pass through holes drilled in the box's ends: the power connection and the leads for the 9-volt battery snap connector. The power connection can be in the form of insulated wires, the positive of which you'll connect to your car's fuse box. Or, if you're feeling flush, you can pay a couple of dollars for a cigarette lighter plug (use a 3-amp fused model), which will allow a more convenient connection for your charger. In any case, tie a figure-eight knot in each pair of wires just inside the case to prevent tension on the circuit board.

If you decide to hard-wire your charger into the car, you'll have to choose between a switched and an unswitched circuit. The former will be turned off when you shut your engine off, thus preventing drain from your car's battery; the latter will allow continuous charging whether or not the engine is on. Frankly, the amount of power the charger uses is so small that it would take months to pull down a healthy car battery.

You can connect a pair of AA batteries (in a Radio Shack battery holder, No. 272-382+) or a 9volt directly to the 9-volt snap connector. To charge C or D cells, take another 9volt snap connector (you're the proud owner of five of these if you bought from Radio Shack), and solder its leads to a battery holder that will handle two cells. Then you can connect the two 9-volt snaps. Bear in mind that the batteries you charge at one time (up to seven will work) must be wired in series (positive to negative, positive to negative, etc.). Only one 9-volt cell can be connected at a time.

One of the most attractive aspects of Mr. Bialy's use of a car's electrical system to charge ni-cad batteries is that most of us use our vehicles daily. Thus we'll be reminded of the cells under charge and avoid wasting power by leaving the batteries on too long. (The charging rate is low enough that the batteries can't be damaged by overcharging.) Those of you who live in cold climates, however, please take note: Ni-cads should never be charged at a temperature lower than freezing; at less than 32 degrees Fahrenheit, the charge rate must be reduced to prevent damage.

If, for this or other reasons, you'd prefer a charger that plugs into the wall, you could adapt this circuit with a 12.6-volt transformer (such as Radio Shack's No. 273-1385). Use a larger project box (Radio Shack No. 270-222 will work), eliminate D2 and C2, and use a 470-microfarad capacitor for C1. Remember that you'll be dealing with line current, and shocks from the 120-VAC side of the circuit can be life threatening.

Whichever way you choose to charge, you'll be doing your part to mend our disposable society . . . and you're going to save a fair amount of money in the process.

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