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Updated 10/20/04

BATTERY CHARGERS

[battery power]

Interior view. In the top center of the enclosure is the 18 volt transformer, below it a bridge rectifier, and above it an inline 3 amp fuse.  On the lid is the ammeter and rheostat.

Here's what most of my battery chargers look like, a black wooden box about 7 by 7 inches, 3.5 inches high, with rheostat, ammeter, and 2.1 mm plug.

From the start I wanted battery power to be practical, and part of that means keeping the cost down.  I have made a number of battery chargers based on the circuitry of older HO power packs: a transformer to reduce 110 volts to 12 volts (or 18 volts), a rectifier to change it to DC, and a rheostat (variable resistor) for charging rate.   I have added an ammeter to monitor charging rate and a connector that keeps the polarity correct.  The whole shebang is mounted in a wooden box.  It's not fancy, it's not pretty, BUT IT WORKS!        About charging: it's OK to charge at a slightly higher voltage than the batteries, but never at a voltage lower than the batteries.  If the batteries get warm, they are either fully charged or they are being charged too quickly.  For maximum battery life, charge batteries at no more than 1/10 the amps of the battery's amp-hour rating.  For example, charge 3-amp-hour batteries at no higher than 0.3 amps.        As the batteries reach their full charge, they will draw fewer amps from the charger. I usually give the batteries a 12-hour charge, but a shorter charge will do OK on Gel Cell and NiMH batteries.  Nicads need to be fully discharged and fully charged each time.

PARTS LIST Transformer:  12 to 14 volt (or 18 to 20 volt for 18 volt charger) 1 to 3 amp........$4.00 Bridge rectifier: 1 to 4 amp, 50 to 200 volt rating.................................................  .50 Rheostat: 40 to 80 ohm, 25 or 50 watt rating.................................................... 10.00* Ammeter: DC 0 to 1 amp range is best, but 0 to 10 amp OK..............................  12.00* Plug: 2.1 (or 2.5) mm Co-ax power plug with 6 foot cable................................   1.00 AC extension cord or patch cord...................................................................    1.00 Box made of plywood approx 6 by 7 inches by 3.5 inches high.........................   1.00 20-22 ga wire, solder, electricians tape, etc.....................................................     .50 TOTAL COST...........................................................................................  $30.00         *(you can lower the cost if you can find rheostats and meters at surplus sales)

TOOLS Wirecutters Needlenosed pliers Drill Saw and hammer (for enclosure) Soldering gun or iron Voltmeter (just a simple $10 meter will do)

PARTS SOURCES Allectronics.com Hosfelt.com Mouser.com Radio Shack www.surplussales.com (for rheostats at low prices)

SCHEMATIC DIAGRAM

CONSTRUCTION 1.  Build a plywood box, 1/4 inch top and bottom about 6 by 7 inches, sides 1/2 to 5/8 inch to match, 3.5 inches high. 2.  Mount the transformer.  An alternative is to use a DC wall-mount transformer. 3.  Drill a hole near the base to fit the patch cord or extension cord. 4.  Mount the meter and the rheostat on the top panel.  Make sure they clear the transformer. 5.  Drill a hole near the base on the right side for the low-voltage cable. 6.  Wire according to the schematic above.  I suggest wiring one part at a time, checking voltage with a meter at each completed step, then unplug before wiring the next component. Solder all connections. 7.  Before using as a charger, make sure the output is 12 to 14 volts with the center of the plug positive and the outside negative.  If you are using a DC wall transformer, the voltage may appear higher when there is no load on the circuit.  8.  Connect to a battery set, making sure the battery set's jack is wired with center post positive.  Turn the rheostat knob and check that the meter is indicating a change in current (amps).  Charge the batteries at about one-tenth their amp-hour ratings, that is, a 3-amp-hour battery set should not be charged at higher than 0.3 amps.  I usually give the batteries an 10 to12-hour charge.