BATTERY CHARGERS
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Updated
10/20/04
    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)
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.
PARTS SOURCES
Allectronics.com
Hosfelt.com
Mouser.com
Radio Shack
www.surplussales.com (for rheostats at low prices)
SCHEMATIC DIAGRAM
[battery
power]
TOOLS
Wirecutters
Needlenosed pliers
Drill
Saw and hammer (for enclosure)
Soldering gun or iron
Voltmeter (just a simple $10 meter will do)
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.
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.