The chargers depicted in Figure 2 are identified as follows:
BD3 -Black & Decker Dustbuster charger rated at
4.4V @ 125 mA carrying the number 133353 00.
BD4 - Black & Decker charger from unknown application
rated at 5.8V DC 1125 mA and carrying the
SEARS - Sears screwdriver P/N 350930 rated
at 5V 0.35 A.
MILL6 -Millennium charger Model No. CH72RC rated at 8.7V 2W and designed for charging an
R/C car 6 cell 1400 mAh pack.
What useful charging jewels can we draw from this information? Both the
Black & Decker and Searschargers would do a reasonable job charging some of the higher capacity 4 cell flight packs.Sears
would be better at 125 mA. They would also work fairly well on each others products. If you are turned on to using 5 cell
Ni-Cd flight packs these three chargers offer a range between 50 and 90 mA.
How do you find out if your
particular charger orphan will work on something that needs to adopt a charger?
Simple.just connect it to the battery
pack you want to charge after checking the polarity to make sure that plus goes to plus and then measure the current with
the digital multimeter I told you to buy and see if it is in the range you need. Remember that current will be flowing from
the charger into the battery (unless you are trying to make a battery discharger which I don't recommend, as it may make
the charger unhappy and the battery equally unhappy). The positive lead of the meter should go to the positive of the charger
with the negative lead going to the plus side of the battery. Complete the circuit by connecting the negative of the charger
to the negative of side of the battery.
Let it charge for some time as the current will start out higher
on a discharged battery and then taper off some as the battery reaches full charge. To be on the safe side keep an eye on
the charger the first time you use it in its new role to make sure it is not over heating. Warm is OK. Hot (uncomfortable
to hold) is a problem. UL approval would assure that the charger would not overheat but on the off chance that it may not
have this approval, it's better to be safe than sorry.
Not all of the plug in the wall chargers provide
DC current required to charge the battery. They are just a step down transformer and will need a diode, or better a diode
bridge, inserted as a rectifier to convert the AC to DC. The diode should "point" from the charger (either lead)
to the positive lead of the battery. If you don't have the foggiest idea of what this is all about don't mess with
it or get one of your electrical buddies to
give you a hand. You can't get into too much trouble, as this is very
low voltage stuff. The Ni-Cd pack, on the other hand if you have not already discovered it, can do a number on your wiring
harness if you get things mixed up and short it out.
There are a couple of great chargers no longer on
the market that you should keep your eye out for at flea markets. The Ace H/D500 is a nice variable constant current source
that will handle anything up to and including a 12-volt battery charging up to 500 mA. For twice the fun a dual unit (Model
DMVC) is available.
These handy units can form the heart of a charger characterization system. Connect
it across 2 10 ohm 20 watt resistors in series. Vary the current to set the voltage across the resistors to represent the
voltage of a Ni-Cd pack under charge (1.35 volts/cell). Then connect the charger you want to study across the same resistor
string. The charge should be passing current through the string in the same direction as the current source. By varying the
current source and switching one of the 10 ohm resistors in or out of the circuit you can cover quite a range of charge voltage/current
situations and plot what the test charger is capable of. You must make provisions to measure the voltage across the resistors
and the current coming from the charger under test. By this time you should have gone out and bought that digital multimeter
before they are all gone.
CLS 4-93 rev 7-03
Notes:The "wall wart" chargers while designed to charge
a specific number of cells at a specific current. This is usually the rating seen on them. The consist
of nothing more than a transformer, and a diode to rectify the AC input. The windings of the transformer are designed to have
the necessary resistance (or some resistance may be added) to set the current at the right level for the specified number
of cells. In actuality, because a true constant current source would be quite expensive, the "wall wart" is a compromise
in design. If you were to measure the voltage without being connected you would find it is somewhat higher
that the voltage of the battery to be charged. A Futaba charger for instance, measures 6.7 volts* at the receiver plug, while
it says the voltage is 4.8 volts and current is 50 mA. Even his really not true as the current is around 60 mA at the beginning
of charge and then settles down to 50 mA as the battery reaches full charge and the voltage approaches 6.0 volts.This is how
you can use a "wall wart" charger marked for a specific voltage/current on a battery with more or less cells (voltage).
You just have to make sure that the resulting current at the higher or lower voltage is proper for that battery. A simple
current measurement while on charge will take care of this.
*keep in mind this is an unfiltered half wave voltage measured
on a digital volt meter, so not actually a true DC.