Wall Chargers!

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Red Scholefield

You were cleaning up your shop and somebody stepped on you charger and broke the plug off.Your significant other wants to toss all those useless charger transformers you saved that were left over from by gone days of cordlessegg beaters, carving knives, mixers, shavers things you can’t even remember. You still have as a remembrance the charger from the dustbuster you kids use when they tried to suck a mud puddle dry.

On the other hand you are still trying to figure out how to charge the 850 mAh pack you made up from the guts of a new video pack that you salvaged after Uncle George backed over it with his '68 Caddy at your cousin’s wedding. Somewhere in your shop you just cleaned up, you can't find the charger to your glow plug driver and it's very dead as you found out as you tried to sneak in an flight before work at the field and you were the only one there.

GOOD NEWS!! (and there is no bad news so take heart) You are surrounded by charging opportunities. If we ignore the banal warnings on your collection of orphan chargers warnings like USE ONLY ON HINKENDORF MODEL XL199-06 OR SEVERE DAMAGE COULD RESULT. These warnings are in the same category as USE ONLY GM APPROVED PARTS or FOR INDOOR USE ONLY. When was the last time your charged your transmitter in the rain?. Notice that all of these little charges have a UL approval mark. It means that these chargers when short-circuited will not do nasty things like catch on fire or electrocute the owner. They may blow an internal thermal fuse, but they will not cause a hazardous condition.

I have, at no small expense of time (at least 30 minutes), taken on the task of characterizing the chargers lying in and around my workshop to see how they would work in other applications. The term “characterizing" is the term used when you are getting a ridiculous compensation for doing something rather simple. In fact as a reward for reading through this I will tell you how you can characterize your very own charger(s).

In addition to several Futaba R/C system chargers, I had at my disposal a Black & Decker Dustbuster charger, a Sears cordless screwdriver, A Black & Decker charger of unknown vintage and application an R/C car pack charger for 6 cell packs. Since those of you reading this, must by definition, have a least 1 charger that came with your R/C system and it's a good chance that it is a Futaba charger, let's look at one of them first. Figure 1 shows how the receiver and transmitter portions of this charger work with different numbers of Ni-Cd cells.


The chargers were tested to determine at what rate they would charge a range of 1 to 10 Ni-Cd cells. (The capacity of the cell didn't matter since the charge voltage curves are essentially the same as well as the end of charge voltage.) Note from the curve in Figure 1 that for 4 cells the charge current for the receiver portion of the charger is about 50 Ma right where it should be. Likewise for 8 cells in the transmitter section it is charging at the rate it was intended to approximately 50 Ma. But you have acquired a 4.8 volt 1000 mAh flight pack from somewhere and you would like to charge it in something less than a couple of days.

Just make up an adapter to fit the plug on the transmitter side of the charger (Radio shack has matching plugs) and plug into the 1000 mAh pack. From the curve you can see that the charge rate will be 90 mA and much better suited for the higher capacity battery. This should not harm the charger as the power is at the same level. Note that the receiver side of the charger will nicely handle charging your 1 cell glow plug driver at 100 mA. For a 10 cell battery (nominally 12 volts) the transmitter side of the charger would do a reasonable job of recharging you 12 volt lead acid field box battery although it would take about 5 days to fully charge a discharged 5 AH battery. You can tell you are fully charged when the voltage reaches 14.7 volts. If you don't have a digital voltmeter and plan on doing any experimenting you are working blind! They are available from Radio Shack for under $40.They are far better and much more versatile than any expanded scale meter.(See article on Loaded Digital Voltmeter – Better than ESV)

What about those other old chargers lying around. Can they enhance your R/C life style? Figure 2 illustrates their performance on various cell packs.


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

number 86755.

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


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.

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