Keefer

My Feedback: (2)

What size resistor should I use in chargine a 110, 140 or 150 mah 4.8v nicd or nimh pack with a standard wall charger?

Can I do this with my Hobbico AccuCycle? Or my Turbo charger?

Red Scholefield

My Feedback: (9)

You really can't get there from here. A wall charger is essentially a constant current charger (albeit a rather sloppy one). Adding resistance makes it even slopier and more of a constant potential device, which you don't want for charging NiCds or Ni-MH. What you want to look for is a constant current charger in the 10 to 20 mA range. This would mean a relatively high source voltage (4 or 5 times the battery voltage) and then setting a resistance to give you the desired current. As a starter you could play around with the transmitter side of the wall charger by adding resistance in series with the pack.

feihu-RCU

Red S -

I don't seem to understand your statement "Adding resistance makes it even sloppier and more of a constant potential device, which you don't want for charging NiCds or Ni-MH." The addition of resistance changes constant current to more of a constant potential?

I have used the rec'vr output from my wall charger ( the output of which is 6v @ 60ma) and made a jumper with a 47 ohm resister to drop the output to 19 ma for a 4 cell 150 mah battery pack, and I have had no problem. Are you saying that the xmtr output of 12v @ 60 ma with ~100 ohms would be more efficient/effective because of the higher potential of the xmtr output?

I recall years back, when the xmtr and rec'vr chargers were just a series resistor in line with a diode in a 110 v line, but that has since changed.

I am not questioning the validity of your statement, but just trying to understand the rationale.

feihu

laryboy

My Feedback: (2)

do you mean a standard futaba rec/tran/ wall charger? or just some wall charger that you may find in wall mart ?

feihu-RCU

laryboy -

Standard JR radio wall charger for xmtr and recvr.

feihu

laryboy

My Feedback: (2)

if the charging rate is printed on the charger, and it is 50 ma like a futaba charger, then it is ok to use it for even a 100 mah battery. at 50 ma the battery would be charged in 2 hours. i would not leave it on much more than that.

Keefer

My Feedback: (2)

Ahh - so for a 110 mah pack, I could use a 50 ohm resistor with a 6.0v charger to give me about a 12ma charge rate?

Thanks.

laryboy

My Feedback: (2)

the math would indicate 125 ma. here is an approximate equation...........voltage of charger...minus voltage of battery will equal voltage across the resistor. divide that result by the value of the resistor. the result will be the charging current

feihu-RCU

Keefer and laryboy -

The equation is a good place to START; however, not all 6v chargers are the same, nor are all 110mah or 100mah packs the same. Small variations in the hardware can cause large variations in calculations.

In my case, the charger lable is: input 115vac 60hz, output rx: [email protected], output tx: [email protected]. Looking inside the charger is a 115vac dual output of 15vac and 8.5vac. A diode is used to generate dc and an led w/ parallel resistor brings the dc down about 2 volts. So I assume about 6v dc out to the rx.

I want to charge my 4cell 150mah pack at 15ma.
6v - 4.8v = 1.2v
1.2v/15ma = 80 ohms

I inserted a 100 ohm pot and an ammeter (0-200ma scale on my digital vom) in series in the charging circuit, and rotated the pot until the meter read 15 ma. The pot read about 50 ohms and so I used the nearest fixed resistor of 47 ohms which gave me 19 ma.

Some resisters have a 20% tolerance, and that can make a difference too.

Anyway, I believe that what my calibrated VOM reads is the proof of the pudding, and it works for me.

Hope this helps.

feihu

feihu-RCU

This is a continuation of my post #9 above.

What works for me may not work for you. I use the 4cell 150mah battery for the radio receiver when flying my 1/2A planes, and make usually 3 flights. Batteries charged the night before flying and have had no problems. In considering what RedS said in his post #2, it is possible that my battery was not charged to max capacity, but charged sufficiently for my case.

By adding a resistor to decrease the charge current, you are also decreasing the potential difference between the charger and the battery. As the battery capacity is increased with the charging, so is the battery voltage, which can reach as high as 5.6 or nearly 6 volts depending on your particular battery. With a questionable 6v charging potential, there may be insufficient potential difference for the charging to continue. Therefore, quoting RedS "As a starter, you could play around with the transmitter side of the wall charger by adding resistance in series with the pack"

So for reducing the charge current to 12ma from the transmitter side of the wall charger, you might try, for STARTERS :
12v - 4.8v = 7.2v
7.2v / 12 ma = 600 ohms.

Then find the right resistance for 12ma.

feihu

Rodney

You can take an LM317 integrated circuit (available at Radio Shack) and hook it up as a three terminal device with the proper resistor to give you the current required. I believe that the package even has the schematic on it to show you how to do this. You can power it with the transmitter side of any wall wart (the receiver side will even work in some cases) or you can use any 6 to 12 volt source for power. If using the 12 volt side, you may need to mount the LM317 on a heat sink. A real simple solution.

Keefer

My Feedback: (2)

Are there any commercially available "smart" chargers in the industry that will charge these smaller batteries? I use a Hobbico AccuCycle, and a Dimond TurboCharger for most of my 500 mah and above packs. There has to be something for all these micro-systems that are around.

feihu-RCU

Keefer -

Since I have an assortment of resistors, I thought I would hook up the radio charger and battery to see what gives, and here are the results:
Using an ACE radio wall charger, the Tx output, I connected a series resistor and a 4cell 150mah battery together.
A series resistor of 220 ohms resulted in a charge current of 15 ma.
A series resistor of 330 ohms resulted in a charge current of 10 ma.
A quarter watt resistor would be satisfactory.

feihu

Keefer

My Feedback: (2)

feihu - did you happen to note the charger voltage during the charge?

feihu-RCU

Keefer -

Hooked up the charger w/ resistor just now to take rreadings.

Voltage at the TX output, before the 220 ohm resistor is 9.13v
Voltage after the resistor,same as across the 4c 150mah batt. is 5.95v
Ammeter still reading 15ma.

And the math checks out 3.18v/220 ohms = .01445 amps.

The battery was fully charged last night.

feihu

Keefer

My Feedback: (2)

feihu -
How about using the Rx side of a charger?

laryboy

My Feedback: (2)

http://www.hobbico.com/chargers/hcap0100.html look at this charger. it wll carge with as little as 10 ma if thats what your looking for

feihu-RCU

Keefer -

I pulled out the 150mah pack and 47 ohm resistor and connected them up to the rx output of the Ace charger.
The battery was still charged up and here were the readings;
6.62 volts before the resistor
5.94 volts after the resistor
The ammeter read 15 ma
The math checks out

I discharged the battery for a while to bring its voltage down somewhat and redid the measurements.
5.75 volts before the resistor
4.82 volts after the resistor
The ammeter read 19 ma.
The math checks out

This clearly shows that as the battery gets charged up, the battery voltage rises.
By charging at the rx output, the charge current decreases as the battery voltage approaches the source voltage.
By charging at the tx output (with 220 ohms), the charge current remains constant because of the greater difference of the source voltage.

I'm glad you asked, and I see the difference.

Thanks,

feihu

Keefer

My Feedback: (2)

feihu

A most excellent comparison of theory to practice. Well done! Now I'll make a trip to Radio Shack.

Keefer