RC Helicopter Charger Fix

[TheAppleMac]

Hi again everyone,

I have an RC Helicopter which I bought from the US, which worked well, until the mains charger broke. The company which sells them doesn't have an online manual or even replacement parts.
So, I am thinking to set about making a charger. The original charger said output: 3.6V 450mA.
I have a 5V 1A supply, so to lower the voltage, I am using a 10k potentiometer and then a darlington pair to boost the current output out of the pot.

My question is: will this setup work? The Heli probably has a Li-Po battery, does it just charge with DC? I am REALLY cautious about Li-Po batteries, so I don't want to jump the gun here.

Can I just use a pair of transistors like that to boost current output, and will the input voltage be relatively stable?

Edit: I just realised that two transistors will drop the voltage 1.4V as required, is the pot even needed?

 

[cowboybob]

Lithium batteries require a very carefully metered charge. Improper charging can cause a fire.

Google "Lithium charger circuits".

 

[TheAppleMac]

I've already had a look, and they're complicated! The more complicated, the less likely anything will work.

The old charger just says it outputs dc 3.6V 450mA.

Will the regulation circuit be on the heli itself, or inside the mains plug?
If inside the mains plus, I may try to squeeze it open and get to the regulator circuit.

 

[cowboybob]

I've already had a look, and they're complicated! The more complicated, the less likely anything will work.

The old charger just says it outputs dc 3.6V 450mA.

Will the regulation circuit be on the heli itself, or inside the mains plug?
If inside the mains plus, I may try to squeeze it open and get to the regulator circuit.

It could be in the heli.

Not knowing exactly which heli you've got makes it tough to suggest solutions. What does your manual for it indicate (if anything)? What sort of warnings?

 

[alec_t]

The Heli probably has a Li-Po battery

Can you check the battery label? Then we'd know where we're coming from.

 

[audioguru]

At first the charger MUST measure the Li-Po battery cell voltage and if it is less than about 3.2V then it tries a test charge with a low current. If the voltage of the battery does not rise normally then the charger should disconnect the battery and indicate a bad battery.

A Li-Po battery MUST have its charging current limited to its mAh rating or less. Some expensive high-tech batteries are an exception.
The charging voltage MUST be limited to 4.20V.
The charging current is monitored and when it drops to 3% of its mAh rating then the charger MUST be disconnected.

 

[TheAppleMac]

It's an exRC Skycrawler.
As mentioned, there's very little documentation online, especially since the link to the manual on their website is broken. I don't have the manual, I left the box in the US to save space. Doh! (Ok, hopefully my first and last mistake)

Shown by AudioGuru, the charging circuit is extremely precise and li-po batteries are extremely temperamental. Although what you describe looks to be doable, it looks a bit too complex for my time frame. This is why I don't really want to build a charger (yet).

I have just opened her up. There are no markings on the battery. So I guess that means it's not certain it's a li-po anyway, I think that's what alec_t meant.

The mains charger is just a wall plug, there's no large box, if that's why the li-po chargers are like.
I took a photo of the board. Does it show anything?
There is a thin grey rectangle at the bottom which is the battery, if that indicates anything as well.

I think that the other end of the black connector on the far right in the back connects to the charging port. After, this connector then is manually connected to another loose connector on the leg to complete the circuit and power the heli.

Thanks for all the replies!

 

[audioguru]

I think the charging circuit is in the helicopter.
The charger is simply an unregulated power supply that is 3.6V when the charging current is high then its voltage increases as the charging current normally drops.

Today at the park a guy had a quad-copter that had GPS, a video camera with transmitter and an altimeter. It could fly stationary even when the wind was trying to blow it around.
I didn't watch his copter much because I was busy looking at and chatting with his pretty teen daughter and her pretty teen girl friend, hee hee.

 

[TheAppleMac]

Haha audioguru- I don't even know how to reply to that!
Thanks for the info though.
Can you easily tell from the pic?

Edit: I guess I'll have to open the charger to see if there's a charging circuit. If not, I can assume it's onboard. If so, I can connect my own power supply to the circuit.
Is this correct?

 

[audioguru]

I do not know from looking at your pic if some of those many parts are in a charger circuit. I am not looking at pretty girls yet today.

I do not see a Li-Po cell. I see a big cylinder on the left side of your pic that might hold three 1.2V button Ni-Cad or Ni-MH cells that can be charged from 3.6VDC.
I am guessing that the power supply the helicopter came with has poor voltage regulation because 3.6VDC is not high enough to charge a Li-Po battery cell.

I have a small RC helicopter that is charged from USB which is 5.0V and I have RC airplanes that have their Li-Po cell charged with a charger circuit powered from a 6V battery.

 

[cowboybob]

That looks like a SMPS to me.

The large gray cylinder to the left looks like a capacitor (probably in-line with the mains connector). I can clearly see a couple of diodes. Don't know what the "27M" device is. The 8 pin IC could be a charge controller.

I'm pretty sure the cap looks as though it's blown out, which would explain why the PS has quit working. NOT an unheard of occurance for a PS circuit of this sort.

If that were the only damage, replacing the cap might fix the problem.

TheAppleMac, think you can do a capacitor replecement job? My guess is that it would only require de-soldering two wires and then re-soldering them back onto a new cap.

I should point out that since this involves mains power, thus it carries a significant electrocution risk if you don't take appropriate precautions!

 

[TheAppleMac]

I'm pretty sure the cap looks as though it's blown out, which would explain why the PS has quit working. NOT an unheard of occurance for a PS circuit of this sort.

Hi cowboybob, just to check, you know this is a picture of the heli? The reason I'm asking is because the charger outputs no voltage at all. That's why I want to know if the heli has any spefic circuitry to control the charger. I've not even connected the charger to the heli yet.
But if we are in the same page, then yes, I guess a capacitor solder should be ok. I'd rather have a problem with the charger, as it would be harder to fix anything in the heli than a charger.
I'm taking everything to the workshop tomorrow to get some more advice and do tests.
I'll keep you updated.
Thanks,
TAM

 

[cowboybob]

You're welcome, TAM.

In or out of the heli, still looks like a PS.

I will say though, must be a fairly large heli if the PS and the battery are in the copter.

I take it that the mains connector is nothing more than a cable that plugs into the wall and then into the heli (via those two slightly bent prong looking connectors on both sides of the PCB), is that correct?

 

[audioguru]

I have a very small IR controlled helicopter and the gyro, PWM steering, motor speed control and charging circuit for its Li-Po battery are all in the helicopter. It is charged from 5V USB. The charging cable has an LED that glows during charging then turns off when the charging is finished.

Duh! Of course the battery is in the helicopter.

 

[cowboybob]

Gimme a break, AG.

I was emphasizing the oddity (compared to my heli) 0f having the mains powered PS in the heli...

 

[TheAppleMac]

I was emphasizing the oddity (compared to my heli) 0f having the mains powered PS in the heli...

I have done some testing and researching on LiPos etc. and I now understand what you mean, it makes sense that the charging circuit is not in the heli, like you say Cowboybob,
It turns out that as well as a dead charger, the battery had also died (I'm guessing from the charger).
and after much reluctance, I have ordered another LiPo and built a charging circuit with a LM317 as an adjustable voltage output, keeping in mind what Audioguru said.

At first the charger MUST measure the Li-Po battery cell voltage and if it is less than about 3.2V then it tries a test charge with a low current. If the voltage of the battery does not rise normally then the charger should disconnect the battery and indicate a bad battery.

A Li-Po battery MUST have its charging current limited to its mAh rating or less. Some expensive high-tech batteries are an exception.
The charging voltage MUST be limited to 4.20V.
The charging current is monitored and when it drops to 3% of its mAh rating then the charger MUST be disconnected.

Thanks so much for your help.
TAM