Charging LiPo with Wall-wart

[trennonix]

Hello,

I got a RC Electric Helicopter that came with its own charger. Unfortunatly, the provided charger can only charge at 800mA.
My batteries have a capacity of 2200mAh, so, each battery requires almost 3 hrs to charge.
One option would be to go and buy a professional expensive charger.
Before I do that, I'd like to try to find an alternative way.

My question:

I have a wall-wart rated: 12VDC max 2.0A
So, if I hook it up to one of those LiPos the current will never exceed 2A (<2.2A) and I can get the voltage up to 12V.
After that, the provided charger will get the voltage up to the 12.4V mark (4.13V/cell).
Would that work?
The only problem I see is balancing the individual cells. (maybe I'll use 3 wall-warts )

Thanks

 

[Reloadron]

You have the beginnings of a recipe for disaster.

Lithium Polymer or LiPo batteries are a great new way of storing energy for portable devices from cell phones to RC helicopters.

They’re great because they can store 350% (approximately) more energy than a typical Nickel Cadmium (NiCd) battery pack and weigh 10% – 20% less. They can also discharge much more current than a NiCd battery and be fully charged in about an hour. LiPo batteries also don’t develop memory or voltage depression characteristics like NiCd batteries, and do not need to be discharged before being charged.

But they’re not without their downside. Mishandling of these batteries can lead to fire, explosions and toxic smoke inhalation. In the rest of this guide, we’ll discuss how to charge, store and handle lithium polymer batteries safely so you can enjoy them again and again.

Please note that the information contained in this guide is for informational purposes only. You should consult your batteries manual for specific instructions regarding the handling, charging and safe usage of your lithium polymer batteries.

The above quote was taken from here. There are a good number of considerations in charging LiPo batteries that really should be observed. You need to get familiar with them. Addition a Google of "Charging LiPo batteries" will bring up a wealth of information. In a nutshell, no what you are considering is a bad idea.

Ron

 

[trennonix]

"You have the beginnings of a recipe for disaster."

Haha, well I thought so...

I read that a LiPo charger functions in 2 stages:
-Constant current
-Constant voltage

The constant current stage keeps the current from going over 1C and the constant voltage stage allows the battery to reach 100% charge.
Is my understanding of the workings of a LiPo charger correct?
If so, the wall-wart will limit the current to a maximum (constant current stage).

I know that using wall-warts seems crude and I honestly have no hope of this working but I just can't see the flaw (technical flaw, not safety flaw) in my idea.

 

[alec_t]

Just because the wall-wart is rated at 2A doesn't mean it will limit the current to that. If the wall-wart (assumed to be a linear type, not an SMPS) has an excessive load the current may be well above 2A, limited perhaps only by its winding resistance. Watch out for the smoke and/or listen for the bang!!

 

[Reloadron]

In reality (and as a reality check) plug in the wall wart and measure the unloaded output voltage. I would be curious as to wht the actual output voltage is.

Ron

 

[audioguru]

As the Li-Po battery charges its current decreases. But then the voltage of the cheap wall-wart increases until the Li-Po catches on fire.
Also a Li-po must never be trickle-charged. Its charger must turn off when it detects a full charge.

A 12VDC wall-wart will have a low current output of 18V or more. Then the Li-Po will burn for sure.

 

[trennonix]

Ron, the voltage on the provided wall-wart (w/o load) is 12.23V, another wall-wart gave 14.85V.

alec_t, if the rated current is not the limit, than what does it mean?
I would've liked to measure the current if I short the output but my multimeter can't measure DC current, only AC must get another one.
However, I know that if I run a small DC motor off the wall-wart, it runs much slower than off a battery (same voltage of course) so it must be current limited (not just by the winding resistance).

audioguru, I don't understand how will the voltage go over the unloaded voltage (unless in your case 18V is the unloaded voltage).

 

[audioguru]

A cheap wall-wart has resistance in the windings of its transformer. The resistance forms a voltage divider with the load resistance which reduces its output voltage when loaded.
So the transformer manufacturer uses a transformer with a higher voltage so the voltage is correct with a load.

I have a cheap 9V wall-wart that is marked and measures 9V/100mA. With no load its output voltage is 17V.

 

[Reloadron]

OK, since I don't often charge LiPo batteries and the ones I do use I have chargers for I may be off on what I say. Anyone feel free to correct me.

LiPo battery charging consist of a few phases. The process begins with a CC (Constant Current) where the current is slowly ramped up to the charge current. This is generally 1C or in the case of your batteries about 2.2 amps. The charge will be CC running for about 45 min or so. A wall wart as you describe is not a CC device.

Following the CC phase of charge the charge will go into a CV (Constant Voltage) mode. This actually is when the cells reach 4.2 volts I believe and in the CV mode the charger should hold or maintain the cells at 4.2 volts. When in the CV mode the current will slowly drop off. Once it drops to a predetermined level the process stops and the battery is charged.

A basic wall wart is neither a CC or CV device. When LiPo batteries are charged they require a carefully monitored charge process to do it right. Will LiPo batteries charge by just tossing a wall wart across them? Yes, I would think so but you run the risk of a fire (they have been known to do that) a small explosion (they can do that too) or simply destroying the batteries. That is the reason myself and likely nobody else here will advocate charging a LiPo battery using a wall wart.

Some more advanced systems also use temperature monitoring during the charge process.

Ron

 

[audioguru]

When your Li-Po batteries catch on fire because you don't charge them correctly then please don't put water which makes them burn hotter. Instead, make a movie of the fire and post it here. Don't breathe the toxic smoke.

I saw a brand new big and expensive ($500.00) Li-Po battery catch on fire. It was put in a pail of sand and burned for about 15 minutes. Some of the sand melted into glass.

 

[trennonix]

According to my understanding, the CC phase charges at maximum safe current (1C), so as long as the current is smaller or equal to 1C all is fine (the current doesn't have to be constant).
The transformer can only produce a limited amount of current (say 1C); the transformer will, almost always (during the 45 min mentionned by Ron), be operating at its maximum output current.
The current might not be constant but it is below 1C.
I'm not saying that the wall-wart will do a safe job, but it will do...
I assume I'm missing something; where did I go wrong?

 

[tytower]

Unfortunately all of the sites I visit seem to have these people trawling daily for posts they can caution you on . "Wowsers" I call them because they would outlaw all experimentation which is not within their own restrictive set of criteria.

Now you won't get rid of them and the sites don't yet police to remove them and it will be a difficult moderator problem. I think soon though it will be recognised that the Internet has opened up a mass of opportunity for "Wowsers " to stifle and ridicule peoples ideas and in doing so discourage legitimate research and experimentation which we all use to learn . Listen to them and you would never do anything for yourself . Motor mechanics and auto electricians have been doing it forever that I remember.

So I find it is a teach yourself how to ignore the "Wowsers" and proceed anyway.

On your battery , the warnings are everywhere and so too are the horror stories . Every " Wowser " makes up their own or a better version of what they have heard or read . Some may be genuine . I have not seen any evidence of it . I have not put a nail through the center of a fully charged LiPo yet either, just in case, but I have had a bit of a look for myself and you will find some photos on the link .



https://www.electro-tech-online.com/threads/inside-a-lipo-battery.115676/

Oh and I suggest you charge each cell separately if you can as I think they don't naturally accept charge uniformly from a pos /neg feed to the output line but rather tend to overcharge and destroy the first cell encountered.

 

[Reloadron]

I told you that the wall wart would indeed charge the battery. I also said I would not be the one to tell you to do it.

Ron

 

[audioguru]

A wall-wart does not limit the current. When the battery is discharged then its voltage is low so it draws too much current from a wall-wart. Too much current might cause the battery and/or the wall-wart to catch on fire.

The wall-wart does not measure the battery voltage then refuse to charge a battery that is already fully charged so then the battery will severely over-charge and might catch on fire.

My Li-Po chargers regulate the current then limit the voltage to exactly 4.20V per cell. They stop charging when the battery cell is fully charged. They refuse to over-charge a battery that is already charged.

 

[alec_t]

alec_t, if the rated current is not the limit, than what does it mean?

If the power unit is from a reputable manufacturer it means the maximum current which can safely be drawn continually without the power unit over-heating and while maintaining the rated output voltage.

 

[audioguru]

A cheap unregulated Wall-wart does not limit the voltage and does not limit the current. It is supposed to produce its rated voltage when it has its rated load current. But a battery will draw too much current from a wall-wart when it is discharged.

 

[trennonix]

Okay... a wall-wart cannot act as a current limiter.

Here's a page that pretty much says the same thing and provides current limiting circuitry:
http://sparksandflames.com/p6.html
http://sparksandflames.com/p12.html

Finally, I thank you for your help and patience

 

[audioguru]

The circuits for charging a 9V battery are confusing since a 9V rechargeable battery is not a Li-Po type. It is probably a completely different Ni-MH type that limits its own voltage. A Li-Po battery needs to have its voltage balanced and at no more than 4.2V per cell.

 

[tytower]

Read up lots on recharging them

For all batteries that are rechargeable the manufacturer suggests certain maximums and they should know.
Well thats what most of us would assume .
However these days one has to be a bit more cynical or even suspicious, after all the manufacturer wants to sell batteries and once sold the sooner it breaks the better.

So personally I test them myself and with lead acid for example I look for the point where much gassing of the cell occurs and set a voltage limit a bit down from that.
I think thats what aguru is trying to tell you. The other indicator is heat produced in the cell and if it heats up you are overcharging it.

So with LiPo's my approach is put any voltage in even say 6 Volts but limit the current at the start then turn it all off and check the voltage in the cell and if 4.2 V or above then stop there . If it heats up you are damaging it and from experience the cell will grow in width so stop immediately if you see any swelling.

Then use the cell and see if it holds up . If it does not then find out why and write about what you find . Then we all learn

 

[trennonix]

tytower, I will not do any tests on my LiPos because they are near and dear to my heart
I only have 2 batteries and I use them to fly my heli; I don't want to have to go buy new ones...
However, after using them, they tend to get really hot (can't hold them for more than 20 sec) and they swell.
They get back to their normal size after about 10 min.
I assumed that a small amount of swelling was normal because it happened on both my batteries, plus the batteries I use have a higher discharge rate then needed.
Is that normal?
You got me worried when you said: ANY swelling
Here's a picture