Chinese (rubbish?) Li Ion Battery Charger Schematic and Advice Please

[Jules_Theone]

I've recently bought a two cell 600mA battery charger from China for £3.49 to charge some cells I've got from dismantling laptop batteries. I've removed the protection circuits from the packs because I want different voltage / currents. Anyway, I've figured out the schematic for it to understand it more - can anyone see any obvious errors with it or suggest the value for two resistors with "?" marks next to them or the zener diode ZD1 voltage as it doesn't say explicitly what they are.

Also, it doesn't seem complicated enough. I was under the impression it needed three different stages for charging - constant charge current until the cell reaches 4.2V then constant voltage until the current drops to 10% of the capacity, then boost when the voltage drops to 4.0V. I understand this is cheap charger and probably only performs the first stage, am I correct to assume that?

It came with two AA size cells (900mA each) which it appears to charge in parallel, but I plan to charge 2200mA cells. Is there any problems with charging larger cells and is there a limit to how many i can charge at once? Is it a bad idea to charge two cells which are at different voltage levels with this charge - could the higher charged cell discharge into the less charged one at high amps (blowing internal fuse(s)?)

Thanks,

Jules

 

[Diver300]

You shouldn't charge Li Ion batteries beyond 0.1 C if the voltage is very low.

You are right that the circuit doesn't do all that it should, but it's probably not too bad. The voltage regulation will be as good as you could expect with as it is set to 4.2 V with the TL431, the 6.8 k and the 10 k resistors. The current will be limited by the transformer circuit. I can't quite work out how, but I am sure you won't get a big current into a short circuit.

The LED2 will go out when the current is very small, but it's not possible to say exactly how small the current is.

So the charger will limit the current by its design and transformer size, and it has as good voltage limit. It turns off the LED but doesn't actually shut off so it holds the voltage at 4.2 V until disconnected, and it could charge a very low battery too fast. So it's not too bad for normal use, but might damage batteries if they are very flat or if you left it a very long time.

There is no limit to how many batteries you can charge at once, but the current limit on the charger will mean that the more batteries, the slower they will charge. It is a very bad idea to parallel cells that are not at the same level of charge.

(You have the TL431 drawn wrong)

 

[audioguru]

They don't make Li-Ion battery cells in an AA case because idiots will replace alkaline cells (1.5V) with them (3.7V) and blow up their neighbourhood.

Lithium is dangerous because if it is charged wrong then it catches on fire. An extremely hot fire. You don't want that. The cheap Chinese charger keeps overcharging the lithium cell which every battery manufacturer says is begging for a fire.
Then you don't want a cheap Chinese charger.

 

[Jules_Theone]

I was planning to put a voltmeter and ammeter in the circuit to check everything is working correctly. I measured one of the cells which came with the unit and it only read 1.7V, is that too low to safely charge now and if it can only charged at less than 0.1C is it simply a case of putting a limiting resistor in series and charge it for a longer to recover it or is it not worth the risk?

I couldn't find the TL431 diagram in Altium so I used a thyristor instead but it's pretty close or do you mean that it's connected up wrong?

The cells that came with the charger are exactly the same size as AA's -Li-ion 14500 Cylindrical Rechargeable Cell: 3.7V 750mAh (AA size, 2.77Wh)- UL/UN approved

 

[Diver300]

If you put a 50 - 100 ohm resistor in series, that will limit the current and I expect the voltage will build up to 3.4 V or so quite quickly. There is always a risk that you could have damaged the battery, but it depends what it is used for, and how much damage a battery fire would cause, and how expensive a replacement would be.

I doubt that the TL431 is connected wrong. I would have connected the resistors to the side connection and the LED to the cathode, but it's not important unless you are going to build a copy. I've just noticed that the LED in the opto isolator is the wrong way round as well.

 

[Jules_Theone]

Thanks Diver300, I will try that when I get home tonight. I got this charger and batteries last week, checked the voltage - both around 4.1V and connected them up to power a 7.2V RC car. They lasted about 30 seconds and then the car just became slow and I gave up, I haven't charged them yet. I guess these cells wouldn't have any internal protection circuitry, so is there a simple circuit to stop them being discharged too much next time? I was looking at MCP73843
https://ww1.microchip.com/downloads/en/DeviceDoc/21823c.pdf or that kind of thing but they only come in surface mount devices and might be overkill for what I need.

 

[MrAl]

Hi,

Do you have a 9vdc wall wart laying around? You can build your own using an LM317 and a few other small parts.

 

[Jules_Theone]

I have 23 years worth of junk laying around, I was thinking of building a charger using a PIC based on this Chinese power supply because I'm not certain how to regulate the voltage accurately enough, but i suppose a simple linear regulator will do the job anyhow.

 

[MrAl]

Hi again,


Where did you plan on getting that transformer from?

 

[Jules_Theone]

Initially use the Chinese power supply but use a PIC to control it better but if I need to charge larger cells use a LF transformer and a variable linear regulator set to 4.2V then have the PIC switch that voltage on or off based on voltages / currents in the charge circuit. Unless there is a better way of doing it? I just wanted to see how a "proper" li ion charger does it.

 

[MrAl]

Hello again,


Well there is a basic requirement for charging an Li-ion cell, and that is to limit the current and regulate the voltage. The current is limited based on the size of the cell and the voltage is regulated to under 4.200v. The voltage regulation must be very carefully controlled, while the current limit is a bit more flexible as long as you dont go over the rating for the cell.
As an example, those 18650 cells usually charge at 1 amp with a set voltage limit of 4.15v or so. What this means is that when you first connect the cell it draws 1 amp but its voltage cant get up to 4.15v yet so it ends up being lower like 3.5v or something like that. As time goes on and the cell takes more and more charge, the voltage rises up to 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, and then the voltage regulator starts to take over and limit the voltage so the current starts to drop, 0.9 amp, 0.8, 0.7, 0.6, ..., 0.3, 0.2, and then finally the current drops to 0.1 amps and that would be considered fully charged for a 2 Ahr cell (C/20).
So the basic idea is to limit the current until the voltage gets up near 4.2v and then regulate the voltage. Because of the way voltage regulators work anyway, the voltage regulator will not kick in until the voltage gets close to the set point anyway, so it's basically a current limted voltage regulator circuit. These circuits are quite simple to build and so maybe you are looking at too complex of a circuit to begin with anyway. If you have a wall wart that can put out say 500ma you can build your own much simpler and probably more reliable too.

A little note about safety...
Most problems with Li-ion cells occur during charge, so a cell should never be left alone while charging. These things can actually catch fire so they can cause a lot of problems if something goes wrong with the charging process.

 

[Jules_Theone]

Is this too simple for the charger, I know there isn't any cutoff but I can add that with a PIC later.

 

[MrAl]

Hi again,


Yes, that's the basic idea, except you dont use a diode on the output because that causes an uncontrollable voltage drop.
You also may have to adjust the 560 ohm resistor for a more exact output voltage setting.

Although i have not tried this yet, it may be possible to go higher on the 240 ohm resistor and thus also the 560 to draw less current if the power goes out. This could very well be possible because the cell to be charged will always draw more than 6ma. Something to think about, although the 240 and 560 doesnt draw THAT much current anyway.

 

[audioguru]

You can see exactly how a lithium battery can be charged at www.batteryuniversity.com .
The voltage must be higher than 3.0V or the charger must reduce the charging current until the voltage reaches 3.0V then the current must be limited to what the battery manufacturer recommends. When the voltage reaches 4.20V the voltage must be limited and the current is monitored until it reduces to 1/40th its rated current when the charging must be turned off (trickle charging is not allowed).
A battery charger IC does all this stuff.