In need of some Lithium-poly charger Circuit Design Hints.

[Zanderist]

I've been looking all over the internet(Google) for some idea on making a lithium battery charger, or the basic principle behind it without the need of complex micro controllers.

What I've been finding is completed designs but not really any in-depth theory or formulas.

What I'm basically looking for is how to make a constant 4.2 volts and 420mA lithium charger circuit. I eventually want to be able to vary the current further down the road. I also would like to have something that would indicate when the battery is charging and when it has hit full charge.

What my limitations are that I only have LM317 regulators and no low ohm resistors. I also don't have a cross reference to some of the European transistors used in some designs.

I do also have tons of 2N3904 and 2N3906 transistors.

One other problem I have is I want to give is some kind of automation with a window comparator but I'm not too sure on how to sense a charge of battery without causing a current drop.

I haven't ventured that far out from mutlisim (i.e real breadboard prototypes ) for fear of damaging the limited parts I have.

 

[Audioguru]

Go to www.batteryuniversity.com to learn about battery charging and battery care.
It is simple to make a Lithium battery charger from an IC that is designed to do it.

To make your own charger circuit you can use an LM317 adjustable voltage regulator to make 4.20V and to regulate the maximum current. Use an opamp to sense when the charging current drops to a low amount then it can turn off the charging circuit.

 

[Zanderist]

Alright will do.

Its the Op Amp part I'm a little stuck on because as I've seen in Google searches it involves a sense resistor which I'm afraid will effect the constant voltage and current parameters I'm trying to work to.

 

[Audioguru]

Its the Op Amp part I'm a little stuck on because as I've seen in Google searches it involves a sense resistor which I'm afraid will effect the constant voltage and current parameters I'm trying to work to.

An opamp has a voltage gain of about 200,000 times. If the charging current drops to 3% of 430mA then it is 13mA. A sense resistor value of 1 ohm develops a voltage of 0.013V across it then it can drive the output of an opamp to saturation that will turn off the charging. The battery will not notice the 0.013V tiny voltage lost.

 

[iimagine]

Take a look at this: http://circuit-diagram.hqew.net/Auto$2dOff-12V-NiCd-Battery-Charger_4894.html

 

[Audioguru]

Take a look at this: .....

DO NOT use this 12V Ni-Cad battery charger to charge a 3.7V Lithium battery cell.
A Ni-Cad battery is fully charged when its voltage rises and this circuit detects it.
But a Lithium battery is fully charged when its charging current drops low that this circuit does not detect.

 

[iimagine]

Well, that was just to prove how much I know about batteries :/

Did some reading at this site: https://readious.blogspot.com/2012/12/battery-guide_9173.html
Its much more complicated then I had thought

 

[Zanderist]

I've been doing some searching for creating a lithium battery model in mutlisim ( so far with no luck)

But I did find this interesting Google Book,


https://www.techgig.com/topics/NI M...Batteries-Science-and-Technology/v4F5NrFgaTUC

- - - Updated - - -

I've been doing some searching for creating a lithium battery model in mutlisim ( so far with no luck)

But I did find this interesting Google Book,


http://www.techgig.com/topics/NI%20Multisim/books_search/NI%20Multisim/Lithium-Batteries-Science-and-Technology/v4F5NrFgaTUC

 

[Audioguru]

Multisim does not seem to know anything about modern electronics.

 

[Zanderist]

Okay so I've come up with a simple circuit and I'm pretending the battery is a lithium Ion battery.

The 4.2 source is supposed to be a LM317T and the 3.6 battery is the Li-Ion.



So what happens is that the voltage reference is set approximately 99% of full battery charge. When it reaches it it cuts out the power source and only will turn back on when it drops down past Vref.

 

[Audioguru]

The Lithium battery IS NOT fully charged when its voltage reaches 4.2V. It is only about 70% fully charged. The charger must monitor the charging current and turn off only when the charging current drops to about 3% of the battery mAh rating.

You have an NPN transistor as an emitter-follower. So when its collector is +4.2V then its emitter comes close to +4.2V only when its base is 0.7V HIGHER than its collector and emitter.

The output of the opamp will go only as high as about +3V and the LED will reduce it to about +1V to the base of the transistor.

Most opamps DO NOT WORK when an input voltage is close to the positive supply voltage anyway.

 

[Zanderist]

Okay I've added a little more in. This time I have an extra fuse blown indicator, and now the Op Amp has more power from the FWB.

 

[Audioguru]

Please get rid of the NPN emitter-follower. An emitter-follower makes a lousy switch. Replace it with a PNP transistor used as a high side switch. Both of the transistors need a series base resistor.

Your opamp compares the battery voltage to a 4.2V reference which is useless. Instead, set the trimpot on the voltage regulator for an output of 4.20V and use the opamp to detect when the charging current drops low then it turns off the transistor.

What does R7 do??

 

[Zanderist]

R7 is just to discharge the battery it no real application in the final design.

I'm still presently working on the design.

 

[Zanderist]

Here's an update.

I've ditched the transistor switch all together in favor of a power mosfet.

The automated part of it has changed as well, it is not a difference amplifier checking the charge across the cell. The problem with feeding it directly into the comparator was that comparator was not acting in a 'either-or' state. Instead as it approached the threshold it started to go half off. In order to correct this I put the output of the difference amp to another opamp set up as a schmitt trigger.

Now in the simulator, it works great. However in the real world bread board build I have...it doesn't. In fact it does nothing.

Another thing I need to point out that the resistor value is incorrect, after doing a KVL-loop calculation from the LM317t shows in fact to achieve .42mA I actually need a resistor of 1.5 approximately

-4.2+(.42*R)+3.6=0

Here's some simulator pictures of what I got and for anyone else that might be able to take this further.

 

[Audioguru]

You have nothing limiting the charging current. I see a switch but nothing operates it to detect the battery voltage and current.
The diode does not prevent a fully charged battery from feeding the output of the turned off voltage regulator.

I do not know why your circuit has so many parts when simple Li-Po battery charger ICs are available.

 

[Zanderist]

I do not know why your circuit has so many parts when simple Li-Po battery charger ICs are available.

The circuit is this way in order to gain an understanding as to what goes into the making of the chip. In short reverse engineering the ic and putting it on display in an exploded display.

 

[Audioguru]

A charger IC is much smarter than your complicated circuit. It first measures the battery cell voltage and if it is too low then it either refuses to set it on fire or it begins charging it with a very low current to avoid a fire. It also has a temperature sensor input to avoid a fire. Its temperature sensor input can be shared with a pressure sensor (swelling detector) input to avoid an explosion.

My Li-Po chargers have smart charger ICs but they all have a label, "do not charge unattended" so I guess they want me to put out the fire when the charger IC fails.

 

[Zanderist]

Are these sensors within the battery or off to the side of the battery?

 

[Audioguru]

Some Lithium rechargeable batteries (in cell phones, in laptop computers and in some toys) have a protection circuit inside. It prevents over-charging and over-discharging. The 3rd terminal on the single-cell battery is the temperature sensor.