3s lipo charger help

[ejector]

hello All,
last night i just blow up my lipo charger. so now i don't want to buy new because they are very expensive here in Pakistan. please check the attachment what i thought to do. my drawing is not very Good. and i forgot to put ground in it. it will have a ground point. please check it that is it correct? and i'll highly appropriate, if someone tells me how to make is balance charger. i want that an LED be there and when the charger starts the LED should glow the LED should me off when battery is full.


Thanks

 

[alec_t]

I'm afraid that circuit is quite unsuitable. Lithium batteries have very specific charging requirements (which is why the chargers can be expensive). If you don't charge the cells properly there is a risk of them exploding or catching fire, or at least being damaged in some way. Do some Googling and check how the cells must be charged.

 

[MrAl]

hello All,
last night i just blow up my lipo charger. so now i don't want to buy new because they are very expensive here in Pakistan. please check the attachment what i thought to do. my drawing is not very Good. and i forgot to put ground in it. it will have a ground point. please check it that is it correct? and i'll highly appropriate, if someone tells me how to make is balance charger. i want that an LED be there and when the charger starts the LED should glow the LED should me off when battery is full.


Thanks

Hi there,

It's not the drawing that is so bad, it's the "charger" design that is so bad. I mean not *so* bad, but really really really bad.
No actually i mean really really really really bad. Well ok, really^50 bad.

Why? Because there is absolutely NO regulation at all of any kind in that circuit, and these kinds of batteries require regulation of the current and voltage in a very carefully planned way. So in short, Li batteries require a much more sophisticated kind of charger.

However, you can build one using an ic chip part LM317 that is not too difficult to build. It requires an LM317 chip and some resistors and a couple capacitors. These chips are fairly cheap. You do have to be careful anyway however in that you should measure the voltage of the cell before you start to charge it to make sure it is above 2.5 volts.

Before you even start you should read up on the dangers of charging Li cells so you know what to watch out for. After that, you could do a search for the LM317 charger right here on this forum. It's the cheapest charger you'll ever find that is safe enough to use as long as you measure your cell voltage first.

 

[ejector]

Thanks a lot sir
can you help me find the charger with this part lm317. i just want to build one. Thanks

 

[MrAl]

https://www.electro-tech-online.com/attachments/li-ioncharger-lm3-gif.40123/

You dont need Q1, D1, etc., short out R3, if you want to build the simplest version.
You can change R1 to a lower value for a higher charge current.

 

[ejector]

Hello Sir MrAl,
Thanks for your kind help. check out the link i liked and what i wanted.
https://www.rcgroups.com/forums/showthread.php?t=215550

please check post#16 by Mr.kodel
i am planing to build this circuit. and praying that i find all the parts here in my city. please check it. will it be fine?

Thanks a lot

 

[MrAl]

Hello again,

I dont believe you need that complexity unless you really have to charge the cells while they are in series. If you do then you might consider that circuit, but if not, then you can use three much simpler circuits.

I'd also take a better look at the 'safety' feature of that three cell charger. It looks like it is sensing the voltage of all three cells in series rather than for each cell individually. If going through all that complexity anyway, why not check each cell.

 

[ejector]

Hello Sir,
It'd be pretty fine, if the circuits become able to check each individual cell.

 

[MrAl]

Hi,

You know what doesnt look right about this circuit is the resistors that go from the output of the op amps to the non inverting inputs. That resistor should go from the non inverting input to the emitter of the respective transistor for that cell.

 

[MrAl]

Here is a drawing...

 

[ejector]

Hello Sir,
thanks a lot now the circuit look pretty high intelligent. what about the component values? and will it be fine if i build this circuit. is it tested?

 

[MrAl]

Hi,

The circuit is not tested yet but it looks like the original circuit was not tested yet either.
The component values have not been selected yet as i assumed you would do that because the original diagram did not have them selected yet either.
The resistors on the op amps could be 10k each. The LED resistors maybe 4.7k each. I'll take another look today and come up with some other values too if you want to build this circuit. You do want to build this circuit then?

 

[ejector]

Yes I definetely want to build a charger for my lipo. And thank God you are helping me. Thanks a lot sir

 

[ejector]

And that circuit I told you was built by its designer. He made the circuit and showed its picture in his post on first pagel

 

[MrAl]

Hi again,

I hope you have at least some experience with building these kinds of circuits as sometimes there are some little details that are left out of the schematic that the author assumes the end user has some idea about already. For example the heatsink(s). Many of the transistors will have to dissipate some power so will need a heatsink, and if the collectors are not common then each transistor needs it's own heatsink.
The power transistors will need heatsinks.

You also need to specify how much current you want to charge the cells with. What capacity are the cells you are going to use with this?

Here's an updated drawing which has been tested in a simulator but not on a breadboard.

 

[ejector]

Hello MrAl,
Yes, i have a bit of experience of building circuits. not much but i have built circuits in number of tenth.
at the moment i'm having 3s 1000mha 25c lipo battery.
and what about op-amps? are the op-amps the same as prescribed in post i told you?

And still there are many component that are missing with component values. like resistor and capacitor values on lm317? and all three potentiometers? or these resistors have to be set so the voltage after them should as written there? and the total power supplied to the circuit is 16v. am i right?

 

[MrAl]

Hi,

What charge current does the manufacturer of your cells recommend, is it 300ma by chance?

What op amps did the original post use?

The components without values are the same as in the other stages, but if you cant see what they are i'll update the schematic again.

 

[ejector]

Hi dear,
the op-amp used in the post i told you was LM2903. it is dual op-amp Ic. and about the charger, i bought charger separately. and i heard that a battery should be charged with 10 times less than its capacity. so i think 100ma or 200 would be fine. this is what i think.
and yes, i can't see the values for resistor and capacitor value with lm317 regulator. and values of 3 potentiometers.

Thanks a lot

 

[MrAl]

Hi again,


The value for the right hand side LM317 resistor would be sized according to the charge current:
R=1.24/I

so for 100ma we would use 12.4 or just 12 ohms, and for 200ma half that value.

For the left hand side LM317, that resistor has to be selected based on the bias current for the pots. If the pots are 1k each then for example with the lower pot adjusted to 500 ohms (center travel) that would mean we need 5.4ma out of the LM317. This is a little too close to the LM317 requirement though, so maybe make the pots 500 ohms each and that would mean around 11ma out of that LM317. For 11ma that would mean the resistor would be:
R=1.24/0.011=113 ohms or close to that, maybe 110 ohms would be good. You should verify that it works this low however.

You cant use LM2903 for the 'op amps'. That's because that is not an op amp it is a comparator with an open collector output.
You could use this for some of the op amps but not for the two that are driving the NPN bases. That means the ones connected to the base via resistor and LED and also the ones connected to the four 10k resistor have to be true op amps. A true op amp is like the LM358.

If you run at lower current like 100ma or 200ma you may not need a heatsink on the transistor if they are the power type and have free air flow across them. They might get pretty hot at 200ma though with out at least a small heatsink.

There are a couple tests you should run before using this too.

 

[ejector]

Hi Dear,
Thanks for you concern Dear, the two things are remaining are
1- the capacitor a take for lm317 is always around 1uf to 10uf. so i'm gonna take 1uf this time. and is there any ic that has as many op-amps as there are in our schematic? or i will have to use 3 or 4 ICs of quard op-amp Ic?
Thanks