Hello, everyone

So I have been trying to figure out a circuit to charge LiFePO4 cells and prevent them from overcharging.
While also maintaining a steady current.
I used a simple LM317 to obtain a steady current of 500mA, and ran that current through 4 cells in series where each one is preceded by a switch.... (this switch will be controlled by an arduino or perhaps with a relay) which either directs the current through a cell to charge it, or through a resistor to keep the current going through the rest of the cells. I used a small resistance of 5 ohms as to not impede the current further and to keep it steady.

This is my circuit and simulation... What do you think? Will it charge?
I'm learning so please show me all my mistakes
Thank you

 

Assuming this works, your charging strategy is 500mA or nothing. I'm not sure that's a good strategy for this battery chemistry. Take a look at battery university.

How will you decide when to throw each switch? What is the actual switch you have in mind?

 

Assuming this works, your charging strategy is 500mA or nothing. I'm not sure that's a good strategy for this battery chemistry. Take a look at battery university.

How will you decide when to throw each switch? What is the actual switch you have in mind?

I'm thinking of using a relay controlled by an arduino
The arduino will take readings of the voltage of each individual cell and will decide whether to switch the relay or keep it as it is

 

I'm thinking of using a relay controlled by an arduino
The arduino will take readings of the voltage of each individual cell and will decide whether to switch the relay or keep it as it is

So the strategy is to charge to a voltage and stop. Again, I'd check at BU if that strategy is appropriate. One thing to consider: When the 500mA charge current is removed, the battery's apparent voltage will drop. You'll need a control algorithm that accommodates this.

 

Yeah, that's basically the strategy.... and thank you I'll check it....
Would you further elaborate though on what an apparent voltage is? Or do you think it's too big a topic and I should do some research

 

Yeah, that's basically the strategy.... and thank you I'll check it....
Would you further elaborate though on what an apparent voltage is? Or do you think it's too big a topic and I should do some research

I was referring to the voltage you measure across the poles of the battery. Each cell has internal resistance and will show a voltage under charge that is higher than the resting, no-charging voltage. The same thing happens during discharge - the cell will read a little lower under load than it will when the load is removed. So it's a common problem with DIY battery chargers that rely on voltage to decide switch positions.

 

Experimenting is good, and I'm not trying to dump all over your ideas, but ...

The charger design you are considering is inappropriate for *any* rechargeable battery chemistry. And of all of the various rechargeable battery chemistries, lithium batteries are the most difficult and most dangerous. Almost a dozen semiconductor manufacturers make hundreds of competing lithium charger controller ICs because there is that kind of demand in the marketplace. For a description of how to charge a lithium battery, see any of the Linear Technology data sheets and app notes. For the consequences of mishandling lithium batteries, see hoverboards, and Sony, and Samsung.

ak

 

For the consequences of mishandling lithium batteries, see hoverboards, and Sony, and Samsung.
ak

... and Boeing Aircraft.

 

Hello, everyone

So I have been trying to figure out a circuit to charge LiFePO4 cells and prevent them from overcharging.
While also maintaining a steady current.
I used a simple LM317 to obtain a steady current of 500mA, and ran that current through 4 cells in series where each one is preceded by a switch.... (this switch will be controlled by an arduino or perhaps with a relay) which either directs the current through a cell to charge it, or through a resistor to keep the current going through the rest of the cells. I used a small resistance of 5 ohms as to not impede the current further and to keep it steady.

This is my circuit and simulation... What do you think? Will it charge?
I'm learning so please show me all my mistakes
Thank you

Lithium full charge terminal voltage is critical - like +/- 0.05V tolerance. They can vent with flaming gas, and the steel jacket type can build up a head of steam that's pretty destructive when it lets go.

If you're charging a single cell, a TL431 based shunt regulator works (if you know what you're doing) - its what I've been using for my E-cig for a few years.

Some lessons are better not learned, so I'd advise at least an off the shelf Li charging chip - or a ready made module, and even more especially if you're charging multiple series cells.

 

Ok, got it
Thank you all