LiFePO4 Battery Charging

[Overclocked]

Let me state the obvious here, LiFe's are different than Li-ion's and LiPoly batteries but are charged in the same manner (CC then CV), just the voltage levels are different. There doesn't seem to be a Off the shelf IC that can charge these batteries, as they require a final voltage of 3.6 to 3.8v. The ones I just bought need a final voltage of 3.6v to finish charging.

For a while, Ive successfully used a Standard Li IC to charge smaller capacity LiFe batteries that needed 3.8V, I just used a Schottky diode to drop the extra voltage (and ofc checked the voltage manually each hr). But since a regular Li battery stops charging at 4.2v, These new Batteries need 3.6v EOC. Can I still use the Same Diode Drop concept to successfully charge the Batteries? Are there any downsides to doing this? Finally, Is there a Easier way to do this since the charger I am using wasnt specially made for LiFe batteries?

Ive seen the popular LM317 Circuit, and LiFe batteries are more rugged than Li-Ion and have thought of using that, the only downside is that it doesnt shut off when the batteries reach their full charge. Ive thought about using a Current sense amp to do this, feed it through a comparator and then a FET to shut the input off once it reaches final charge, but Im unsure if this method will work

(side note: There Is one Charger IC available but its in a QFN or smaller package-impossible for me to solder)

 

[bountyhunter]

The LI-Fe batteries are currently being mass produced and used in motorcycle starter batteries. They have a lot of chargers and charging info avaialble. The other technology using LiFe is for electric cars.

https://www.shoraipower.com/

 

[bountyhunter]

it doesnt shut off when the batteries reach their full charge. Ive thought about using a Current sense amp to do this, feed it through a comparator and then a FET to shut the input off once it reaches final charge, but Im unsure if this method will work

I think it would if you did it right. Sense the charging current drop when the voltage is at the final set value, when it get's below a threshold the battery is charged.

I have used the same technique in lead-acid chargers I have designed. The attached schematic shows an example.

 

[Overclocked]

Hmm I had forgotten about the voltage end of the equation. I had thought about that on the way to work, if I had it shut off at say 10 ma, the charger wouldnt know if its starting up or shutting down! So if I sense when the current drops *and* the voltage is at the required level then it would work. Sounds like I would need a and gate.

One of the great things about LiFe batteries is their temp range and their abuse tolerance. They seem like the best solution for hobbyists use.

 

[Overclocked]

Hmm still, Is there any disadvantages to using a Diode to drop the extra voltage? It seems to simple that something as simple as diode can make it work though. I had another thought though, I know how a SMPS can be used for a constant current load (ie Like a LED) but can the LED be replaced with the battery so the battery will be charged with a constant current? The Only problem is voltage. Is it the charger that varies the voltage, or is it the chemistry that handles that? What Im saying is does the Voltage get closer to its "required value" by it self as the cell gets closer to being full? Ive always been confused by that.

 

[bountyhunter]

Hmm still, Is there any disadvantages to using a Diode to drop the extra voltage? It seems to simple that something as simple as diode can make it work though. I had another thought though, I know how a SMPS can be used for a constant current load (ie Like a LED) but can the LED be replaced with the battery so the battery will be charged with a constant current? The Only problem is voltage. Is it the charger that varies the voltage, or is it the chemistry that handles that? What Im saying is does the Voltage get closer to its "required value" by it self as the cell gets closer to being full? Ive always been confused by that.

You need a charger specifically designed to do what is called "CC-CV" operation. It means it acts like a current source (Constant Current) when the terminal voltage is below the set point. Once the battery reaches the set point, it switches to CV (constant voltage) and tapers off the current as required to hold the voltage there.