Does power supply output current drop as LiFePo4 cell becomes more charged?

[forscience]

I'm charging up a 3.2V LiFePo4 cell with an adjustable converter. I have the voltage set to 3.65V and when I initially started charging, the output current was at 11Wh, but as it charges over time, the power output of the converter steadily becomes lower. Right now it's charging the cell at 5.5Wh. Is this normal?

 

[meetyg]

Yup, that's what a proper CC/CV charger should do.

During the Constant Current phase, the current stays the same, and the voltage naturally rises as the SOC of the battery rises.
Once the voltage of the battery is equal to the PSU voltage you set, the PSU should go into Constant Voltage mode, which will hold the voltage fixed, and current will drop slowly as the battery SOC continues to rise.

With Lifepo4 most of the charging (95% if I recall correctly) is done during the CC phase.

 

[forscience]

Yup, that's what a proper CC/CV charger should do.

During the Constant Current phase, the current stays the same, and the voltage naturally rises as the SOC of the battery rises.
Once the voltage of the battery is equal to the PSU voltage you set, the PSU should go into Constant Voltage mode, which will hold the voltage fixed, and current will drop slowly as the battery SOC continues to rise.

With Lifepo4 most of the charging (95% if I recall correctly) is done during the CC phase.

So when the battery cell is at full capacity, does the output current reading from the converter drop to 0A? Also if I leave it connected to the battery cell after reaching full capacity, is there any danger of over charging?

 

[John Frum]

@forscience https://diysolarforum.com/resources/how-charging-works-in-the-context-of-lfp-batteries.233/

I will be glad to answer any follow up questions.

 

[meetyg]

So when the battery cell is at full capacity, does the output current reading from the converter drop to 0A? Also if I leave it connected to the battery cell after reaching full capacity, is there any danger of over charging?

Yes, it will reach 0A. But you shouldn't go that low... Depending on the size/capacity of the battery, it's enough to charge up untill the charger really reaches a few hundred miliamps. For a one time capacity test or top-balance you could go down to 0A charge, but don't do it regularly.

You should also not leave any Lithium type cells at 100% or connected to a charger for prolonged time. With Lifepo4 its not as dangerous as with Li-Ion, but it will degrade the battery.

 

[Bvillebob]

One great thing about the lithium cells is that they charge at full speed right up to the point that they're full. Lead / acid hits a point where putting additional power into them gets more and more difficult, the last 5-10 per cent of charge seems to take forever, that's the abosrption phase, while the lithium just takes all you throw at it. I have one system with lead/acid and one with lithium, I often see the lithium charging at twice the rate of the lead acid when they're getting full.

In my system, the lithium takes the full 92 amps from my controller until it hits the cutoff voltage, then the amps drop to zero, then after a minute it ramps back up to "float" meaning that they controller is supplying enough amps to cover the demand being taken by the inverter. The float voltage is set low enough that the cells don't get above 3.4 volts.