DC to DC Charger with MPPT controller + lithium iron phosphate batteries

[parmm]

Originally Posted by jwcolby123

I wonder how a constant current constant voltage power supply works. A load is a resistance, which in the case of a camper can change. Think turning on a pump or a light or a microwave. If I want the current to remain constant and the resistance goes u I have to drop the voltage and v.v.

I know that a LiFePO4 battery internal resistance varies as the charge state varies, as it gets closer to full the internal resistance goes up. If the resistance goes up and my voltage remains constant, the current will drop. If the resistance goes up and I want the current to remain constant, my voltage must go up.

Something has to give...

"It all depends on what the resistance is doing in the circuit. If it used for a bias, the output of semiconductor device will get affected. If it is in a timing circui, the discharge time and the time setting will change.
If voltage is kept constant across the resistor itself, its current will reduce. If the resistance is part of oscillator circuit, frequency response will change. If it is in series with capacitor or inductor, it will change the damping effect.
So same resistor in different situations will behave differently, and current will get adjusted accordingly."
from RP Deshpande, B.Tech(Hon), Elec., F.I.E Institution of Engineers India, I IT BOMBAY

[jwcolby123]

Originally Posted by gtinseth

I assume it works like my bench power supply. While the battery voltage is below your selected voltage, give max current. When the voltage approaches the selected voltage, decrease current. Stop at some predetermined current at the selected voltage (small, but related to the AH capacity of your battery - I have to do this myself, but it could easily be automated). I am not sure why you want the current to be at a preset when you have ended up in constant voltage mode where it is normal for current to decrease.

But that defines a constant voltage, not constant both.

[gtinseth]

Originally Posted by jwcolby123

But that defines a constant voltage, not constant both.

I think that is the misunderstanding. My bench power supply does both CV and CC, but not at the same time. None of my battery chargers do both at the same time. It is either one, or the other, depending on the battery SOC. You are right that both at the same time doesn't make sense.

[CarlD]

Here is another way to look at. Start with the power supply set to some voltage. Load the power supply with a variable resistor. Set the resistor to a very large value. The voltage will be at the setpoint and the current will be 0. Start to decrease the resistance value. The voltage will remain constant but the current will begin to increase. When the resistance value gets small enough the current will reach the maximum value. Further decreasing the resistance will cause the power supply to reduce its output voltage but the current will remain constant at the maximum value. When the resistance goes to 0 ohms the voltage will go to zero and the current will remain at the maximum value.

[jwcolby123]

Originally Posted by CarlD

Here is another way to look at. Start with the power supply set to some voltage. Load the power supply with a variable resistor. Set the resistor to a very large value. The voltage will be at the setpoint and the current will be 0. Start to decrease the resistance value. The voltage will remain constant but the current will begin to increase. When the resistance value gets small enough the current will reach the maximum value. Further decreasing the resistance will cause the power supply to reduce its output voltage but the current will remain constant at the maximum value. When the resistance goes to 0 ohms the voltage will go to zero and the current will remain at the maximum value.

Lol, this all started with a power supply described as "constant voltage constant current". I have seen folks describe power supplies this way many times. Obviously not. It may be one or the other at any given time. Never both at the same time... unless the load never changes then ... Duh as my daughter likes to say.

I don't claim to be an engineer... though I did play one on tv.

in 1969, when I was 13 years old, I learned electronics by reading a 1955 ARRL handbook three times cover to cover. At 17 the USN sent me to school to fix their computers. I remained in the fix-it end until I switched to programming in the mid 80s. To this day I build my own computers from parts off of NewEgg. So I have a tad bit of electronics training. At least enough to use E = I*R.

[CarlD]

Originally Posted by jwcolby123

Lol, this all started with a power supply described as "constant voltage constant current". I have seen folks describe power supplies this way many times. Obviously not. It may be one or the other at any given time. Never both at the same time... unless the load never changes then ... Duh as my daughter likes to say.

I don't claim to be an engineer... though I did play one on tv.

in 1969, when I was 13 years old, I learned electronics by reading a 1955 ARRL handbook three times cover to cover. At 17 the USN sent me to school to fix their computers. I remained in the fix-it end until I switched to programming in the mid 80s. To this day I build my own computers from parts off of NewEgg. So I have a tad bit of electronics training. At least enough to use E = I*R.

So I am glad you understand and agree with my description of operation. Obviously you don't have constant voltage regulation and constant current regulation at the same time. Any person educated in electronics knows that. It's just a name for this type of power supply. You could also describe it as a current source with a specified compliance voltage. And E=IR for sure, I watched a professor derive it from Maxwell's Equations. It was most interesting.