# Voltage sources in parallel?

I've looked at other questions "Voltage sources in parallel", but don't answer my specific problem.

Imagine the following circuit:

simulate this circuit – Schematic created using CircuitLab

I have a solar powered charge controller that, ideally, can deliver 30A at 14.0V, and a battery that, ideally, can deliver unlimited current at 12.8V.

Which laws can tell me something about how this circuit will behave?

For example, if the load is a 50W LED, it would need 14V at 3.57A. Would all the current and voltage come from the CC, because it has a higher voltage?

When the total current would be higher than 30A, I suspect the CC would fail to sustain the voltage.

• Btw, I was wondering if the CC could be considered a current source, but the CC is programmed to deliver a specific voltage, and will then provide as much current as it can. (AFAIK) Jan 3, 2017 at 21:00

## 3 Answers

"Would all the current and voltage come from the CC, because it has a higher voltage?"

Exactly.

The higher voltage from the CC biases the battery's diode so it will not conduct.

(Also, the diodes will reduce the voltage by about .7 volts.)

• Ah, of course! Now it's obvious :) Jan 3, 2017 at 22:34
• And those diodes will get pretty hot! Jan 4, 2017 at 0:18

The voltages of the solar cell will not stay at exactly 14V, but will dip down in voltage as you increase the current drawn. This is because it can deliver only so much power. In this case about 420 watts at best. Same with the battery, the voltage will fluctuate.

If your cc cannot deliver the 50 watts for the led, the voltage of the cc will start to dip until it reaches the battery voltage in which case the led will begin to draw more current from the battery. Lets say its a bad day and the solar panel is delivering only 40 watts. The battery then produces the other 10 watts. So that's 3.125 amps from the cc and 0.8 amps from the battery.

The actual behaviour of this circuit will heavily depend on the power supply's voltage in function of current being drawn. And indeed in the case of a solar panel on the amount of light it is receiving. It gets more complicated though if you consider a real circuit!

A "smart" switch mode converter that lets the solar panel operate at its optimal point and outputs a stable voltage would be an advantage for this type of application and would give a pretty reliable circuit that behaves a lot like the ideal circuit above until there is insufficient power from the solar panel to provide the current the load requires. At that point the controller would typically shut off until it's able to operate within the desired voltage range again. Though some might start dropping the output voltage as well, really depends on the controller topology and safety features built in. At that point the battery would take over.

If you actually intend to build this look into a solar panel controller with a built in power management functionality that allows you to switch to a battery.