How does current limiting on a solar panel charger work?

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deonb
 
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How does current limiting on a solar panel charger work?

Post by deonb »

This is a fairly basic question which is intuitively obvious (of course it works like this...), but I can't quite explain why.

I have a solar panel, connected to a charger (basically a voltage stabilizer), in turn connected to a 12V lead acid battery.

If I measure the voltage over the charger (input & output side) I can see it being fairly constant, doesn't matter what the actual sun coverage is (within reason). It's about ~20V input (solar panel side), ~15V output (battery side, with battery disconnected).

However, if I put the battery in, and measure the current between the charger and the battery, I see a large difference. With direct sunshine I measure about 1.5 Amps. When it's cloudy I measure about 0.3 Amps. The battery is empty in both cases.

What's the principle behind this? The internal resistance of the battery is obviously the same, so this would seem to violate ohms law - How can one get 2 different currents on the same circuit given the same voltage.

So what exactly is causing this? Is it that the internal resistance of the charger is changing? Alternatively is it the internal resistance of the solar cell that changes? If so, how does that translate to current on the other side of the charger?

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adafruit_support_bill
 
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Re: How does current limiting on a solar panel charger work?

Post by adafruit_support_bill »

There are many ways to design a charger and we don't know what is inside yours. But the general strategy is to adjust the load to keep the panels operating efficiently.
Here are the design notes from LadyAda's solar charger:
https://learn.adafruit.com/usb-dc-and-s ... sign-notes

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deonb
 
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Re: How does current limiting on a solar panel charger work?

Post by deonb »

Ok, but I still don't understand how the current limiting itself works in this case. Not specifically caring about the charger, but more about the ohms law theory behind this.

Generally when you have a circuit that's current limited, your voltage drops to 0 (or you most of the time effectively form a voltage divider so that in part of the circuit you'd have a voltage drop). But that's not the case here - the voltage on the solar panel as well as over the battery stays very similar, but with a dramatic drop in current.

Full readings:

High-sunshine:
Current between charger & battery: 1.56A
Battery input / charger output - connected: 12.64V
Charger input - connected: 16.67V
Battery - disconnected: 12.52V
Charger input - disconnected: 20.08V
Charger output - disconnected: 14.99V

Low-sunshine:
Current between charger & battery: 0.323A
Battery input / charger output - connected: 11.93V
Charger input - connected: 15.21V
Battery - disconnected: 11.89V
Charger input - disconnected: 19.86V
Charger output - disconnected: 17.53V

So I would expect that you would see voltage drops on the battery input as well as charger input (solar panel output side) as it gets to being current limited. But I don't see that.

How can current be limited in this way without an associated voltage drop?

Put another way - current gets drawn, not supplied. If a source can't supply enough current for a given resistance at a given voltage, there is always an associated voltage drop - mostly as a result of an implicit voltage divider between the ESR of the source, and the resistance of the circuit. But this is behaving differently, for some reason.

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Re: How does current limiting on a solar panel charger work?

Post by adafruit_support_bill »

How can current be limited in this way without an associated voltage drop?

Put another way - current gets drawn, not supplied.
In the numbers you posted, there is indeed a voltage drop in low sunlight. The charger output drops from 0.12v above disconnected battery voltage to 0.04v. Assuming it was a strictly resistive load (which it is not), you would assume that the current would drop to about 1/3 the level.

Keep in mind here that the 'load' is also a voltage source. If the charger output voltage is higher than the battery voltage, current will flow into the battery. If the charger output voltage is equal, the current will stop flowing.

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