volts and charge controller

[Alanthus]

I'm wondering what voltage to make my solar panel, and why does it matter with a charge controller. is there an advantage of having lower volts.

[Mike90250]

It takes a higher than battery voltage source, to recharge a battery.

A 12V lead acid battery needs about 14.5V to get fully charged. If you use a charge controller, to prevent overcharge, you loose a few more volts in the working of the charge controller (but get much longer battery life from not overcharging it)

So a typical panel for charging a 12V battery via a controller, is about 36 cells, or about 20V. Since solar panels loose voltage when hot, in hot areas , sometimes another cell or 2 is added to the panel, to insure enough voltage is present for full charge.

[Sunking]

I'm wondering what voltage to make my solar panel, and why does it matter with a charge controller.

Because there are two types of Charge Controllers being series (PWM and Linear) and Boost-Buck (MPPT). Each of the two groups work completely differently.

With a Series type charge controller input current = output current and what that boils down too is a Series CC is nothing more than a variable resistor and thus the reason they are so inefficient. As stated to charge a 12 volt battery takes a minimum 15 to 16 volt source out of the CC. Since a Series controller is really a variable resistor means it will drop or loose voltage so the the source which is the panel in this case must be higher to compensate for the 1 to 6 volts it will drop across the CC. Minimum loss using a Series controller is 30%, 50% typical.

A Buck/boost MPPT controller is a different animal altogether. INPUT CURRENT DOES NOT = OUTPUT CURRENT. MPPT controllers can buck or boost the current depending on the voltage input level. For example if you have 100 watt 100 volt panel the input current at the controller is 1 amp @ 100 volts. On the output would be 13 volts at 7.46 amps.

Here is what it boils down to. If you go the cheap route, you use a PWM controller. Doing that require you to use a panel made for 12 volt system of 36 cells. The Vmp of the panel will be 18 volts. So if you have 100 watt panel the voltage is 18 volts @ 5.55 amps. On the output will be 12 volts @ 5.55 amp (67 watts)

[Alanthus]

very helpful, except a little more confused, is a "Buck/boost MPPT" controller more efficient then a PWM controller? It seems the output volts and amps are higher on the example of the mppt controller with 100watt panel. why is the 100 watt panel using the PMW controller only outputting 67 watts? What does Vmp mean? Is that like open circuit voltage?

[Sunking]

very helpful, except a little more confused, is a "Buck/boost MPPT" controller more efficient then a PWM controller?

Yes MPPT controllers are high 90% efficient, typically 96%, where as the absolute best with a PWM is 66%, and 50% typical.

It seems the output volts and amps are higher on the example of the mppt controller with 100watt panel. why is the 100 watt panel using the PMW controller only outputting 67 watts?

Because a series regulator input current = output current. Example let's say you have a 100 watt panel rated @ Vmp = 18 volts, Imp = 5.55 amps. So at the input of the controller is 18 volts @ 5.55 amps, and the output = 12 volts @ 5.55 amps. Power = volts x amps. Now you do the math and tell me what the input and output power is. Once you do that you will understand why. Key input current = output current

What does Vmp mean? Is that like open circuit voltage?

Vmp = Voltage at maximum power
Imp = Current at maximum power
Voc = voltage open circuit
Isc - Current short circuit

So if you were to look at a commercial made 100 watt panel made for 12 volt battery systems the specs will be something like this:

Pmax = 100 watts
Vmp = 18 volts
Voc = 22 volts
Imp = 5.55 amps
Isc = 6 amps

What you are missing or not understanding is the term Buck-Boost converter. The voltage can be either bucked or boosted. Two real life examples

a 100 watt panel rated Vmp = 18 volts with Imp = 5.55 amps. On the output @ 96% efficiency is 12 volts @ 8 amps.

Now let's say you have some odd ball 100 watt panel with a Vmp = 10 volts, Imp = 10 amps. The output would be 12 volts @ 8 amps.

But here is the real advantage to an MPPT controller beside efficiency, is it allows you to run very high voltages on the panels. My first example was 100 watts, Vmp = 100 volts, and Imp = 1 amp. Output = 12 volts @ 8 amps.

Now turn it around and let's say we could find a PWM controller that can handle 100 volts input to a 12 volt battery. Using the same 100 watt panel, Vmp = 100 volts, Imp = 1 amp. The output is 12 volts @ 1 amp of 12 watts. You just lost 88% of your power. I can tell you exactly where to find the lost power, just put your hand on the controller and you will find it. Your nose will tell you because you will smell your flesh burning.

[Mike90250]

........I can tell you exactly where to find the lost power, just put your hand on the controller and you will find it. Your nose will tell you because you will smell your flesh burning.

Lovely description ! Thanks.