Charge controller needs more clarity

[Jim Burrow]

Charge controller needs more clarity

I’m looking at the all-in-one 24 v charge/inverter and the 24v LifePo4 power battery unit. - (My system has to look net and clean or my wife will not let my install one)

The question I have with respect to the solar panel, does the output voltage (pv) need to be that of the batteries?

As I understand it so far (and I have a lot to learn) if I use a 24v battery system, the solar array out also must be 24v. - otherwise, how can 12v solar panel charge a 24v battery even though one is going through a charger/inverter unit like the all-in-one unit?

 

[Forbisher]

- otherwise, how can 12v solar panel charge a 24v battery even though one is going through a charger/inverter unit like the all-in-one unit?

Stop calling panels 12 volts or 24 volts.
Look at the panel voltage and amp specs.
So called 12 volt 100 watt panels are about 21 Volt open circuit and 18 volt working output.
Two 21 Volt open circuit panels connected in series are about 43 volts.

The 24 volt all in one units have a MPPT controller with a max 145? Volts open circuit input and a working voltage range.

 

[MichaelK]

Hello Jim
First, there are some misconceptions to clear up. No, you do not use a 24V solar panel to charge a 24V battery. Panels commonly "called" 24V typically put out ~30-37V. Your typical 24V battery is NOT charged at 24V, but 28-30V.

Early generation PWM controllers only act as an on/off switch to cut/connect power from the panel to the battery. They can not regulate voltage, they only regulate current. That means the panel voltage needs to be carefully matched to the battery so it will not be damaged. So, if you have a 12V battery, you can only connect a panel putting out no more than about 18V. For 24V batteries, about 32V. However many amps the panel is putting out is what the battery gets.

More sophisticated MPPT controllers act as a transformer to bring a high voltage panel down to the voltage the battery needs. Let's say you connect a 12V battery to the MPPT controller, then connect a 8A-30V (thats 240W) panel to the controller. The 12V battery wants to be charged at ~14.5V, so the MPPT controller transforms the 30V current down to 14.5V, but at the same time increases the amperage to 16.5amps. In other words, the MPPT converts extra voltage into extra amperage. The real-world numbers might be a little lower, but basicly that's how it works.

The second BIG advantage of MPPT is because it down transforms the voltage, you can string several panels in series to raise the voltage significantly higher. This means you use thinner, cheaper copper wire because the power is transmitted as high volts instead of high amps.

Right now, 30V grid-tie panels are far cheaper per watt than 12V panels. You get more bang for your buck with grid-ties. For my own 24V workshop system, I'm running four 250W panels in series to get 120VDC, which feeds my Midnight200 charge controller. The controller than transforms that 120VDC down to 29.6VDC that my battery bank wants.

I would not recommend though that you get what is now commonly referred as a "all in one system". I built my system from individual components, but tie them all together with a power center. It looks just like this.

MidNite Solar MNXWP6848-2CL150 Pre-Wired Power Panel -RES Supply

Free Shipping! MidNite Solar MNXWP6848-2CL150 Pre-Wired Power Panel, 6800W, 120/240VAC, (1) XW+6848 NA Inverter, 48VDC, (2) Classic 200 Charge Controllers, High Voltage Input Breakers, AC Bypass Assembly

ressupply.com

Each one of the components in the pic are exactly what I have. The power center is the box in the lower middle to ties everything together. That's where all the wiring goes, holds the breakers, and makes it look neat as the spouse demands. Keep in mind though that there are different power centers made for each brand of inverter, so though they function the same, they may look different.

 

[Jim Burrow]

Hello Jim
First, there are some misconceptions to clear up. No, you do not use a 24V solar panel to charge a 24V battery. Panels commonly "called" 24V typically put out ~30-37V. Your typical 24V battery is NOT charged at 24V, but 28-30V.

Early generation PWM controllers only act as an on/off switch to cut/connect power from the panel to the battery. They can not regulate voltage, they only regulate current. That means the panel voltage needs to be carefully matched to the battery so it will not be damaged. So, if you have a 12V battery, you can only connect a panel putting out no more than about 18V. For 24V batteries, about 32V. However many amps the panel is putting out is what the battery gets.

More sophisticated MPPT controllers act as a transformer to bring a high voltage panel down to the voltage the battery needs. Let's say you connect a 12V battery to the MPPT controller, then connect a 8A-30V (thats 240W) panel to the controller. The 12V battery wants to be charged at ~14.5V, so the MPPT controller transforms the 30V current down to 14.5V, but at the same time increases the amperage to 16.5amps. In other words, the MPPT converts extra voltage into extra amperage. The real-world numbers might be a little lower, but basicly that's how it works.

The second BIG advantage of MPPT is because it down transforms the voltage, you can string several panels in series to raise the voltage significantly higher. This means you use thinner, cheaper copper wire because the power is transmitted as high volts instead of high amps.

Right now, 30V grid-tie panels are far cheaper per watt than 12V panels. You get more bang for your buck with grid-ties. For my own 24V workshop system, I'm running four 250W panels in series to get 120VDC, which feeds my Midnight200 charge controller. The controller than transforms that 120VDC down to 29.6VDC that my battery bank wants.

I would not recommend though that you get what is now commonly referred as a "all in one system". I built my system from individual components, but tie them all together with a power center. It looks just like this.

MidNite Solar MNXWP6848-2CL150 Pre-Wired Power Panel -RES Supply

Free Shipping! MidNite Solar MNXWP6848-2CL150 Pre-Wired Power Panel, 6800W, 120/240VAC, (1) XW+6848 NA Inverter, 48VDC, (2) Classic 200 Charge Controllers, High Voltage Input Breakers, AC Bypass Assembly

ressupply.com

Each one of the components in the pic are exactly what I have. The power center is the box in the lower middle to ties everything together. That's where all the wiring goes, holds the breakers, and makes it look neat as the spouse demands. Keep in mind though that there are different power centers made for each brand of inverter, so though they function the same, they may look different.

Thanks, Michaelk for the in-depth explanation. That really helps. My education is 45 years old and all has changed since my college days especially, with respect to Solar and Batteries.

That's why I was wondering if a charge controller has a step-up transformer (or electronic equivalent) that would allow a person to use one solar panel (14 ocv) to transform it up to a level to charge a 24-volt battery pack. For my load, I can get away with one panel if the controller can do that. Else I will have to use two panels in series. - Space and weight is limited for the panels.

 

[gnubie]

Most controllers are built to work with higher voltage and reduce that down to something suitable for the battery and gain current in the process, but you can get MPPT controllers that go the other way, increase voltage but obviously reduce current as a result.

Genasun Boost MPPT's

The Genasun GV-Boost controllers are designed to step-up lower-voltage solar panels to charge higher-voltage batteries. Available for lithium, LiFePO4 and lead-acid batteries.

genasun.eu

 

[Dzl]

The question I have with respect to the solar panel, does the output voltage (pv) need to be that of the batteries?

As I understand it so far (and I have a lot to learn) if I use a 24v battery system, the solar array out also must be 24v.

The simple answer,

1. Before MPPT output voltage of a PV array needed to be matched to battery voltage
2. With MPPT output voltage generally just needs to be higher than battery voltage (with a few exceptions like Gnubie's link above). The MPPT controllers I've looked at (Victron, Epever) advise an array voltage roughly 2x to 3x battery voltage for optimal performance.

I agree with MBR that calling panels 12v or 24v served some purpose in the pre MPPT days, but now mostly just confuses people, and gives the impression they must be matched to battery voltage.

 

[MichaelK]

That's why I was wondering if a charge controller has a step-up transformer (or electronic equivalent) that would allow a person to use one solar panel (14 ocv) to transform it up to a level to charge a 24-volt battery pack. For my load, I can get away with one panel if the controller can do that. Else I will have to use two panels in series. - Space and weight is limited for the panels.

I hotwaxed a ladyfriend's snowboard, and she hated it. Said that it made her go too fast. My response was "umm, that's the point"! Transforming the voltage up seems as backwards thinking as that.

With higher voltage, you can position your panels hundreds of feet away from where your controller is placed. Right now, I'm running 120VDC power more than 125 feet, and I'm seeing virtually no voltage drop.

Exactly why is space limited? Is this on the roof of a RV, or the roof of the house. Exactly how much surface area do you have available? If you have enough space for a single 72 cell panel, those typically put out 37-38VDC, which should be enough to charge a 24V battery under any conditions. Mine run about 72X39". If you can not mount more than 1 single panel, I really question whether or not you can put together a practical system that will actually accomplish anything?