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What Does a Solar Charge Controller Do?

A solar charge controller is a vital piece of a camper electrical system. It is installed between the solar array and the battery bank (or busbars, more often) and regulates the voltage and amperage of the solar array down to a level that is appropriate to charge a battery bank with.

Long story short, the charge controller is essentially a high powered voltage regulator.

Here are the three things we know about charging a battery bank from a solar array:

  • The voltage and amperage of the solar array can be altered depending on how the array is wired.
  • Batteries want to be charged at a very specific voltage.
  • Solar arrays don’t put out a very specific voltage at all.  They can be anywhere from 0 volts at night or upwards of 250V or more depending on the panels used and how the array is wired.

Q: So, how to we charge a battery bank that wants to be charged at a very specific voltage when our solar array puts out an incredibly variable voltage that is potentially much to high to charge a battery bank with?

A: We install a charge controller to regulate the high and variable voltage from the solar array down to a level that is more appropriate to charge a battery bank with. And here’s how it works:

What Does a Solar Charge Controller Do? – Video

Charging a 12V Battery Bank with an 80V Solar Array

A solar array with 4 panels operating at 5A delivered at 20V each, wired with 4 panels in each series-string with 1 series-strings wired in parallel would result in a solar array that operates at 5A delivered at 80V to the charge controller for a total array wattage of 400W (Watts = Amps x Volts).

The Charge Controller will then convert the 80V from the solar array down to the 14.5V that it takes to charge a 12V Battery Bank.  The 400W from the solar array is now being delivered to the battery bank at 14.5V and is therefore charging the battery bank at a rate of 28A (Watts/Volts=Amps).

Charging a 12V Battery Bank with a 40V Battery Bank

A solar array with 4 panels operating at 5A delivered at 20V each, wired with 2 panels in each series-string with 2 series-strings wired in parallel would result in a solar array that operates at 10A delivered at 40V to the charge controller for a total array wattage of 400W (Watts = Amps x Volts).

The Charge Controller will then convert the 40V from the solar array down to the 14.5V that it takes to charge a 12V Battery Bank.  The 400W from the solar array is now being delivered to the battery bank at 14.5V and is therefore charging the battery bank at a rate of 28A (Watts/Volts=Amps).

Effects of a Higher Voltage Battery Bank on the Charge Controller

A solar array with 12 panels operating at 10A delivered at 40V each, wired with 4 panels in each series-string with 3 series-strings wired in parallel would result in a solar array that operates at 30A delivered at 160V to the charge controller for a total array wattage of 4800W (Watts = Amps x Volts).

The Charge Controller will then convert the 160V from the solar array down to the 14.5V that it takes to charge a 12V Battery Bank.  The 4800W from the solar array is now being delivered to the battery bank at 14.5V and is therefore charging the battery bank at a rate of 331A (Watts/Volts=Amps).

Now… This is really too many amps for a single charge controller to handle So… We could either use multiple charge controllers to handle this many watts coming from the array OR we could investigate using a higher voltage battery bank.

Understanding that the wattage from the solar array gets divided by the battery bank charging voltage to get the battery bank charging amps, we can see that as we increase the voltage of the battery bank, the amps charging the battery bank will decrease as the watts stay the same.

WATTS LAW

X

=

0

This would mean that the same array as earlier could be charged by a single 100A charge controller because the increased voltage of the battery bank as decreased the amperage demand even though the charging wattage remains the same.

A solar array with 12 panels operating at 10A delivered at 40V each, wired with 4 panels in each series-string with 3 series-strings wired in parallel would result in a solar array that operates at 30A delivered at 160V to the charge controller for a total array wattage of 4800W (Watts = Amps x Volts).

The Charge Controller will then convert the 160V from the solar array down to the 58V that it takes to charge a 48V Battery Bank.  The 4800W from the solar array is now being delivered to the battery bank at 58V and is therefore charging the battery bank at a rate of 83A (Watts/Volts=Amps).

Charge Controller Operation Calculator

Here is a calculator that you can play around with that will show you how various array configurations and battery bank voltage interact with each other. (This calculator is NOT to be used for accurate sizing of a charge controller as it does not account for max voltages, amperages nor temperature compensation.)

20
5
2
2
12

SOLAR ARRAY

Operating Voltage: V

Operating Amperage: A

Wattage: W

BATTERY BANK

Charging Voltage: V

Charging Amperage: A

Charging Wattage: W

A solar array with panels operating at A delivered at V each, wired with panels in each series-string with series-strings wired in parallel would result in a solar array that operates at A delivered at V to the charge controller for a total array wattage of W (Watts = Amps x Volts).

The Charge Controller will then convert the V from the solar array down to the V that it takes to charge a V Battery Bank.  The W from the solar array is now being delivered to the battery bank at V and is therefore charging the battery bank at a rate of A (Watts/Volts=Amps).

 


James

Sunday 27th of November 2022

I think I may be a bit unique but would like your thoughts. I'll catch all kinds of grief for this idea but...I know I can make it work; just gathering ideas and feedback.

People only show ways to charge batteries when in fact solar panels can produce A/C directly at the panel. That energy can feed the A/C appliances directly without charging batteries. Batteries are used only for low light and night time needs or power outages and of course all DC requirements.

This is basically a physical home installation converted to a mobile posture. Up to (13) 395 watt LG panels - reduces direct heat from roof which directly reduces energy requirements Up to (13) Enphase IQ8+ micro inverters (110 volts to house with excess going to charge controller(s)). Electricity needs to come in as 2 legs to get the 220 volts needed for current power panel. I have designed a box to house solar panels that does not exceed 13.6' above ground (most commercial truck height); at or just below the height of current AC units on roof. I have also designed a hydraulically operated rail system to lift rails above AC units (3) so all panels are on a single plane above all shadow producing items on roof. My panel design has IQ8+ attached to each panel with wiring connected via spring loaded connectors; thereby transferring all power A/C through panel string back to combiner box, etc..

Have you ever thought about doing it this way? And if not, why not? A/C doesn't need the 10 gauge wire and directly feeds all A/C appliances directly.

Jim Palensky

Saturday 23rd of July 2022

Nate, Help! I've been abandoned by my captain. I bought a 1979 saber 34 sailboat in February and my delivery captain was rehabbing the solar array. He left in the middle of the project and I have nothing but parts everywhere. What I have is a four 100 watt Renology solar array panels that are connected in parallel array, and a fifth 100 watt Renology panel that is not connected . The four 100 watt panels are connected through an Victron mppt charge controller that is rated 100/30. I also have another Victron charger controller 75/10 (new and in box) and a 2000 watt inverter. This is a typical boat / RV setup where there are two battery banks. Bank one is three lead acid starting Marine batteries. Bank 2, which is hooked to the solar array, is two 150 amp hour lithium batteries. I also have a 27 horsepower diesel engine with 2-30 amp alternators. There is also shore power. The shore power tripped the marina pedestal circuit the one and only time I plugged into a marina power source . I am not looking to plug in and plan to be off the grid and sail the Bahamas and the Caribbean. I need some design help please! I need both battery banks to be charged by the solar array. I am a veterinarian and do not speak electrical or solar fluently. I have binge watched all of your videos and I feel like I can do the install if I have the design and the components that I need. I know no other way to get a hold of you because the link on your videos to reach you took me into internet error hell. Please reach me at jamespalensky@gmail.com

Seth Harris

Monday 20th of June 2022

Hey, I have 2x 200ah 12v lithium batteries in parallel. Each battery has a max charge current of 100a and a recommended charge current of 50a.

Are these numbers important when choosing a charge controller? I am planning to purchase the Victron Smart Solar MPPT 100/30 based on your solar calculator.

Great work! Thank you in advance :)

Peter Andrrews

Thursday 19th of May 2022

Hi Just discovered your site excellent, I have a DC to DC charger (JayCar 20 Amp) which has a maximum input of 25 volts, to increase volts from solar array is it possible/ practical to use a higher voltage MPPT controller to step higher voltages down for the DC to DC charger. I know this could complicate the system by the DC-DC also automatically connects and disconnects from alternator as required.

Peter for Perth Australia

Chad Brewster

Thursday 21st of April 2022

I have 6 interstate gc2 deep cycle batteries that are connected in series, I think! (I have a diagram I can send?) to a Xantrex Freedom 458 - 2500w inverter. I need to purchase a Charge controller and need to know how to connect it to the batteries? Is one charger enough? Do you recommend a brand for those batteries?

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