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.
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.)
Brad
Friday 24th of March 2023
Nate, super helpful chart. Thank you so much. Saved me from burning down the cabin. haha. Brad
Wayne Davey
Saturday 4th of March 2023
Hi Nate from the other side of the world in sunny Australia. Thanks for your great blogs and youtube videos. You have taught me so much. I have a tech question, I have five 275w 22v panels that were wired badly by a supposed professional. I have two solar controllers 100v 40a I want to rewire the panels in series, so can I have 3 panels on one string through one controller and 2 panels on the second string through the other controller? My system is in a caravan with 600a lithium battery bank. There is ac-dc charger and also a dc-dc charger. I have redone the whole system inside the van and it looks fantastic. When I get the panels sorted it should perform fantastic as well. Thanks Nate in advance, Cheers Wayne.
Bev Kimpel
Saturday 14th of January 2023
Hey Thanks so much for all of the amazing and CLEAR information. I have been a carpenter for 35 years but electricity has always made me seize even without crossing wires. No more! Plus any excuse to buy tools makes me happy. Anyway my question is if I am adding a collapsible ground solar panel to augment my array can I use the same charge controller calculator? I am using your diagram which employs the availability of a large solar array but happen to have an extra victron 100/30 solar charge controller. Also curious how and where you put the connector for this externally on the transit. Perhaps I missed it in the videos. Thank you! Bev
Nate Yarbrough
Sunday 15th of January 2023
The calculator doesn't care where the panels are mounted. The external solar mount is covered in this video: https://youtu.be/E5q7_4FH4LQ
Sean
Wednesday 7th of December 2022
Hey Nate! Amazing stuff here and just recently found your channel and website and have been trying to learn as much as possible. I recently bought an old ambulance that I am converting and plan on covering the entire roof with solar as I want a full sustainable off grid system and won't be near shore power, and will use alternator as a secondary. I can fit (9) Newpowa 220w panels on my roof perfectly. I want to run them in parallel as I have read when they are in series, shade kills them drastically, as since I plan on being in the woods more often than not, I think parallel is the better route to take. These specific panels are rated at 220w, 12.52 Voltage (pmax), 12.6 current (imp), 20.52 open circuit (voc) and 13.41 short (isc). So this gives me 12.52 operating volt and 113.4 operating amps. This will put me at roughly 1420 charging watts and 98 amps.
I was looking at the 250/100 charge controller, but from my understanding with my current example above, it puts me just a little over the threshold for that controller, I think. 250/100 controller, 250 volt max, and 100 amp max. I heard you can have a little wiggle room with the amp but not the volts.
Am I okay with using this controller? Would it be more beneficial to use two smaller controllers? I can get two 150/100s and split it 5 and 4. But obviously would like to just use one to keep it "simple" if possible.
Let me know your thoughts.
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.