Here is a recent system we put in a friend’s work van. Much more than Rebelbase is wanting but it can help explain the system and how they work. One can adapt these basics to their needs. It can be adapted to be a 24 volt system also with 24 volt rated panels. And the 12 volt batteries can be hooked up in series to make 24 volts. Pre-charge controller voltage reduction with the buck converters was only needed because of the repurposing of the 40 volt home panels we acquired and it was too much for a 12 volt charge controller, it would not have needed the buck converters for a 24 volt system and charge controller.
Originally posted in another forum:
“Finally got around to sharing the solar system we set up in my friends van. But it could be set up the same stationary also and as 24V as an option.
I’m not going to show the panels because they all look the same, just the heart of the charging works. And he has 6 deep cycle marine batteries in his bank hooked in parallel, and a 120 inverter. Standard common system so no need to show those either. But he is rated enough to run an electric chain saw and still only tax the inverter to about 50% of available power output. And run it for at least an hour from the batteries even without sun on the panels.
The panels are four old take offs from a large 36 panel home system that had to be replaced with new panels. They are rated as follows:
X4 - 40 Volts - 150 Watts - 5 Amps. ( So that is 0 - 40 Volts, 600 Watts total, and 20 amps total.)
(If these had been new panels we could have divided the two circuits which are set up in series in the panels. But because these were used and not putting out what they should they would only give us a little over 12V even in the best of sun when we divided them up to try as parallel. And a 12V battery system needs an input of at least 13.7 volts to charge efficiently).
So the 34V - 40V panel voltage rating was an issue for setting up as a 12V system rather than a 24V system. But we needed to have it 12V so that it would also integrate with the 12V vehicle system and be an addon system to work with the vehicle also aside from the 120 AC inverter output. This way not only could he pull from the solar system batteries for the vehicle DC system, but could also use the vehicle charging system to help recharge the battery bank while he is driving. And he can flip a switched solenoid to jump the starting system on the van from the solar system if the van battery went dead for some reason. Redundancy for both systems.
When setting up a system you have to work backwards, from what voltage and current rating your battery bank will be, and what voltage and current your charge controller will need to be. In this case we went overkill with the charge controller, that way if he ever needs to use it with more panels, or in a stationary application it is already rated for more. And now days they use very very little to keep the controller powered up anyhow. So the charge controller is rated as follows:
Max 36v input when used on a 24V system. And max 24 volt input when used on a 12V system at max 80 amp input. Now it might (risky) have handled up to 40V input, but it would have probably caused overheating issues and digital control issues. We did not want to overdrive it, better to be safe than sorry with electronics that could cause fires. So we ran the 40V panel output through buck convertors to step down the voltage going into the charge controller. The buck convertors are rated as follows:
X2 - 20A 300W CC CV Step Down Module Adjustable DC 6-40V to 1.2-36V Voltage Regulator Buck Converter Constant Current Power Supply. The efficiency can be up to 95%,(measured at 20A, converting 24V to 12V).
Because of the max 300 watt rating of each buck convertor we had to use two and divide the output of the four panels (600 watts) into two sets of two panels (300 watts each set) going into the buck convertors. All that was left to do after hooking it all up was to tune the adjustable buck convertors on a good day with max sun available on the panels. A tip for buck convertor efficiency is this... You want to try and set the convertor output just under the max input to the charge controller. The more you step the voltage down the harder the convertor has to work, they operate hotter, draw more themselves, and are less efficient. Max input of the charge controller is 24V so we set the output of the buck convertors at 22V output. The only 2V difference is easy on the buck convertors and the heat sinks get warm, but not hot.
Now if one wants to make a small system like this as backup for a home there only a few important additional technical changes. A grid main switch needs to be installed so that when you turn on the solar system, it simultaneously physically cuts the grid power. Last thing you want is for the grid power to come back on while you are on your solar inverter. And unless you foot the bill for a home system inverter, portable inverters can be used but will fault if the ground is hooked up on the receiving home system, so the male plugs into the inverter need the ground post deleted. And there is a bit of careful wiring in the AC power distribution box so that you can pipe in two separate 120V circuits from the inverter. And unless you have a big high rated portable inverter you will want to temporarily shut off and live without all your 240 appliances.”
He can run a couple skill saws or cut off saws or 120 screw guns through an all day project and still be almost topped off in the batteries if it is a sunny day. It will take quite a load and hang in there well. This would make an excellent smaller scale system for a home. You could run smaller appliances like microwaves and such with no problem.