24v and 12v system

[Bigdaddybearpotts]

I have 2 12v systems, but a friend of mine has a bunch of 24v panels that are very slightly used and is selling them to me for dirt cheap. hes going to do 48v systems. can I use them on my 12v system, I'm going to build a 24v system but there is alot of panels.

 

[DerpsyDoodler]

I have 2 12v systems, but a friend of mine has a bunch of 24v panels that are very slightly used and is selling them to me for dirt cheap. hes going to do 48v systems. can I use them on my 12v system, I'm going to build a 24v system but there is alot of panels.

Short answer is yes. There are efficient and inefficient ways to do it, though. An MPPT charge controller would be one efficient way to make it work. There may be other, cheaper, simpler ways, though.

 

[Short_Shot]

Just for the sake of second opinion, there's nothing stopping you from using 300 volts of panels to charge a 12v battery as long as the charge controller can handle that input.

Just make sure the charge controller you use is rated for the maximum "voc" of whatever panels you use.

This includes taking into account whether they're in series as well as the temperature coefficient which causes the voc to increase as panel temperature decreases.

Usually I've seen people recommend the controller is rated for 1.25x the nominal VOC of the string per your panel specs as a rule of thumb to simplify this.

 

[DerpsyDoodler]

My response stressed efficiency. You can get a PWM, plug it up, and you'll get charging. however, 24v panel -> pwm -> 12v battery you're losing 1/2 your solar output power. That said, a dc-dc converter in front of the pwm would make it more efficient. Could you cobble that together cheaper than an MPPT? Probably. YMMV

 

[Short_Shot]

Also valid. I just assumed an mppt was the default to be honest.

 

[chrisski]

When you get these new panels, just make sure you batteries can handle them.

I have 458 ah of 12 volt lead acid batteries that can be charged to a max rate of 13% or 60 amps. With 1350 watts of panels, I can reach that, and actually hit 74 amps. With a Lithium bank I am building, I can charge those to 50%.

Don’t want these new panels exceeding the charge capacity of whatever your battery bank is. Until that capacity is reached, you can just throw another MPPT and panels at it until it does.

 

[Steve_S]

Let me put it another way.
Solar Panel Watts, voltage & amps belong to the panels and not much else, it is agnostic.
The Solar Charge Controller RULES the roost. Virtually all SCC's can handle 12/24/48V Battery Packs, they sense the voltage on Startup and work accordingly. Each SCC has INPUT LIMITS and Specs, it takes the incoming DC Voltage & Amps and steps it down to BATT Voltage and uses teh Amperage from the panels for delivering Charge Amps... (very kiss explanation).

I run 24V, my SCC Takes 2kw of solar, which puts out just under 200VDC and up to 2100W on a nice good day @ High Sun. My SCC converts that into 28.2V/80A Charge rate.

MPPT is the Smart prefered option because they manage solar input far better than the old PWM tech (which is really cheap).

Typically when putting together a system, it is Highly Recommended that you calculate for the Lowest Sun Hour Days (Dec/Jan in North America) and not during June/July Max Sun Hour Days. You want to be able to charge your batteries on the low days and that usually means a bigger array. Excess Power, once cells are fully charged. charging is shifted to "Float Mode" which allows the batteries to level up and balance out nicely, while any usage of the house is covered by the Solar Input directly up to the amount of amps available. If Solar is not generating enough amps, the balance gets pulled from the batteries, as as soon as the load is gone, the SCC will return to Charge Mode and then to balance in Float once the batteries are back to 100%.

The BIG RULE !
Never Ever exceed the Input Limits of the SCC from the solar panels, that will cook the SCC and make for a very bad day.