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24v Solar Panels to Charge 12v Battery Bank?

jsmit209

jsmit209

New Member
Joined
Nov 24, 2022
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I've got a 12v battery system (4x200ah 12v batteries in parallel). I'm planning to purchase 6x100watt 12v solar panels to wire in series pairs, and then parallel to generate 300watts at 24v. If my math is correct, this should produce roughly 12.5a at 24v.

If I pick up a 20a MPPT charge controller, will it be able to receive the input from the 24v panels and charge the batteries at 12v?
 
The 20A of the MPPT refer to its output.
Also, unless you have very particular space constraints, you'd be better off with ~400W panels - they're usually the best bang-for-the-buck.
-
 
Don B. Cilly said:
The 20A of the MPPT refer to its output.
Also, unless you have very particular space constraints, you'd be better off with ~400W panels - they're usually the best bang-for-the-buck.
-
Click to expand...
I'm still a bit confused. Does the input voltage matter then for a charge controller? At what voltage is the output amperage measured? How would you set up the solar for this battery bank?

Any recommendations on 400W panels?
 
jsmit209 said:
I'm still a bit confused. Does the input voltage matter then for a charge controller? At what voltage is the output amperage measured? How would you set up the solar for this battery bank?
Click to expand...
It does matter as charge controllers are only rated for so much PV input voltage which you never EVER want to exceed. The output voltage is going to be whatever the battery bank calls for, in your case about 14v to the 12v nominal batteries.

To size the charge controller you're going to want to divide your solar panel capacity (1000w) by your nominal battery voltage (12v) to tell you how big your charge controller needs to be (1000w / 12v = 83.3a!).

That's all assuming you're running an MPPT controller. If you're using PWM controllers then it's going to just cut off any voltage over the battery and you'll limit yourself to whatever amperage your panels output. At that point you're going to lose about 50% of your potential panel capacity.
 
jsmit209 said:
I'm planning to purchase 6x100watt 12v solar panel
Click to expand...
Is there a reason you are choosing a bunch of small panels instead of a couple big panels?

You only need to stay a bit below the SCC's max input voltage. Look for big cheap panels on craigslist if you have room for them (as opposed to tetris on an RV roof).
 
jsmit209 said:
I've got a 12v battery system (4x200ah 12v batteries in parallel). I'm planning to purchase 6x100watt 12v solar panels to wire in series pairs, and then parallel to generate 300watts at 24v. If my math is correct, this should produce roughly 12.5a at 24v.

If I pick up a 20a MPPT charge controller, will it be able to receive the input from the 24v panels and charge the batteries at 12v?
Click to expand...
600w panels deliver up to 600w. No matter how you wire them for voltage. Lets say the Voc rating of those panels is 22v You wire 2 panels in series gives you 44Voc and the same amperage as 1 panel is rated at. You make up 3 parallel pairs of 2 panels wired in series (2S3P) and you stay at the 44Voc but you must multiply the amperage by 3. So guessing at the amps of about 5a per panel that would give you 15a that you would want to size your wire for.

Now the SCC has ratings for max amperage it will charge the battery at. Say it is 20a, as in your question, 20a at 24v = 480w. If your panels could make more than this it will not happen because there is not sufficient load from the SCC. So you would want a bigger amp rated SCC to get closer to the panel output rating. a 30a SCC would give you 30a X 24v = 720w
 
Mattb4 said:
600w panels deliver up to 600w. No matter how you wire them for voltage. Lets say the Voc rating of those panels is 22v You wire 2 panels in series gives you 44Voc and the same amperage as 1 panel is rated at. You make up 3 parallel pairs of 2 panels wired in series (2S3P) and you stay at the 44Voc but you must multiply the amperage by 3. So guessing at the amps of about 5a per panel that would give you 15a that you would want to size your wire for.

Now the SCC has ratings for max amperage it will charge the battery at. Say it is 20a, as in your question, 20a at 24v = 480w. If your panels could make more than this it will not happen because there is not sufficient load from the SCC. So you would want a bigger amp rated SCC to get closer to the panel output rating. a 30a SCC would give you 30a X 24v = 720w
Click to expand...
Thanks! I was asking about wiring on the solar array, mostly because of wire sizing. Your answer makes sense for that.

In your final paragraph, you mention the SCC amps at 24v. Does the math change if it's charging my 12v battery bank?
 
MisterSandals said:
Is there a reason you are choosing a bunch of small panels instead of a couple big panels?

You only need to stay a bit below the SCC's max input voltage. Look for big cheap panels on craigslist if you have room for them (as opposed to tetris on an RV roof).
Click to expand...
This will be on the roof of a Ram Promaster. Do you know of larger panels that fit well on a van roof? Going to need space for a vent fan and AC unit as well.
 
MisterSandals said:
Is there a reason you are choosing a bunch of small panels instead of a couple big panels?

You only need to stay a bit below the SCC's max input voltage. Look for big cheap panels on craigslist if you have room for them (as opposed to tetris on an RV roof).
Click to expand...
Tetris is fun!

If it is an RV application, a bunch of 12v panels would be best... correct?

I'm currently designing a 2000 watt system for my RV that is going to consist of 20 100w "12v" panels. Yes, the initial installation is more time consuming and costly but the long term benefits are HUGE!
 
jsmit209 said:
Thanks! I was asking about wiring on the solar array, mostly because of wire sizing. Your answer makes sense for that.

In your final paragraph, you mention the SCC amps at 24v. Does the math change if it's charging my 12v battery bank?
Click to expand...
Yes. Watts=Volts X Amps

So if you are charging 20a at 12v = 240w
 
jsmit209 said:
So, for the 12v batteries and 600w of solar, my best bet would be a 50a SCC?
Click to expand...
Likely you will not find a 50a SCC. More common is 60a. Understand that there is another side of the equation you need to keep in mind. Some batteries have a maximum and recommended amp charge amount. You mentioned 4 -200ah batteries. That is 800ah by 12v = 9600wh

To charge these from solar power could be a problem with only 600w panels (max) in a day depending on your loads and the discharge level. Say you get 5 hours at 600w for a total of 3000wh. If your batteries were fully discharged you would near 4 days of charge time. No loads during that time.
 
Mattb4 said:
Likely you will not find a 50a SCC. More common is 60a. Understand that there is another side of the equation you need to keep in mind. Some batteries have a maximum and recommended amp charge amount. You mentioned 4 -200ah batteries. That is 800ah by 12v = 9600wh

To charge these from solar power could be a problem with only 600w panels (max) in a day depending on your loads and the discharge level. Say you get 5 hours at 600w for a total of 3000wh. If your batteries were fully discharged you would near 4 days of charge time. No loads during that time.
Click to expand...
Yeah, that was also a concern. I'm planning to install a second alternator in the van to charge the batteries.
 
EWesch said:
If it is an RV application, a bunch of 12v panels would be best... correct?
Click to expand...
The SCC manages the charge voltage so panel voltage is moot since the advent of MPPT controllers. Just stay below max input voltage of SCC.
Using biggest panels that maximize watts on roof is usually best because mounting fewer panels (it’s a pain) is better in my opinion.
If you start your own thread for your project we can keep all that conversation together and separate from this thread.
 
MisterSandals said:
The SCC manages the charge voltage so panel voltage is moot since the advent of MPPT controllers. Just stay below max input voltage of SCC.
Using biggest panels that maximize watts on roof is usually best because mounting fewer panels (it’s a pain) is better in my opinion.
If you start your own thread for your project we can keep all that conversation together and separate from this thread.
Click to expand...
This is the thread that I started for my project.

Can you recommend any specific larger panels that would be good for a van roof?
 
jsmit209 said:
This is the thread that I started for my project.
Click to expand...
Yep, i tried to get EWesch to make his own thread, sorry for my slight venture into his questioning.

Look on craigslist or whatever local marketplaces you have for big cheap panels. Get their dimensions and see if they fit on your roof. If money is no object, then measure your roof (or take cardboard cutouts up to the roof) and see what you can fit and buy/ship whatever you get that matched.

My point is that you don't need 12V panels (its just a marketing term anyway for 18V panels) just because you have a 12V battery system. Mounting fewer big panels is usually far better than a bunch of small panels.
 
MisterSandals said:
Yep, i tried to get EWesch to make his own thread, sorry for my slight venture into his questioning.

Look on craigslist or whatever local marketplaces you have for big cheap panels. Get their dimensions and see if they fit on your roof. If money is no object, then measure your roof (or take cardboard cutouts up to the roof) and see what you can fit and buy/ship whatever you get that matched.

My point is that you don't need 12V panels (its just a marketing term anyway for 18V panels) just because you have a 12V battery system. Mounting fewer big panels is usually far better than a bunch of small panels.
Click to expand...
Great! I really appreciate the advice.
 
Sorry... I felt like I was asking a question pertinent to jsmit209's project then simply offered information on a personal build to encourage discussion.

Bigger isn't always better! One thing to take into consideration is redundancy, dependability, and expand-ability.

jsmit209...
  • Is there anything on the roof that is going to stick up further than your solar panel(s)? (You mentioned A/C)
  • If so, is it substantial enough (standard/low profile/ultra low profile) to cast a shadow large enough to compromise the performance of a large panel, or panels in series?
  • Why do you feel you need a second alternator vs a DC/DC charger?
2 400w panels in series will produce 800w under perfect conditions or ZERO if one is shaded or fails/is damaged.
8 100w panels in parallel will produce 800w under perfect conditions, 700w if 1 is in the shaded, 600w if 2 are shaded or a tree branch falls and breaks them.

Same with your charge controllers...
  • 1 charge controller that can handle several panels through a single 10 gauge wire at high voltage/low amperage
  • 3 charge controllers on three 10 gauge pairs at low voltage/high amperage.
1 charge controller fails, you get ZERO solar
3 charge controllers and 1 fails, you get reduced solar

My recommendation given the amount of battery you have...

6-10 (whatever you can fit on the roof) Renogy 100w panels ($85 each Black Friday Sale) - 4 or 5 Harbor Freight 100w panels (can usually get for $99 each when on sale) to deploy. (note HF panels consistently outperform Renogy)

3 Victron 100/30 ($200 each) 2 for roof array and 1 for mobile array... you could safely over-panel (not bad) each controller and put up to 5 "12v" panels in parallel on each. (30 amps short circuit and 24 volts open circuit per string)

2 Victron 12/12-30s ($225 each) for up to 60 amps of charging while the engine is running (again, redundancy)


... the other benefit is that you start small and add as you go.
 
Last edited:
EWesch said:
Sorry... I felt like I was asking a question pertinent to jsmit209's project then simply offered information on a personal build to encourage discussion.

Bigger isn't always better! One thing to take into consideration is redundancy, dependability, and expand-ability.

jsmit209...
  • Is there anything on the roof that is going to stick up further than your solar panel(s)? (You mentioned A/C)
  • If so, is it substantial enough (standard/low profile/ultra low profile) to cast a shadow large enough to compromise the performance of a large panel, or panels in series?
  • Why do you feel you need a second alternator vs a DC/DC charger?
2 400w panels in series will produce 800w under perfect conditions or ZERO if one is shaded or fails/is damaged.
8 100w panels in parallel will produce 800w under perfect conditions, 700w if 1 is in the shaded, 600w if 2 are shaded or a tree branch falls and breaks them.

Same with your charge controllers...
  • 1 charge controller that can handle several panels through a single 10 gauge wire at high voltage/low amperage
  • 3 charge controllers on three 10 gauge pairs at low voltage/high amperage.
1 charge controller fails, you get ZERO solar
3 charge controllers and 1 fails, you get reduced solar

My recommendation given the amount of battery you have...

6-10 (whatever you can fit on the roof) Renogy 100w panels ($85 each Black Friday Sale) - 4 or 5 Harbor Freight 100w panels (can usually get for $99 each when on sale) to deploy. (note HF panels consistently outperform Renogy)

3 Victron 100/30 ($200 each) 2 for roof array and 1 for mobile array... you could safely over-panel (not bad) each controller and put up to 5 "12v" panels in parallel on each. (30 amps short circuit and 24 volts open circuit per string)

2 Victron 12/12-30s ($225 each) for up to 60 amps of charging while the engine is running (again, redundancy)


... the other benefit is that you start small and add as you go.
Click to expand...
Thanks for all of this. I hadn't considered a DC-DC charger. Does that link the starter battery to my battery bank and charge off the alternator when the engine is running? In this case, any risk to the starter battery?
 
Yes, it connects the house to the starting battery. It changes nothing that came from the factory and allows you to charge your house batteries without damaging your factory alternator.

You can start with one and parallel them up to whatever amperage your alternator can handle. If you desire more amperage, you can simply upgrade your factory alternator (in the factory location) and add another DC/DC charger... much less expensive and, again, creates redundancy and can get bigger as the project and/or money allows..
 
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