Thread: How Many 12v Solar Panels Needed To Charge 24v Battery?

1. How Many 12v Solar Panels Needed To Charge 24v Battery?

Part of me doesn't even know why I'm asking this question, but just need to confirm this with a few people.

I have 2x 12v batteries hooked up in series to produce 24v, and I need to confirm that I only need 2x 12v solar panels in order to properly charge them? Of course I'll have a charge controller and etc., but I'm only focused on the question above.

I remember when I asked someone how many panels I needed to charge a 36v battery, I originally thought I needed only 3x 12v batteries, but according to them that would not be enough, so that's why I'm double checking about this.

Don't know why this wasn't a popular Google question

2. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Nominal Voltage-wise, 2 times 12 = 24.
Vmp-wise, 2 times 17.5 = 35.
Providing they are standard "12 Volt" panels two in series will produce sufficient Voltage to charge 24 Volts of battery.

The only missing bit: how big are the batteries and how big are the panels?

3. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Originally Posted by Cariboocoot

The only missing bit: how big are the batteries and how big are the panels?
I'm thinking of getting 2 of these 30W solar panels since they are on sale: http://www.ul-solar.com/30_Watt_12_V..._p/stp030p.htm

As for the batteries, they are 12v, 35Ahm batteries.

Does the battery size change something, as if so, maybe that's what they were referring to?

4. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

The Amp hour capacity of the battery determines how much you need in Watts for proper recharging:
V * A = W
So a 12 Volt, 35 Amp hour battery would be recharged at (minimally) 14.2 Volts * 1.75 Amps = 24.85 Watts. Now, panels don't actually put out their "nameplate" rating all the time; they put out an average that's usually 80% or so of the rated Wattage. A 30 Watt panel would average 24 Watts - so it is just about the minimum for recharging that battery.

Put two of those batteries in series and you get 35 Amp hours @ 24 Volts. Put two of those panels in series and you have the minimum for recharging the 24 Volt battery bank.

5. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Originally Posted by Cariboocoot
The Amp hour capacity of the battery determines how much you need in Watts for proper recharging:
V * A = W
So a 12 Volt, 35 Amp hour battery would be recharged at (minimally) 14.2 Volts * 1.75 Amps = 24.85 Watts. Now, panels don't actually put out their "nameplate" rating all the time; they put out an average that's usually 80% or so of the rated Wattage. A 30 Watt panel would average 24 Watts - so it is just about the minimum for recharging that battery.

Put two of those batteries in series and you get 35 Amp hours @ 24 Volts. Put two of those panels in series and you have the minimum for recharging the 24 Volt battery bank.
Okay, so where did you get the 1.75Amps from? Like is there some type of table you go by that tells you the minimum value?

Also, would it be better to just get in two of these 20W 24v panels so that I'm not so close to the minimum value: http://www.ul-solar.com/product_p/stp020p-wp.htm

Or should I go the 12v 30W route. This is something I'm a little fuzzy on as well. As the 20W 24v panels can only deliver .61A which is going to increase the charge time, where as the 30W 12v panels can deliver 1.77A.

6. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Ah, the magic formula! The recommended charge rate of 5%-13% of the battery's Amp hour rate. You can read about this and many other fun facts in the Battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

What you'd really like to have is twice that 30 Watts for the 12 Volt, or four times for the 24 Volt:

3.5 Amps (10%) * 14.2 Volts = 49.7 Watts, less derating = 62.125 Watts.

7. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Originally Posted by Cariboocoot
Ah, the magic formula! The recommended charge rate of 5%-13% of the battery's Amp hour rate. You can read about this and many other fun facts in the Battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm

What you'd really like to have is twice that 30 Watts for the 12 Volt, or four times for the 24 Volt:

3.5 Amps (10%) * 14.2 Volts = 49.7 Watts, less derating = 62.125 Watts.
Well if space is limited, and I mean, very limited, which route would you go with if you had to pick one, as both will work, but as you've just calculated, it will be charging at the very minimum...but it will charge the batteries, which is the important thing for me. I just want to check to see which one is faster, since we have an Amp difference.

8. Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

More Amps = faster charging.
The other thing to remember is net charge rate is the gross charge rate less any current going out to supply loads. So the 5% rate works for a system that doesn't have large or continuous loads during recharge time.
My rule-of-thumb is to "shoot for" 10% and you'll get a workable rate between 5 and 10 percent.

9. wwwindsock6 Guest

Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

I have a 48v solar system, can I mix a 36v 280 watt panel with a 12v 15 watt panel to get 48v ? I am new at this trying to learn, Thanks

10. BB.
Just some guy
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Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

No, not really--It is like connecting a 12 volt car battery with a set of "D" cell flashlight batteries in series to get 24 volts.

Yes, you will get 24 volt's -- but only at the current level of the battery cells--not the big car battery.

-Bill

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