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

Solar Expert Posts: 75 ✭✭✭✭
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

## Comments

• Banned Posts: 17,615 ✭✭✭
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?
• Solar Expert Posts: 75 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

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_Volt_multicrystalline_solar_panel_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?
• Banned Posts: 17,615 ✭✭✭
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.
• Solar Expert Posts: 75 ✭✭✭✭
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.

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.
• Banned Posts: 17,615 ✭✭✭
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.
• Solar Expert Posts: 75 ✭✭✭✭
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.

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.
• Banned Posts: 17,615 ✭✭✭
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.
• Posts: 6,290 admin
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
• Super Moderators, Administrators Posts: 33,553 admin
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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,009 ✭✭✭✭
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

A 93 watt 12 volt panel would be a good match for the mentioned 280 watt 36 volt panel.
• Solar Expert Posts: 75 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

I just noticed something after deciding to go with 20W panels instead of 30W panels due to the overall size of the panels.

The 24v 20W panel produces 0.61 Amps as can be seen here.

Hooking two of those 24v 20W panels in parallel is going to double the Amps to give us: 1.22A total.

The 12v 20W panel produces 1.17 Amps as can be seen here.

Hooking two of those 12v 20W panels in series is going to keep our Amps the same and still give us 1.17A total.

So in this case, it's better to go with the 24v setup above correct? Again, I had to change from 30W to 20W panels because again I'm limited on space I found after measuring, the 30W panels would be a little too long.
• Banned Posts: 17,615 ✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Vmp on the "24 Volt" panel: 33
Vmp on two "12 Volt" panels in series: 34.4

What this means in terms of charging a 24 Volt battery with a PWM controller: not much difference.

The current the panel produces at its Imp is not necessarily the same as the current the battery charges at once the controller and all the wire is in place.

Typically a "24 Volt" panel will have a Vmp of 35. The two "12 Volt" units in series come closer to this by 1.4 Volts. But that could make up for the Voltage drop through the wires.
• Solar Expert Posts: 75 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
Vmp on the "24 Volt" panel: 33
Vmp on two "12 Volt" panels in series: 34.4

What this means in terms of charging a 24 Volt battery with a PWM controller: not much difference.

The current the panel produces at its Imp is not necessarily the same as the current the battery charges at once the controller and all the wire is in place.

Typically a "24 Volt" panel will have a Vmp of 35. The two "12 Volt" units in series come closer to this by 1.4 Volts. But that could make up for the Voltage drop through the wires.

So basically, you are saying it's better to go with 2x 12v 20W panels hooked up in series instead of 2x 24v 20W panels hooked up in parallel?
• Banned Posts: 17,615 ✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
rss2q wrote: »
So basically, you are saying it's better to go with 2x 12v 20W panels hooked up in series instead of 2x 24v 20W panels hooked up in parallel?

It looks that way to me. There's about .9 Watts difference in the choice.
• Posts: 6,290 admin
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
A 93 watt 12 volt panel would be a good match for the mentioned 280 watt 36 volt panel.
Thanks for the info, I was unsure whether to purchase a MX60 or solar panels to balance the voltage.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
rss2q wrote: »
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

normally 2 12v pvs of the proper current rating through a controller will charge a 24v battery bank. it really is governed by the vmp the pvs output and if not enough voltage is there to overcome resistive losses, operating losses of the cc, absorb voltage point of the batteries involved (and in the case of fla types the eq voltage), and temperature effects then putting another like pv in series can be done with at least trying to match up the imp ratings in the case of differing pvs. there are pvs out there that can put out enough to operate at a 24v potential without the need to series other pvs, but these are in the higher wattage range as well as expense range.

i don't recall the question in the other thread going into the number of 12v pvs it would take to charge a 36v battery bank specifically and i believe if i'm remembering correctly that you wanted to split the batteries into differing voltage loads which you should not do on a battery bank. simply, if a pv can properly charge 1 12v battery then suffice it to say that 2 of the same pvs will charge 2 of the same 12v batteries. do realize too that when we refer to a pv as being a 12v pv that the output vmp of that pv is far above 12v and that 12v rating is for the typical charge requirements presented at that nominal 12v rating that is used by batteries.

somehow, i feel you are leaving pertinent questions out of this like is a 20w pv enough to charge your particular battery and what these batteries are to power.
• Solar Expert Posts: 75 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?
niel wrote: »
i don't recall the question in the other thread going into the number of 12v pvs it would take to charge a 36v battery bank specifically and i believe if i'm remembering correctly that you wanted to split the batteries into differing voltage loads which you should not do on a battery bank. simply, if a pv can properly charge 1 12v battery then suffice it to say that 2 of the same pvs will charge 2 of the same 12v batteries. do realize too that when we refer to a pv as being a 12v pv that the output vmp of that pv is far above 12v and that 12v rating is for the typical charge requirements presented at that nominal 12v rating that is used by batteries.

Well the method for charging that 36v system has come a long way since that thread and a solution has already been found as far as how to successfully charge that portion of the system.
niel wrote: »
somehow, i feel you are leaving pertinent questions out of this like is a 20w pv enough to charge your particular battery and what these batteries are to power.

I can't remember who it was here, but these have been covered as well.
• Registered Users Posts: 2
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Hello,

I've got a system similar to the original poster's that I am trying to set up. I've got two 30W 12v panels wired in series that I'd like to charge two 12v 55Ah gel acid batteries, so it's really a 24v system on both ends.

The problem I'm having is finding a solar charge controller that could handle this set up, does anyone have any recommendations? I have a cheap instapark 24v 10A one in place, and while it passes the battery voltage to my load, it doesn't seem to charge the batteries at all. The solar array is sending out ~38v.

Thanks for any help you can give me.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

without more specs for the panels, one cannot give a definitive answer.... but... you are shy by at least 2 more panels probably as much as 4 short or more... that is why they are not charging, not enough amps total

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Super Moderators, Administrators Posts: 33,553 admin
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Another issue is the charge controller itself.

It will stop current flow if it thinks the battery is full... Some controllers are set with a switch/jumper between 12 and 24 volts, some will even automatically configure based on the battery voltage--So if you had "dead" 24 volt batteries, it may think they are supposed to be 12 volts.

In any case, what is the battery voltage at the terminals of the charge controller? Either it is less than ~25.4 volts (for charger to turn on--fixed, had 24.4 volt typo before. -BB) or over ~29 volts (charger reduces/stops current flow). And, in between the controller may choose to let current through or not.

You can also just put a jumper from PV + to Batt + and see what current flow you have--It should be the maximum supported by the panels/sunlight conditions.

And, how are you measuring the current? Ideally, I would suggest a DC current clamp meter if you can justify it (here is a ~\$60 one). A DMM set to 10 amps can work--but it may also reduce current flow (especially if set to less than 10 amps full scale or there is a blown fuse).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 2
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

Panels are Vpm 17.5, 1.68A each.

Hadn't thought of that, I'll bring a multimeter with me tomorrow try draining the batteries a bit to see if the controller kicks in.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: How Many 12v Solar Panels Needed To Charge 24v Battery?

before you start, you need to do a Voltage check on the battery. If it has sat overnight you will get a 'good' reading as to its SoC. if below 12.6/ 25.2 it did not get a good/full charge or it is failing. if below full charge and you do a load test watch it closely. take readings every 15 minutes, write it down and report back. The truth is in the numbers.. Been there done that...

HTH

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Registered Users Posts: 2
Please how many Solar panels will i need to charge a 36v battery, I have 150watts rated 18-22v each, or 250watts rated 36-44v
• Solar Expert Posts: 891 ✭✭✭✭
Please how many Solar panels will i need to charge a 36v battery, I have 150watts rated 18-22v each, or 250watts rated 36-44v
Between 1 and hundreds, depending on required charge voltage (which depends on cell count and battery chemistry) and capacity (in amp hours.)
• Registered Users Posts: 474 ✭✭✭✭
Please how many Solar panels will i need to charge a 36v battery, I have 150watts rated 18-22v each, or 250watts rated 36-44v
You should start a new thread, rather than tacking on a comment on a 5 year old thread. The folks here have lots of expertise, but your question will get better responses if you give some details on what you are trying to accomplish: What your loads are, when they are running, etc.

Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.